Diversity of living things
Many small animals can be found in the schoolyard environment. Animals are capable of actively moving from place to place at some stage in their life cycle, and they feed by consuming other living things—or parts of them.
Most plants are fixed to one place and need to make their own food by a chemical process called photosynthesis. This chemical reaction usually occurs in the green leaves and uses energy from sunlight (carbon dioxide and water form glucose sugar and oxygen). This process of using carbon dioxide and producing oxygen only occurs in sunlight.
The schoolyard might have animals with a backbone (vertebrates) such as:
- birds (avians—feathers, lay eggs)
- lizards (reptiles—dry scales, lay eggs)
- frogs (amphibians—moist skin and lay eggs in water)
- and some mammals such as cats and dogs.
All mammals have fur and produce milk for their young. There are three groups of mammals:
- placentals (e.g. humans) that feed their unborn babies through a placenta
- marsupials (e.g. koalas and kangaroos) that feed their babies with a milk teat in a pouch
- monotremes (echidna and platypus) that lay eggs and feed their young by secreting milk onto the mother’s belly fur.
Many animals in the schoolyard are, however, likely to be very small and without a backbone (invertebrates) such as earthworms, snails, ants, ant lions, slaters, beetles and spiders. Their exoskeleton provides protection and a framework to which muscles are attached and which allows them to move.
The arthropods include:
- crustaceans, such as slaters and crabs
- arachnids have eight legs, a head and abdomen, such as spiders and ticks
- myriapods have many legs, such as centipedes and millipedes
- insects, the largest groups of arthropods, have a head, thorax and abdomen, six legs and antennae, such as ants and bees.
Scientists have developed different classification systems for animals based on the features and/or the origin of species. These are being revised constantly as new knowledge emerges.
Observing and understanding the diversity of all living things allows students to group living things so that they can identify patterns and change over different time scales. This allows function and relationships within systems to be identified and, models to be used to predict the consequences of change.
On this page:
- Browse these tabs to learn about powerful scientific ideas in biological sciences.
Life cycles
All living things have the potential to reproduce, and the offspring grow and develop through a series of stages. In some, the offspring look like small versions of their parents from the outset, but in others there are very different stages as the young develop. Many insects in particular go through a series of amazing changes.
Every flowering plant starts life as a seed. With the right amount of warmth, air and moisture, a seed starts to germinate by sending roots down into the soil and a shoot up towards the sunlight. If the plant receives enough light it grows to become a seedling, and eventually an adult plant. When it is time for the plant to reproduce, it produces flowers. After pollination and fertilisation have occurred, the flower develops into a fruit containing seeds. If the seeds experience suitable conditions for germination, the life cycle starts over again.
Every living thing goes through life stages. Although a developmental pattern is predictable for most animals, each living thing has a unique sequence of life stages that can make it difficult to make direct comparisons. The purpose of making these comparisons to gain an appreciation of the diversity among living things.
On this page:
- Browse these tabs to learn about powerful scientific ideas in biological sciences.
Living vs. non-living things
Looking at the world around us, we instinctively seek to identify living and non-living things. However, it is hard to absolutely define life.
Scientists agree that life on Earth generally has characteristics, including:
- Movement: All animals move, at least at some stage in their lives. Some plants can open and close their leaves, and sunflowers orient their flower to follow the Sun. However, for many plants their ‘movement’ is their ‘growth’. For example, roots explore the soil by growing into it.
- Respiration: Cellular respiration is a scientific term that describes the release of energy stored in organic compounds, for example, sugar. This is sometimes confused with our respiratory system (lungs) which is the area where the gas necessary for our cellular respiration (oxygen) and the by-product of the reaction is released (carbon dioxide). All living cells need to be able to release the chemical energy in sugars or other chemical molecules. Animals need to eat to gain these molecules, while plants use sunlight to produce their sugar molecules (photosynthesis). Both plant and animal cells use oxygen to break down the glucose sugar to produce energy (cellular respiration).
- Sensitivity: A living thing gathers information about its environment and reacts in consequence. For example, we avoid things that cause us pain. Plants react to their environment by growing towards the light or even by releasing alarm hormones when eaten by a predator.
- Growth: Living things have the ability to grow. Non-living things can also grow, such as stalactites, but it is an external process (the deposit of minerals on a spike) rather than an internal process (growing by means of absorbed energy and nutrients that are reorganised).
- Reproduction: Living things come from other living things and can often create new living things. A worker bee is sterile but is born from a fertile queen and is therefore alive. Plants have the ability both to reproduce sexually (creating seeds) and asexually, for example, runner plants.
- Excretion: Living things excrete things such as excess gases, salts and waste, in order to keep their internal composition constant.
- Nutrition: Living things need to acquire the necessary elements for growth and reproduction from the world around them. Animals need to eat other things to acquire energy to survive (heterotrophs). Plants need to absorb certain minerals, for example, phosphorous, in order to capture energy from the Sun (photosynthesise).
On this page:
- Browse these tabs to learn about powerful scientific ideas in biological sciences.
Adaptations
The first stage of understanding the diversity of living things is identifying the unique structural features of living things. The next step is to ask 'why' all living things are different. Other ways to ask this are 'how does this feature help the living thing survive' or 'what is the survival advantage of this feature'. For example, some animals that live in the desert have large ears. These features allow them to listen for predators so they can feed at night when it is cooler. Some plants have stick-like leaves that help minimise water loss. These features, which are important to the survival of an animal or plant in its native environment, are called ‘adaptations’. Adaptations can also be behaviours, such as the instinct to run from danger or the unfurling of leaves in sunlight. Adaptations evolve by means of natural selection.
Within populations there is variation among individuals, such as in the size of ears and the size of the leaves. If the conditions in the environment favour particular traits, such as larger ears that help to avoid predators at night and narrow thin leaves that help slow dehydration in water-poor environments, then the individuals who have them are more likely to survive and reproduce. If offspring inherit these traits then future generations are more likely to have larger eyes and waxier leaves.
Generally, scientific adaptations are identified at a population level. If a single individual has a mutation (change in the DNA that causes a change in its features), this changed feature may allow the individual to survive more easily to breed. If the individual’s offspring (children) inherit the same feature, they will also survive more easily in the environment. Gradually the new feature spreads through the population and can be considered an adaptation.
The ability to survive in an environment is only one thing that determines what features are common in a population. Other examples include the following possibilities:
- The necessity to reproduce to pass on genes. If individuals selectively choose their mating partners then traits might evolve due to preference. For example, if female birds prefer red chests then the population might evolve red chests. If there is competition to mate then males might develop special characteristics for fighting even if these make it harder to survive. For example, some species of stags fight each other with antlers and therefore have large antlers which can make it harder to walk around in a forest.
- If all individuals have the exact same genes for a particular feature then it will not change or disappear even if it can hinder survival. For example, even if it might be advantageous to have red fur, if all the individuals of the population have black fur then that feature cannot evolve unless the right mutation occurs.
- A mutant might have a structural feature that makes it more difficult to survive, but if the other individuals die from an unexpected event, for example, an avalanche, then the population might end up with that feature.
- Sometimes it doesn’t matter either way—structural features and instincts might not affect an individual’s ability to survive. In that case the population might end up sharing a structural feature or behaviour because of chance.
Species might have very different adaptations to the same environment, and not all these adaptations might be the ‘best’ adaptation. The adaptations that living things can evolve are constrained by traits they cannot change because they are integral to their development, such as plants not being able to develop muscles to walk to a water source and amphibians not being able to evolve fish scales. They are also constrained by traits that are not variable, for example, there might not be any individuals in the population with the ability to store fat in a hump, so even if it is a remarkable adaptation for camels it might not be an adaptation that other species will adopt.
On this page:
- Browse these tabs to learn about powerful scientific ideas in biological sciences.
Biological systems are interdependent
Biological systems are interdependent and interact with each other and their environment.
Living things have basic needs
Living things, including humans and other animals, have needs that must be met for them to stay alive. Some needs, such as food, shelter, gases (e.g. oxygen) and water, are common to all living things, although relative needs may vary. For example, a koala rarely needs to drink fresh water, while a fish cannot survive without it.
While most living things can survive in a wide range of conditions such as temperature, most will have a narrower range of preferred conditions in which they can thrive. For example, humans can live in a variety of conditions from the Antarctic to the Sahara desert. The human population living in these extreme environments is much smaller than in the narrower range of preferred climates. This is due to the limited amount of food, shelter and water present. This is true for animals other than humans.
The word ‘animals’ includes groups such as mammals (including humans), fish, insects, sponges, corals, amphibians, reptiles, birds, spiders, crabs, snails, clams, leeches and worms. All of these animals have different characteristics and abilities which allow them to meet their basic needs in different ways. For more information on animals, see Form and features of living things.
Some needs depend on the type of organism and the environment in which it lives. For example, some plants need to acquire nitrogen from external sources, such as well-fertilized soil. Other plants have root nodules in which special bacteria live that provide them with nitrogen.
Growth and repair of tissues, movement and reproduction are processes that require energy and nutrients. Carbohydrate and fat molecules (‘food’) are stores of chemical energy. Plants and animal cells can break down the complex molecules to release the energy. When they use oxygen to do this it is called aerobic respiration.
Plants can use the energy in the Sun’s rays to produce carbohydrates (sugars). This process is called photosynthesis and is carried out by specialised parts of plant cells called ‘chloroplasts’. Chloroplasts contain chlorophyll pigments which are generally green in colour. The process uses carbon dioxide gas and water and produces oxygen and glucose (sugars), and needs to occur in sunlight. The sugars are stored in plants’ tissues or converted into other molecules, for example, cellulose, to build cell walls. When a plant is creating more sugar molecules than it is breaking down, it produces more oxygen than it consumes.
One of the reasons plants need water is to transport nutrients from where they were absorbed, generally the soil, to where they are needed, generally the leaves. Our heart pushes blood (including water and nutrients) through our arteries and veins, but plants rely on different processes. One process is to open up pores (stomata) on their leaves. Water then evaporates into the air, cooling the leaves and causing water, and its dissolved nutrients, to rise up through small tubes in the stem called xylem vessels. This process is known as transpiration. Sap, which includes dissolved sugars produced by the leaves, circulates through a system of live tissue called phloem.
The timing of the supply of water, in particular having adequate water supply during flowering, is important for getting good grain yields. There are visible effects of over and under watering a plant that indicates that a plant is under stress. For example, when plants are over watered their leaves can turn yellow because roots need air in the soil to function; too much water displaces all the air in the soil and the roots drown. When plants are underwatered they wilt and close their stomata, which means they can no longer capture energy from sunlight using photosynthesis as they no longer get carbon dioxide from the air.
On this page:
- Browse these tabs to learn about powerful scientific ideas in biological sciences.
Living things interreact
No living thing can survive in isolation. Living things need food, water, air, and shelter to survive. Every species relies on other living and non-living things to fulfill these needs. For example, spiders attach their webs to trees and rocks, and micro-organisms obtain energy by decomposing fallen tree leaves. Sometimes the relationship between the living things is only beneficial for one of the organisms, for example, tall trees preventing sunlight from reaching smaller plants.
Living things need energy for movement and other body functions. Many living things obtain energy by eating other living things. The movement of energy through a community can be described in the form of food chains. Most food chains start with light energy from the Sun, which is transformed into chemical energy by a producer (plant) before it is eaten by consumers (animals). Most food chains are 4-5 steps as energy is transformed into movement, heat, and chemical energy at each step.
A decomposer is an organism (living thing) that gain their energy from the chemical energy left in dead organisms. Decomposers are linked to each part of the food chain as all organisms eventually die.
Not all relationships involve one living thing eating another living thing. Pollination and the formation and dispersal mechanisms of seeds are important components of the flowering plant's life cycle. Plants have developed relationships with animals to attract pollinators to visit them and animals to help spread their seeds. When living things have relationships with one another it is called symbiosis (meaning ‘living together’ in Greek). There are a number of different types of symbiotic relationships between living things.
Mutualism and cooperation are symbiotic relationships where two organisms—plant and animal, plant and plant, plant and fungi or animal and animal—mutually gain benefit from their interactions. In a cooperative relationship the organisms can survive in the absence of one of the organisms. Bees and flowering plants have a beneficial relationship due to the fertilisation of the plant when the bee is seeking nectar. Some orchids have a mutualistic relationship as they can only be pollinated by one species of bee. Many plant and bee relationships are cooperative as the flower can be pollinated by other pollinators and the bees can harvest pollen from different flower species. Ants may benefit some plants when they disperse their seeds, and the seed helps the ant by providing packets of food for the ant to consume.
On this page:
- Browse these tabs to learn about powerful scientific ideas in biological sciences.
Living things are affected by their environment
Living things generally require a specific set of physical or environmental conditions to survive, such as temperature ranges, rainfall, energy source (food or sunlight) and shelter. Even when plants or animals live in the same or similar conditions, their needs vary. For example, in the deserts of Australia, brown snakes need to shelter on and around northeast-facing rocks in the desert, which hold heat to help warm their cold bodies in a morning, while bilbies need soft, sandy soil to dig deep, cool burrows to sleep in during the day.
Animals and plants have adaptations to help them survive in their specific environments. For example, Eucalyptus leaves hang straight down to help reduce exposure to the harsh midday sun. This leaf adaptation prevents water loss from heat, while ensuring the leaves capture enough sunlight for photosynthesis and growth.
Living things can require very specific environmental conditions to survive, without which they might stop growing, or become unhealthy from accumulation of minerals or toxins, or even die. Some living things are more tolerant to variations in environmental conditions, for example, cockroaches can thrive in a number of extreme conditions. Many species can survive in sub-optimal conditions, but they do not grow as big or strong and might not reproduce successfully. The health of certain living things (bioindicators) can help indicate the health of an ecosystem.
Salt can have adverse effects on plant health and growth. When water available to plants contains higher concentrations of salt, the roots of the plant absorb less water. If concentrations of salt are high enough, the salt water around the roots will draw water out of the plant. The whole plant loses moisture and suffers stress. Symptoms of high salt damage are similar to those of high moisture stress: leaf tip dieback, leaf edges yellowing, scorching and turning brown or black, followed by leaf fall of dead leaves. Salt can also accumulate in the leaves, causing them to die.
Living things also rely on others for survival. A change in physical conditions might not directly affect them but it may still affect their food sources. For example, bilbies do not need to drink water to survive (they get all their water content from the food they eat), so a change in rainfall does not directly affect them. However, rainfall does affect the plants that they eat which can become less nutrient-dense, stop growing, or even dry out and die.
On this page:
- Browse these tabs to learn about powerful scientific ideas in biological sciences.
Form and features of living things
The form and features of living things are related to the functions that their body systems perform.
Living things can be described based on their features
We classify living things into groups to make sense of the world around us and to communicate about it. The classification system used by scientists today is similar to the one devised by Carolus Linnaeus in the 1700s. He grouped living things on the basis of observable external features and created a ranked hierarchical system of identification. The highest level of classification he identified is the kingdom.
Today, scientists recognise that there are more kingdoms, for example, the kingdom of fungi, the kingdom of plants and the kingdom of animals. Scientists are also working to re-classify species to reflect shared ancestry—to group things that are closely related on a molecular level (DNA) and not just things that look related.
External features of plants
The kingdom of plants is comprised of multicellular organisms that transform the energy of the Sun into chemical energy through photosynthesis. They generally have structures to capture light, for example, leaves, and structures to capture water and nutrients from the environment, for example, roots. Plants rely on external forces to move them from place to place, for example, wind, water or animals dispersing seeds.
Although they vary widely in appearance, virtually all flowering plants have three main parts: roots, a stem, and leaves.
- The root is the part of a plant usually found below ground. Roots anchor the plant in the soil and absorb the water and nutrients it needs to grow.
- The stem is the part of the plant usually found above ground. It provides structural support to lift the leaves up into the sunlight and transports nutrients between the roots and the leaves.
- Leaves are specialised for photosynthesis and are often thin and flat to maximise the amount of sunlight captured for photosynthesis, but they can be a variety of other shapes.
Flowering plants produce flowers and fruit as part of their reproductive cycle. Flowers are the reproductive organs of a plant and usually contain both male and female parts. After fertilisation, the female parts of a flower develop into seed-containing fruits.
External features of animals
The kingdom of animals is comprised of multicellular organisms that must eat other things to survive. They generally have body structures such as claws, teeth and digestive systems for catching and eating their food. All animals are able to move from place to place using internal structures, such as muscles and skeletons, at some stage in their life cycle. More detail is provided in Diverse range of living things/Diversity.
On this page:
- Browse these tabs to learn about powerful scientific ideas in biological sciences.
Features and behaviours can aid survival
Looking at an animal or a plant, you can identify structural features and their probable function. For example, some animals have large eyes that allow them to see in darkness and some plants have waxy leaves that help minimise water loss. These features, which are important to the survival of an animal or plant in its native environment, are called adaptations. Adaptations can also be behaviours, such as the instinct to run from danger or the unfurling of leaves when sunlight hits them.
Generally, scientific adaptations are identified at a population level; one individual with a difference is a mutant who might survive better in the environment. If the individual’s children inherit the trait and also survive better in the environment, then the mutation will gradually become ‘normal’ in the population and be considered an adaptation.
The ‘environment’ for a population is the physical habitat, for example, a desert, and also the community of other livings things in which it lives. For example, the abilities of predators determine the adaptations of prey: if a species’ main predator has eyes that primarily detect motion then the instinct to freeze when spotted is an adaptation to that ‘environment’.
Science seeks to justify with evidence and reasoning why features or behaviours can be considered adaptations to an environment. The desert environment is good for studying adaptations to survive because it is so harsh and so it is rare to find features and behaviours that don’t help a population’s survival. Many desert species are ‘specialists’ (adapted to live in the specific conditions of the desert, for example, the thorny devil) rather than generalists (adapted to live in most environments with varying degrees of success, for example, common rats).
Some plants have adaptations to deal with salinity. For example, mangroves can secrete excess salt through salt glands in their leaves. Although not a complete solution to the problem of salinity in Australia, some crossbred hybrid species of rice and wheat grow reasonably well in low levels of salty water.
More information on adaptations can be found at Diverse range of living things/Adaptations.
On this page:
- Browse these tabs to learn about powerful scientific ideas in biological sciences.
The earth system
The earth system comprises dynamic and interdependent systems; interactions between these systems cause continuous change over a range of scales.
Weather
The term ‘weather’ refers to the local, short-term characteristics of the atmosphere of a particular place. Weather events occur regularly on Earth and include wind, rain, thunderstorms, hail, snow and fog. These events are the result of different masses of air coming together. When air masses meet, differences in temperature, pressure and the amount of moisture in the air might lead to the formation of clouds and precipitation in the form of rain, hail or snow. Average weather conditions tell us about the climate of a particular place.
The term ‘climate’ refers to the long-term or prevailing weather conditions in a particular place. For example, a desert location that has no rain for most of the year might be described as having an arid (dry) climate, even if the weather on a certain day is wet or rainy. To know about the climate of a particular place, we have to observe and record the weather over a long period of time; this gives us an idea of the most common weather conditions, and what to expect in the future. The science of studying the atmosphere and predicting the weather is known as meteorology.
A very important part of meteorology is recording atmospheric data over long periods. Meteorologists use this data to detect patterns in the weather and climate trends. Long-term records not only enable scientists to know about the past, but also help them to better predict weather patterns in the future.
Meteorologists use a wide variety of instruments for observing the weather, including:
- thermometers to measure air temperature
- barometers to measure air pressure
- anemometers to measure wind speed
- hygrometers to measure air humidity—how much moisture (water vapour) is in the air
- weather radars to detect approaching rainfall
- weather satellites to monitor cloud cover, surface temperatures of land and sea and other data about the atmosphere.
Clouds
Clouds are formed when warm moist air rises to where it is cooler and the pressure in the atmosphere is lower. As the warm moist air cools (all gases cool when they expand under low pressure), some of the water vapour in the air changes from a gas to a liquid (condenses), forming tiny water droplets. A mass of billions of these tiny suspended water droplets is visible as a cloud.
"Cumulus humilis clouds" by Toby Hudson (Wikimedia Commons) licensed under CC BY SA 3.0
Rain develops when water droplets join together and become too heavy to remain in the cloud, suspended in air currents. Gravity pulls the droplets towards the surface of the Earth, where they fall as precipitation.
When estimating cloud cover, meteorologists divide the sky into eighths. If they estimate that eight-eighths are covered by clouds, they describe the sky as having total cloud cover. If they estimate that zero parts are covered, they describe the sky as being clear. In weather forecasts, the terminology used is simpler: clear (no cloud), sunny (little chance of the Sun being obscured by cloud), cloudy (more cloud than clear sky) and overcast (total cloud cover).
Temperature
Temperature is the degree of hotness. It is a measure of the intensity of heat rather than the amount of heat. When meteorologists discuss temperature, they are specifically referring to air temperature.
There are three temperature scales used to accurately measure and describe temperature. Australia and most other countries use the Celsius (sometimes called centigrade) scale. In this scale, zero degrees Celsius (0ºC) is the freezing point of water, and 100 degrees Celsius (100ºC) is the boiling point of water. Points below freezing are quoted as negative numbers.
In the Fahrenheit scale (still used in the United States), 32 degrees Fahrenheit is the freezing point of water and 212 degrees Fahrenheit is the boiling point of water.
In the Kelvin (also known as the ‘absolute’) scale, zero degrees is ‘absolute zero’, which is the point at which there is no heat whatsoever in the object being measured and that is as low as temperature can get. In this scale, 273 degrees Kelvin is the freezing point of water and 373 degrees Kelvin is the boiling point. There are no negative absolute temperatures.
The Kelvin scale is most often used by physicists and chemists because certain relationships are simpler to describe when absolute temperature is used. The Celsius and Fahrenheit scales are commonly used to measure everyday temperature.
Wind
Wind is the movement of air over the surface of the Earth. Winds are driven by the heat from the Sun, which warms the air and causes it to rise. Where warm air rises, cool air flows in to take its place. This motion is felt on the Earth’s surface as wind. Wind is described in terms of its strength or speed and the compass direction from which it is blowing. Wind speed can be expressed in kilometres per hour, metres per second, knots, or as a force on the Beaufort Scale.
When weather forecasters describe the wind they usually include information about both its strength and direction. For example, a fresh south-westerly wind is a wind blowing from the south-west at an average speed of 30–39 kilometres per hour (the speed of ‘fresh breeze’ on the Beaufort Scale). The term ‘gusty’ is often used in weather reports to describe winds that have sudden increases above the average speed for short periods of time.
Knowledge of wind strength is useful for recreational activities like sailing, the safety of people working in high places or at sea and for forecasting the movement of pollution and smoke from bushfires to populated areas.
On this page:
- Browse these tabs to learn about powerful scientific ideas in earth and space sciences.
Seasons
Seasons are caused by three phenomena:
- the revolution of the Earth around the Sun once per year,
- the tilt of the Earth’s axis of 23.5 degrees relative to the perpendicular to the plane of the orbit around the Sun, and
- the location on Earth.
Countries on or close to the equator experience very little seasonal temperature variation because the Sun’s rays are very direct all year round, the tilt of the axis having little effect at the centre of the globe. The poles experience the most extreme seasons, with approximately six months of sunlight and six months of darkness and the greatest differences in angle of the Sun’s rays.
When the Earth is tilted towards the sun, the days are longer and nights shorter. The greater time for direct sunlight exposure heats the ground and atmosphere.
Different places and cultures in Australia observe different seasons. Being such a large country, with locations close to the equator in the north and closer to Antarctica in the south, many locations in Australia experience the seasons differently.
Some people in northern tropical areas of Australia identify two seasons: the wet and the dry. Others, such as the Nunggubuyu people, identify five seasons. In the south of Australia people commonly identify four seasons, each lasting approximately three months: spring, summer, autumn and winter.
Official dates of starting seasons vary across the globe. For example, the start date is close to the 20th day of the month for many European countries but always on the 1st day of the month in Australia. Seasonal changes are variable and depend on many factors.
On this page:
- Browse these tabs to learn about powerful scientific ideas in earth and space sciences.
Soils, rocks and minerals
Rocks and minerals
The outer crust of the Earth is made up of rock. In places the rock is covered by soil or water. This outer shell isn’t rigid and events such as earthquakes and volcanos help shape the landscape as they impact on the rocky layer and bring new material to the surface. The word ‘rock’ in common usage can refer to large cliffs and boulders as well as to small stones or pebbles. Scientists are more precise when they use the word ‘rock’ and understand it to mean an aggregate of minerals.
A rock is made of mineral materials in a solid state. Geologists use features such as the mineral composition and the shape, size and orientation of the fragments in the rock to classify rocks according to their origin. Both the composition of the minerals and the circumstances under which it was formed determine the features and properties of the rock.
Some commonly measured properties include:
- Density: the ratio of mass to volume for a material. Measuring density tells you if a material is heavy for its size. A rock that floats in water is less dense than water.
- Hardness: how resistant solid matter is to various kinds of permanent shape change when a force is applied. A common measure is to see whether the rock scratches different surfaces to determine if it is harder than the material it is being scratched against. Hardness is usually measured for minerals rather than rocks. The hardest mineral known is diamond whilst the softest is talc. The Mohs hardness scale developed by Fredrich Mohs ranks 10 common minerals based on their hardness.
Rocks are made up of a variety of minerals put together in different ways resulting in different colours and textures. Geologists classify rocks based on their texture and composition. The texture of a rock refers to the size, shape and arrangement of the constituent mineral grains, whilst the composition of a rock is based on the chemical composition of the minerals it contains. Minerals have physical properties, such as cleavage, hardness, specific gravity, colour and streak (the colour of the powdered mineral).
These rocks are made up of mineral salts.
The term ‘property’ refers to an attribute of an object or material, normally used to describe attributes common to a group. In the case of rocks and soils it is the minerals they are composed of that exhibit these properties. For ease of understanding it is suggested that the term ‘feature’ is used to allow students to describe aspects of an object or material. Typically, a student description of a rock might mention some properties of minerals (hard, jagged, colour) and many that are not properties (big, small, round, smooth).
Geologists (scientists who study the origin, history and structure of the earth) classify rocks into three basic types depending on the way that they were formed: igneous, sedimentary and metamorphic.
- Igneous rocks are formed from the solidification of the minerals found in magma.
- Sedimentary rocks are formed at the Earth’s surface from the accumulation and consolidation of sediment.
- Metamorphic rocks are formed from pre-existing rocks within the Earth’s crust by changes in temperature, pressure and by chemical action.
A volcanic rock is formed when the molten minerals (magma) inside the Earth arrive at the surface through, for example, volcanic activity or sea rifts. Not all magma is extruded at the surface—some remains beneath the surface as intrusions into surrounding rock where it can cool. At the Earth’s surface magma becomes lava. As it cools, the minerals change from liquid to solid and form igneous rocks. The nature of the materials formed depends on the rate of cooling. If lava has a long time to cool, then individual minerals separate as distinct crystals to produce, for example, basalt with many different grains of crystals. However, if the same lava cools quickly, the molten mineral mix will harden and form a glassy black rock called obsidian. If it cools quickly and captures air pockets, a light crumbly stone called pumice is created; this is not dense and floats on water. All these rocks will have the mineral composition of the lava that created them, which might also be specific to different regions.
Sedimentary rocks are the most common type, covering approximately 75% of the surface of the continents. Some examples are: sandstone, siltstone, shale, limestone, chalk, gypsum and coal. Their formation involves weathering of pre-existing rock, transportation of the material away from the original site (erosion) and depositing the eroded material in the sea or in some other sedimentary environment. Sedimentary rocks typically occur in layers or strata that cover large parts of the continents. The Grand Canyon in the US is a good example of sedimentary rock strata. Sedimentary rocks are formed from sediments that have been compacted and cemented to form solid rock bodies.
Metamorphic rocks are formed from rocks that have been altered by heat, pressure and chemical action to such an extent that the diagnostic features of the original rocks are modified or obliterated. Some examples of metamorphic rocks include slate, quartzite and marble.
Soil
Soil is composed of small particles of rocks and minerals, plus varying amounts of organic material (derived from living things), water and air. The particles are of different sizes ranging from sand to silt to clay. Sand makes a soil feel gritty; silts are similar to clays but have slightly larger particles; and clay feels silky to the touch because it has the smallest particles. The mixture of these particles gives soil its texture which influences how much water a soil can hold. Generally, the smaller the soil particles (the more silt and clay), the more water a soil can hold. Soil scientists use texture to classify soils, such as sand, loamy sand, loam, clayey sand and medium clay. The ideal soil texture for growing plants is loam, a mixture of clay, silt and sand.
Soil composition is different in different places. These differences can be seen in a very small distance, such as from one side of a garden or farm to the other, as well as from country to country. The differences depend on the type of rocky material from which the soil was made and the kinds of organisms that live in, around and on the soil.
Colour is a simple method of classifying soil. Black or dark brown soil is generally fertile soil for growing plants. Plain brown or yellow soil often indicates that the level of nutrients and organic matter is low and the fertility of this soil is low. Pale soils need plenty of organic material and mulching to become fertile. Red soil usually indicates extensive weathering and good drainage, but often it needs nutrients and organic matter to be fertile. The red colour is due to the oxidising of iron compounds (‘rusting’) in the soil. Organic material that can still be decomposed is called compost, whereas organic material that is stable is called humus (a Latin word meaning earth or ground).
On this page:
- Browse these tabs to learn about powerful scientific ideas in earth and space sciences.
The water cycle
Water is a natural resource of Earth. Chemically, it is a molecule made up of two atoms of hydrogen and one atom of oxygen, represented by the chemical formula H2O. It is one of the few materials that can naturally be found as a solid, liquid and gas in the temperature range usually found on the Earth’s surface.
Water is abundant on our planet. About three-quarters of the Earth’s surface is covered by water. It is found in oceans, lakes, rivers and dams. Ice, snow and clouds are forms of water.
97% of the water on the Earth’s surface is found as salt water in oceans, which teem with marine life. The remaining 3% is fresh water (containing only a small amount of dissolved materials, such as salt and minerals), but two-thirds of this is found as ice and snow at the poles. This means that of all the water on Earth about only 1% is useable fresh water.
Water:
- helps living cells keep their shape
- moves dissolved substances around our bodies in our blood
- is used in many important biochemical reactions
- is necessary for plants to grow.
Animals and plants consist mostly of water.
Fresh water is formed from water that has evaporated from the oceans, rivers, lakes, wet soil and vegetation on the Earth’s surface. When salt water evaporates, the salt is left behind and pure water vapour rises into the air. The water vapour in clouds condenses around dust particles, forming larger and larger droplets of water. When these drops become big enough and heavy enough, they fall out of the air as rain (precipitation). If the temperature is cold enough, the droplets will become snow or hail. Fresh water eventually makes its way back to the ocean, mixes with the salt water there and the water cycle begins again.
Water is constantly cycled through our environment.
Some of the rain runs across the Earth’s surface into creeks and rivers and from there into lakes and dams. This water is called surface water. Some of the water soaks into the ground and collects in the cracks and crevices there. This water is called ground water. Ground water feeds underground streams and aquifers. It is often pumped back up to the surface through bores and is an important source of water for industry, agriculture and domestic homes and gardens. Surface water and ground water are the two main sources of water for use in Australian homes and schools.
On this page:
- Browse these tabs to learn about powerful scientific ideas in earth and space sciences.
Weathering and erosion
The surface of the Earth is constantly changing. Land is being uplifted through processes such as plate tectonics and volcanic activity. Landforms are then further shaped through the processes of weathering and erosion.
As matter cannot be created or destroyed, the matter in rocks must be constantly cycled.
Weathering
Weathering is the process by which rocks are chemically altered or physically broken into fragments, and involves little or no transportation of the fragments. Rocks can be weathered, for example, by ice formation in rock cavities which breaks the rock apart, and changes in temperature causing the rocks to expand and contract producing fractures.
Rocks can also be weathered through chemical processes, for example, the mild acidity of some rainfall can cause minerals in the rock to slowly dissolve. This is how caves such as the Jenolan Caves in New South Wales were formed. Acid can also be released from living things, for example, the decay of organic material, or by direct secretion of acids (lichens).
"Wave Rock (Western Australia)" by Gabriele Delhey (Wikimedia Commons) licensed under CC BY SA 3.0
This rock formation is a result of weathering by wind and sand.
The rate at which weathering occurs depends on three main factors: climate, the susceptibility of minerals to weathering and the amount of surface exposed to the atmosphere. The Devils Marbles in the Northern Territory are an example of rock bodies modified by weathering.
Erosion
Erosion is the removal and transport of rocks and weathered material, for example, soil. It helps to shape landscapes. Agents of erosion include water, ice, wind and gravity. Erosion is affected by variations in the Earth’s surface. For example, if a creek flows over a cliff, a deep pool often forms at its base because the impact of the water falling from a height causes more erosion. Once there is mild erosion, subsequent water tends to flow in the same place, creating deeper and deeper river beds. Human activity can also lead to erosion, for example, the removal of vegetation and constructions such as fences or dams affect erosion rates.
Erosion is a natural process that can be affected by human activity. Erosion, especially in Australia, is a serious concern given the slow-forming ancient soils prevalent in this country. Farmers are particularly concerned about erosion if it removes the soil required for crop growth. Living things, for example, plants both help and hinder erosion. Plant roots bind soil, reducing wind erosion, but can also help weather rocks. Farmers in windy areas often plant rows of trees between their fields. This slows down the wind at ground level, which reduces the amount of soil lost to erosion.
Trees bind soil with their roots, slowing erosion, but their roots can also assist to break up rocks. The decay of once-living things (organic matter) can cause water to become more acidic, inducing chemical weathering of rocks. Rainfall and wind corridors are in turn influenced by the landscape.
By clearing plants away and leaving soil bare after harvest, farmers can leave soils vulnerable to erosion. The top layer of the soil is the most susceptible to being blown away. This is also the richest source of nutrients for crops grown by farmers.
Transportation and deposition
Erosion takes particles away, but they are eventually deposited somewhere. Many particles end up in the sea or in lakes where they accumulate and form sediments that can eventually become new (sedimentary) rocks.
Layers of sediment can harden over a long timescale to form new rock.
Time scale of change
Many rocks take a long time to be broken down and worn away in natural conditions. However, some rocks can be weathered very quickly once they are exposed. For example, the limestone of the Twelve Apostles formation in Victoria developed around 20 million years ago on land. Between 7,000 and 10,000 years ago, at the end of the last ice age, sea levels rose and the limestone was exposed to the sea. Since then, the force of the waves has slowly weathered and eroded the cliffs to create isolated stacks of rocks. In 2005, one of the stacks collapsed, leaving eight standing. The rate of erosion at the base of the limestone stacks is approximately 2 cm per year. Due to wave action eroding the cliff face, existing headlands are expected to become new limestone stacks in the future.
 "Apostel Panorama" by Paul Haydock-Wilson |  "Twelve Apostles, East view" by Daniel 'DXR' (Wikimedia Commons) licensed under CC BY SA 3.0 |
The creation of soils can take hundreds of years, but the erosion of soils can happen very quickly. For example, a mound of loose soil on a hill can be destabilised by water or by vibrations from an earthquake. The hillside might then suddenly collapse due to gravity. The presence of plants such as trees provides a buffer against wind, and all plant roots can help bind soil, reducing the amount blown or washed away.
On this page:
- Browse these tabs to learn about powerful scientific ideas in earth and space sciences.
Earth is part of a system
Earth is part of an astronomical system; interaction between Earth and celestial bodies influence the Earth system.
The Solar System
The Sun, Earth and Moon belong to our Solar System, which includes all the space objects—planets, moons, comets and particles of dust—that are in orbit around our Sun.
A ‘space object’ (or celestial body/heavenly body/space body) refers to naturally occurring objects in space, such as planets, asteroids and comets. Space objects in the Solar System are visible because light from the Sun reflects off them to reach our eyes.
The whole Solar System is moving at great speed through the Galaxy. Our Solar System is part of the Milky Way Galaxy, which contains tens of billions of stars. The universe comprises billions of galaxies.
Stars
Stars are space objects and some are similar to our Sun. They are much further away—the light from the nearest star takes more than four years just to reach us, whereas the light from the Sun takes eight minutes. The light from distant stars, therefore, appears to be not as bright in the sky as the Sun, which is why stars cannot be seen during the day. Sometimes the first ‘star’ to be seen at dawn or dusk is in fact the planet Venus, reflecting light back from the Sun to our eyes.
The stars appear to be in groups, which we call constellations. Different cultures have named some star groups as constellations, based on their mythology. People observed that during the night different constellations passed overhead, and some constellations were more visible at different times of the year. Different cultures have used this observation to track time and/or have explained the patterns in the sky through myths and legends. Some of these myths and legends link the events in the sky to seasonal events at the Earth’s surface.
Planets are kept to their orbits around the Sun by the force of gravity between them and the Sun. The further a planet is from the Sun, the longer it takes to go once around the Sun (its year). The four inner planets (Mercury, Venus, Earth and Mars) consist of dense rocky material. The four outer planets (Jupiter, Saturn, Uranus and Neptune) are collections of gases and are much less dense. Earth is the only planet to have liquid water at its surface.
Moons of planets are kept in orbit by the planets’ gravity. Mercury and Venus do not have moons; Saturn and Jupiter have dozens. Saturn and Uranus have rings made of a huge collection of small orbiting fragments. Generally, moons are made of rock and have little or no atmosphere. However, some moons of gas planets, for example, Jupiter, are very large and are almost like small rocky planets themselves. There are more than 160 known moons in the Solar System.
Between Mars and Jupiter there is a belt of asteroids that orbit the Sun. Asteroids are smaller, rocky space objects. Some are quite large and have other asteroids orbiting them. The largest asteroid, Ceres, has been classified as a dwarf planet, similar to Pluto. Beyond the last planet, Neptune, there is another ring of small bodies similar to the asteroid belt, called the Kuiper belt. Space objects within the Kuiper belt are rocks and ice objects. Three of the larger bodies within the Kuiper belt—Pluto, Haumea and Makemake—are currently classified as dwarf planets. The Solar System also includes human-made objects, such as space probes (robotic spacecraft sent to collect information), space stations and satellites.
Sun
The Sun is a rather ordinary star in an immense galaxy, the Milky Way, which contains about 100,000 million stars. Our galaxy is shaped like a flat spiral, and our Sun is about two-thirds of the way out from its centre. The galaxy is so immense that light takes about 100,000 years to cross it. Our galaxy is only a tiny part of the universe. The estimated number of galaxies out to the edge of what we can see with our largest telescope is about 100,000 million.
The Sun is approximately 70% hydrogen gas, which undergoes constant nuclear fusion to produce helium at its core and release energy as visible light and heat. Because the Sun is so massive, everything else in the Solar System is attracted to it by the Sun’s gravity and everything revolves in orbit around it.
The Sun is a medium-sized star that is in the middle of its life cycle (4.6 billion years old). The Sun is always heating and lighting the Earth, but only the side of the Earth facing the Sun experiences daylight. The rest is in shadow. The reason we experience alternating night and day, or the apparent rising and setting of the Sun, is that the Earth is spinning on its axis, once every 24 hours.
Earth
The Earth is a planet in orbit around the Sun. This orbit takes slightly more than one year—365¼ days—so we add an extra day to our calendar, 29 February, in every fourth year, which we call a leap year.
As the Earth slowly orbits the Sun, it rotates on its axis once every 24 hours (a day). The rotation of the Earth causes the apparent rising and setting of the Sun and is the reason we experience alternating night and day.
The Earth is the only habitable planet in the Solar System and, as far as we know, in the universe. The Earth’s atmosphere contains oxygen, which is necessary to support life. It also contains carbon dioxide, which acts like a blanket, keeping the Earth at a temperature that will support life. Earth is also the only planet in the Solar System with liquid water on its surface.
Earth and its moon
For a long time, the Earth was thought to be flat, because that is how it appears to an observer standing on the ground. Because of the Earth’s immense size, it curves over large distances, which makes it nearly impossible to observe its curvature with the naked eye.
In the fourth century BC, Aristotle proposed that the Earth was round, based on three observations. Firstly, ships sailing away from land appeared to vanish hull-first into the ocean as they sailed into the distance. If the Earth was flat, we would expect these ships to get smaller and smaller until they disappeared. Secondly, the further south a person travelled, the higher the southern constellations rose in the night sky, meaning the person’s angle of view was changing, which is not possible on a flat disc. And thirdly, the shadow of the Earth on the Moon during a lunar eclipse was always a circle, and only a sphere always casts a circular shadow.
Moon
The Moon is a satellite of the Earth. It is held in orbit by the Earth’s gravity and goes around the Earth relatively quickly because it is close to the Earth. The Moon’s gravitational pull on the Earth is not nearly as strong as the Sun’s, because the Moon is less massive, but it is enough to draw the Earth’s oceans towards it and cause the tides.
On Earth we only see one side of the Moon.
We always see the same face of the Moon from Earth, because the Moon spins on its axis once each time it goes around the Earth. The Moon is made of rock and has virtually no atmosphere. The Moon itself does not produce light; we can see it from the Earth only because the Moon reflects light from the Sun.
The Moon reflects the light from the Sun
On this page:
- Browse these tabs to learn about powerful scientific ideas in earth and space sciences.
Sun, moon, planets and stars
Day and night
Night and day are the result of the Earth spinning on its axis. The Earth makes one complete rotation on its axis approximately every 24 hours. The side of the Earth facing the Sun is in daylight, while the side facing away is in the Earth’s own shadow (night). Although the Sun appears to move across the sky during the day, it is not movement by the Sun but the Earth’s rotation that causes the apparent movement of the Sun across the sky. The Earth spins in an anti-clockwise direction when viewed from the North Pole, so that the east coast of Australia moves into the sunlight first in the morning and sunrise is experienced two or three hours later (depending on daylight saving) on the west coast.
The Earth's rotation can be illustrated through the movement of shadows. A shadow stick can be any tall, narrow object that can be set up vertically. In the early morning, as the Sun appears to rise in the East, the stick will cast a long shadow towards the West. In the late afternoon, as the Sun sets in the West, a long shadow will point eastwards.
If the length and direction of the shadow are marked at regular intervals throughout the day, the shortest shadow can be determined; this occurs at solar noon. Solar noon is the time of day, halfway between sunrise and sunset, when the Sun is at its highest point in the sky. This might not be at 12 noon as solar noon is usually different from clock noon because of artificial time zones and daylight saving arrangements.
At sunset, daylight gradually decreases until there is no light to illuminate objects for us to see. We say it is dark at night because there is an absence of light. If the Moon is in a position where it is fully or partially illuminated by the Sun, we will see it at some time during the night and will experience some objects illuminated by moonlight. We can see stars gradually seem to appear as darkness falls. They are always present in the sky but the Sun’s brilliant light blocks the more distant starlight during the day, so they seem to disappear. The stars produce their own light and ‘twinkle’ because they are tiny points of lights that are affected by their passage through the Earth’s atmosphere by turbulent winds. Some of our solar system’s planets are also visible at night. Venus and Mars are easy to spot with the naked eye. They reflect light from the Sun and are big enough that the effects of the atmosphere cannot normally be seen and they do not ‘twinkle’. The planets change their positions in relation to Earth as their own orbits progress around the Sun.
Moon phases
The Moon also rises in the east and sets in the west, but at different times to the Sun because the Moon revolves around the Earth once per month. Sometimes the Moon is on the same side of the Earth as the Sun, and we will see it rise and set during the day. When it is on the opposite side of the Earth from the Sun, the Moon will rise at night and set in the morning. The Moon also rotates on its axis once per month. The result is that we only see one side of the Moon from Earth.
The Moon cannot produce its own light but is visible when it reflects light from the Sun to Earth. We see the Moon’s phases because we only see the part of the Moon that is illuminated by the Sun. When the Moon is in a position in relation to the Earth and Sun where its whole face is illuminated, we see a full moon at night. When the Moon is on the same side of the Earth as the Sun, we will see a new or crescent moon, with very little of its face to Earth illuminated. During the rest of its revolution around the Earth we will see differing amounts of the Moon illuminated at differing times of the day and night. This pattern repeats itself approximately every 27.3 days.
The moon phases are a result of position of the Sun (and not the shadow of the Earth).
Summer and winter
The Sun itself will change position in relation to its observers on Earth and appear to move in an arc across the sky from east to west. This arc will vary in height depending on the season, due to the tilt of the axis of rotation of the Earth relative to the plane of its orbit around the Sun and its position as it revolves around the Sun. In summer the Sun’s rays are more direct and the arc is higher in the sky. In winter the rays are more angled and the arc is lower in the sky.
On this page:
- Browse these tabs to learn about powerful scientific ideas in earth and space sciences.
Changing substances
Substances change and new substances are produced by rearranging atoms; these changes involve energy transfer and transformation.
Physically changing materials
Physical changes affect some of the properties of an object, for example, stretching a hair tie changes its shape but not necessarily its colour. Physical changes can also change the properties of the material the object is made of, for example hair ties often become larger over time as they no longer bounce back to their original shape after many periods of stretching (the material has lost elasticity). Scrunching and folding may change the physical shape of an object but not change the material at all.
A chemical change is where a substance is transformed into a new substance (or substances) at the molecular level. When you burn a piece of toast, the bread changes into charcoal, carbon dioxide and water. Other examples of chemical change include rusting and mixing sodium bicarbonate with acid, such as citric or tartaric, in water to create carbon dioxide and water.
In the early years, students study whether different actions (combinations of forces) change the shape of objects, and whether these physical changes affect the material the objects are made of. The actions include:
- Bending: causing an object, such as a wire or a pipe cleaner, to become curved. Bending involves parts of the object being pushed or pulled towards each other, for example pushing your two hands together, or pushing down on a sheet of cardboard that is held on a table by a heavy object (force arrows indicate direction of force).
- Stretching: making an object wider or longer by having two opposing forces pulling it.
- Scrunching: crunching, crushing or crumpling the object by pushing it towards itself.
Bending, stretching and scrunching will often cause an object to change shape. Different objects will be changed in different ways depending on their shape, the materials they are made of and the strength and location of the forces applied. For example, rubber bands will return to their original shape and size after being stretched, thin copper wire can be bent and keeps its shape, and a sheet of glass might break if twisted. These are all examples of physical changes. Some physical changes are reversible, such as bending a paper clip. Other physical changes, for example, snapping a popstick, are not reversible.
On this page:
- Browse these tabs to learn about powerful scientific ideas in chemical sciences.
Changes in state
Heating a substance might cause it to change from one state (solid, liquid, gas) to another. This includes changing from solid to liquid, and liquid to gas. The material remains the same but has a different appearance or shape. By removing heat, the change can often be reversed, that is, the material will change from liquid to solid or gas to liquid. For example, when chocolate is heated sufficiently it changes from a solid to a liquid. When the melted chocolate is cooled it changes back to a solid. In this case, the change is reversible as a new substance was not formed: the liquid is still chocolate.
When a material changes from the solid state to the liquid state, it is called melting. When it changes from the liquid state to the solid state it is called freezing. For each material there is a specific temperature at which this change of state occurs. This is called the ‘melting point’ or ‘freezing point’ of the material depending on which way the state of matter is changing.
Different changes occur at different rates. A major factor in most change processes is the temperature. Many changes will occur at a faster rate if the temperature is raised. The size of a piece of chocolate and the surrounding temperature are also factors that affect how fast the chocolate will melt. A smaller piece of chocolate will melt faster than a large piece because the heat does not have to penetrate as far. A piece of chocolate in a hot place will melt faster than a piece of chocolate in a cooler place. In order to melt, the temperature of a substance must be raised above its melting point. For this reason, chocolate will never melt in a refrigerator.
When a material changes state, the atoms or molecules do not change. It is the way the atoms or molecules are spaced and held together that changes. Physical changes of state are easily reversible when the materials are ‘pure’ (only containing one type of atom or molecule).
When a non-pure solid (a physical mixture of substances) melts and becomes liquid, sometimes the components can separate. Therefore, when the liquids are put back into the freezer, the original solid might not be re-created. For example, melted and refrozen ice-cream becomes two separate solids: ice and frozen cream.
Some solids undergo chemical reactions when heated and the atoms or molecules react and produce new substances. These reactions are called chemical changes and are not examples of changes of state. For example, wood does not melt when heated but burns (combines with oxygen) instead. Some complex liquids, such as egg white, cook and become solid when they are heated. The nature of the egg white is chemically changed as proteins are broken up, recombine and form new proteins.
On this page:
- Browse these tabs to learn about powerful scientific ideas in chemical sciences.
Reversible and irreversible changes
Physical change is a change in which no new substance is formed. The object itself might not remain the same, such as, a rock could be ground to powder or a mug could be smashed to pieces, but the material particles are still present. There is still rock and porcelain. Physical change occurs when an object receives or loses energy. This might be from a force, for example, by being hit, or when a substance gains or loses heat energy, such as when an ice cube melts or liquid water freezes.
Chemical change is a change that results in the conversion of the original substances to form new substances. The new chemical substances might be in the form of a gas, liquid or solid. For example, it is easily seen that charcoal is created when toast burns, but the combustion of the cellulose in the bread also produces invisible carbon dioxide and gaseous water. When a substance undergoes a physical change, however, its chemical composition does not change—water is H2O whether it is in the form of a gas (steam), liquid (water), or solid (ice). Common salt—solid sodium chloride—can be recovered from salty water by evaporation and dissolved again to form salty water in which the two components of salt are still present. Sugar crystals can similarly be recovered from sugar-sweetened water without chemical change.
In Year 6 students investigate reversible physical changes (melting, evaporating and dissolving) and irreversible chemical changes (burning and the formation of gas). However, some chemical changes are reversible, for example, the chemical reaction which drives rechargeable batteries. Some physical changes are also irreversible, for example, grinding a log into sawdust.
Some other common changes are:
- Rusting iron, which is caused by the iron particles reacting with water and oxygen to form a new substance (rust). The original metal is changed into a new substance. This is a chemical change.
- Cutting wood is a physical change since the wood changes form. However, if the friction of the chainsaw produces enough heat some of the wood might reach ignition point, in which case some sawdust might burn and undergo a chemical change.
- Bending wire is a physical change, since the object only changes form.
- A glowing tungsten filament in an incandescent light bulb is not a chemical change. The heat and light produced are forms of energy, not new substances. The filament is heated to its ignition point but as there is no oxygen, the wire does not burn—it glows.
- Bubbles appearing in carbonated drinks is a physical change. Carbon dioxide gas was dissolved in the drink at high pressure, and when the pressure was released, the particles come out of solution, become a gas again, and leave the drink.
When some substances undergo physical changes, there can also be some minute chemical changes occurring at the molecular level. These changes are only detectable through scientific testing. For example, when sodium bicarbonate dissolves in water the majority of the change is physical. At the molecular level, some of the sodium bicarbonate and water react to produce a slightly alkaline solution.
Dissolving
When a solid (for example, in a powder form) is added to a liquid, it might dissolve. When this happens the particles of the solid completely disperse in the liquid so that they are no longer visible (not to be confused with a suspension of a solid in a liquid). For example, when table salt is dissolved in water, a liquid solution is formed that contains a dissolved salt. The salt is not changed into another chemical substance: the salt remains as salt but is now dissolved in water.
Not all substances will dissolve in water, for example, nail polish does not dissolve in water but does dissolve in acetone. Some solids will dissolve in water but not in other liquids, for example, aspirin tablets will not dissolve in oil. Whether or not things dissolve depends on the properties of the liquids and the solids, for example, fats will not dissolve in water.
If a powdered solid does not dissolve in a liquid, the liquid will appear cloudy when it is stirred and will become clear again when the powder resettles at the bottom. This is because the solid particles block light more effectively than the liquid does. After adding a soluble powder, a liquid might become cloudy when it is first mixed. This is because it takes time for solids to dissolve. Stirring helps the process by allowing the water to reach as much of the solid as possible.
Liquids such as liquid water can only dissolve a certain amount of a solid at room temperature and pressure. That is why some solid might remain undissolved if too much of a solid is added to the water. The salt or sugar dissolves until the ‘solution is saturated’. You can increase the amount of substance that dissolves in a liquid by increasing the temperature or pressure. Water can dissolve more sugar crystals when it is hot. If the sugar solution is saturated when it is hot, sugar will crystallise from the solution when it cools.
Gases can also dissolve in liquids. ‘Carbonated water’, used for soft drinks, is water in which carbon dioxide gas has been dissolved. The water and gas are put under pressure so more gas is dissolved in the water than would be the case under normal atmospheric conditions. When the pressure is released by opening a bottle or can, the excess carbon dioxide escapes from the water as bubbles.
Irreversible fire
Fire is what we see in the chemical reaction of combustion. This reaction requires three things: heat, fuel and oxygen. ‘Getting the fire started’ literally means creating enough heat for the fire to sustain itself while there is fuel and oxygen available. Once started, the reaction produces heat energy. The fuel provides the chemical energy for the reaction. It is generally carbon-based, such as wood, coal, natural gas, oil or wax. Oxygen from the air reacts with the fuel in the heat of the reaction. Combustion reactions usually produce carbon dioxide gas and water vapour, for example:
candle wax + oxygen → carbon dioxide + water
The wick of a candle burns long enough for the wax to start melting. The liquid wax is drawn up the wick to the flame where it becomes a gas. As a gas, it reacts with the oxygen and heat to generate carbon dioxide and water. This reaction releases energy in the form of heat. The carbon dioxide and water produced are gases; however, the water vapour will condense once away from the heat of the candle. Some wax particles begin to react with the oxygen but leave the candle before they are completely changed; these particles create a black smear where they land.
On this page:
- Browse these tabs to learn about powerful scientific ideas in chemical sciences.
Chemical and physical properties
The chemical and physical properties of substances are determined by their structure at a range of scales.
Properties of materials
Scientists use the word ‘material’ to refer to all matter in the universe; this means all solids, liquids and gases (and plasmas) that exist. It includes all the objects and animals we see every day; the water in rivers, lakes and oceans; and the air we breathe. All of the objects encountered and used in daily life are made from materials. Materials take up space and have mass.
The following differentiation can be useful:
- an object is made of material(s)
- a material is made up of substance(s)
- properties are the physical characteristics or attributes of objects (for example, size or shape) or materials (for example, texture or flexibility).
Materials
Some objects are made from a single material, for example, a steel sewing needle or a wooden plank. Others are made from a combination of materials, for example, toys can be made by combining plastic and fabric. Metal saucepans often have heat-resistant plastic handles. The properties of an object are determined by the materials that are used to make it and how those materials are put together. Materials can be classified in a variety of ways, including:
- by origin, such as natural or processed/manufactured
- by type, such as metal, glass, fabric, ceramic or plastic
- by properties, such as porosity or absorbency
- by uses, for example, construction materials.
Plastic is the name given to a wide group of materials characterised by their:
- chemical structure, generally made from long molecular chains, and
- ability to be moulded into different shapes, such as films, fibres and solid objects.
Most plastics are synthetic and are often produced from byproducts of crude oil and natural gas, which are non-renewable resources. Two examples of synthetic plastics are cling wrap and Polyethylene terephthalate (PET) used to make drink bottles. There are also a few natural plastics, for example, natural rubber.
Properties
Objects and materials can be described according to their properties, including their size and shape. The properties of an object can also rely on the materials that it is made of, such as water resistance and transparency. Properties of materials do not rely on the properties of the object. For example, a plastic doll with one missing arm is a different object from a doll with two arms, but it is still made of plastic. The properties of the material have stayed the same but the properties of the object have changed.
Properties of materials can include:
- Strength: a material’s ability to resist forces applied to it. The more force a material can resist, the stronger it is. Tensile strength refers to a material’s ability to withstand being pulled end from end, while compressive strength refers to a material’s ability to withstand being compressed or squashed.
- Hardness: how easily the substance is worn away or scratched. Diamond is the hardest naturally occurring substance known and can only be scratched by other diamonds.
- Brittleness: a material is brittle if it is hard but breaks easily (like glass).
- Transparency: how well light passes through a material. If a material can be seen through, it is described as transparent. If it lets light through but still obscures vision, it is translucent and if it doesn’t let light through, it is opaque.
- Elasticity: a material is elastic if it changes shape when a force is applied to it and recovers its original shape when the force is removed. Rubber and many types of plastics are very elastic.
- Malleability: how easily a material can be bent or shaped. A material that can be deformed or reshaped easily is said to be malleable.
- Conductivity: how easily a material transmits heat and electricity. Most metals are very conductive, whereas plastics are usually good insulators, meaning they are not conductive.
- Viscosity: how much a fluid resists flowing. Honey is quite viscous because it flows slowly whereas water flows easily and is not very viscous.
- Density: the mass of a substance per unit of volume. Lead and gold are very dense, while cork is not.
- Porosity: materials that allow water, air and other fluids to move into them are porous. A washing up sponge is porous. When we want to make something, we take these properties into account (as well as cost and availability) and select the most suitable material for our purpose.
- Absorbency refers to the ability of a material to take in water and other fluids. Absorbent materials have surfaces that attract water (hydrophilic) and contain small spaces called pores into which liquids can enter. Paper, sponge and most woods are absorbent. The higher the number of pores, the greater the volume of liquid that can potentially enter the material.
As language is context specific (i.e. distinguishing between ‘hard’ or ‘strong’, ‘weak’ or ‘light’), it is important that appropriate language is used to describe and compare the properties of materials.
Material changes
Changes occur to materials and objects around us every day. These changes occur because of an input or removal of energy, such as removing heat energy to freeze water or adding mechanical energy to knead dough. Some changes are easily reversible, for example, a melted ice pole can be re-frozen. Others are difficult or impossible to reverse, for example, a cake cannot be uncooked. Physical change is a change to the physical properties of an object or material where the substances remain the same. The object itself might not remain the same, for example, a rock could be ground to powder or a mug could be smashed to pieces, but the substances are still present. There is still rock and porcelain.
Recycling is a way to convert waste materials into reusable materials. Paper, glass, some metals (steel and aluminium) and some plastics can be collected and turned into new materials.
- Cardboard, used paper, and milk and juice cartons can be manufactured into recycled office paper, recycled toilet paper and recycled cardboard.
- Soft drink bottles and other bottles made from polyethylene terephthalate (PET) plastic (marked with a number ‘1’) can be manufactured into detergent bottles, carpet and recycled fabric.
- Glass bottles and jars, sorted by colour and melted down, can be used to make new containers.
- Metal from aluminium and steel cans is used for making other aluminium and steel items.
- High-density polyethylene (HDPE) plastic (marked with a number ‘2’) like milk bottles and shampoo containers can be recycled into wheelie bins, irrigation pipes and air conditioning hoses.
On this page:
- Browse these tabs to learn about powerful scientific ideas in chemical sciences.
Solids, liquids and gases
All matter is made up of very small particles called atoms. These atoms can join with other atoms to form molecules. Every type of material contains specific types of atoms or molecules, resulting in different materials having different properties. For example, gold is made up of gold atoms only. Water is made up of water molecules, a combination of hydrogen and oxygen atoms. The way the atoms or molecules are arranged in a material will affect its state of matter.
States of matter
A material might be found in different states. The most familiar states are solid, liquid or gas. Other states of matter are now recognised, such as plasma and liquid crystal, but these will not be dealt with at this level. The amount of energy the atoms or molecules in a material possess determines the material's state of matter, for example, the molecules in solid chocolate have less energy than those in melted chocolate.
- Solids have atoms or molecules that are held together with strong bonds. The atoms vibrate in place but they do not change position. This means that a solid holds its shape and does not flow, nor can it be significantly compressed.
- Liquids have atoms or molecules that are held together with weaker bonds. They stay close together and so occupy a constant volume of space. One litre of water takes up the same amount of space in any container, but will take the shape of its container, for example, the shape of a bottle, glass or bowl. Thus a liquid can only be compressed a little bit, if at all. However, the bonds are loose enough to let atoms or molecules slide past each other. Due to the force of gravity, a liquid flows and takes the shape of the container into which it is poured.
- Gases have particles that are not held together with strong bonds. In the right conditions they can spread out and fill any available container. They can also be compressed. Because gas spreads out, it cannot be measured in an open container. Instead, gas can be trapped in a sealed container and the volume, temperature and pressure is used to describe the amount of gas present.
Materials change state when they gain or lose heat energy. This is a physical change because there is no chemical reaction or chemical change occurring. Read more about this on the Changing substances page.
On this page:
- Browse these tabs to learn about powerful scientific ideas in chemical sciences.
Particle model
The difference between a material in a solid or liquid state is due to the arrangement of its atoms or molecules. As primary students are not introduced to the concepts of 'atoms' or 'molecules', this page will use the collective term 'particles' instead.
In solids, the particles are linked by rigid bonds, which means the material keeps its shape. The particles in a solid are able to vibrate, but the strong bonds prevent any further movement. When most solid materials are heated sufficiently they reach their melting point and change into liquid materials. The material has the same types of particles, but their interactions are different and so the properties of the material change significantly. This is known as a physical change.
In liquids the bonds between the particles are weaker, allowing the particles to slide past each other due to the force of gravity or external pressures. All liquids occupy a definite volume of space and are able to flow; under the influence of gravity they will spread out and take the shape of the container that they are in or spread out on a flat surface. However, not all liquids flow the same way. The viscosity of a liquid is a measure of its resistance to flowing. When most liquids are heated sufficiently their particles separate from each other and form a gas. This is known as a physical change.
Gases have much more space between their particles and are therefore generally hard to see, although a few can have a slight coloured tinge. When a liquid is heated sufficiently, its particles are so energised that they separate from each other and only have weak interactions. It is easier to see the effects of a gas, for example, by looking at the movements of tiny ash particles in smoke as they float in the air. Because the particles are not bound strongly together, gases spread out and fill the container that they are in. They have comparably low viscosity and density and can be compressed, unlike solids and liquids. Gases vary in how much they can be compressed, the property of compressibility. If the compression from external pressure is strong enough and/or the temperature is low enough, gases will revert to liquid form.
Gases expand when heated and spread out far more than liquids or solids since the interactions between their particles are far weaker. This means that the density (mass per unit of volume) of a gas at a certain pressure can vary significantly depending on the temperature. Hot air balloons use this principle to rise above the ground. The air they contain is much less dense than the surrounding air, which is therefore pushed upwards.
Gases have mass and weight. However, it is hard to demonstrate this in the classroom since air surrounds all the measurement devices, just as a fish couldn’t use scales to measure the weight of water when it is surrounded by water.
On this page:
- Browse these tabs to learn about powerful scientific ideas in chemical sciences.
Natural and man-made materials
In chemistry a mixture refers to a material that is made of two or more substances (or materials) mixed together without combining chemically together. Baking soda dissolved in water is a mixture since the particles of baking soda are present in the water and retain their properties. When baking soda and vinegar are mixed together, they react and create new substances so there is no longer a mixture of baking soda and vinegar.
Mixtures that are uniform in composition are called homogeneous. Two random samples of a homogeneous mixture would contain the same quantities of materials, for example, well-mixed flour and sugar or well-stirred vinegar and water create homogeneous mixtures. Mixtures that are not uniform are called heterogeneous. Such mixtures can have very different elements, such as a mixture of stones and sand, or may have clear zones between the two elements, such as oil and water. Students are not expected to use these terms— they are included here as background information only.
Some mixtures are easy to separate using the properties of the materials. For example, if the substances have particles of different sizes they can be separated using a sieve. However, if the two substances do not have properties sufficiently different from each other, then it is very difficult to separate the mixture. For example, it is difficult to separate a mixture of icing sugar and cocoa powder. Scientists use many different properties of substances to separate them, such as their boiling points, movement through filter paper or ability to dissolve into different solvents.
Many everyday objects are made of materials that are mixtures. Sometimes it might not be evident that a material is a mixture until a change happens. For example, fresh milk naturally separates, however, modern processes homogenise (from the Greek homo = the same) the mixture of fats and water so that it no longer separates.
Natural materials are materials produced by natural processes and changes. Rocks, water and air are natural materials. Sand and soil are natural materials that are produced by natural processes. Cotton fibres, wool fibres, vegetable oils, waxes and wood (timber) have had some processing, but are still natural materials.
Processed or manufactured materials, also called synthetic materials, are made by transforming raw natural materials into new substances, usually involving chemical changes. Making glass from sand, refining metals from ores, making paper from wood pulp and firing ceramics from clay are examples of how natural materials can be used to make processed materials.
Some common types of processed materials include:
- metals, which generally need to be refined from ores
- ceramics, which are mixtures of sand, gravel, water and a binding agent
- glass, which is made from melting several minerals (including silica found in sand)
- plastics, which are made from byproducts of the oil industry.
On this page:
- Browse these tabs to learn about powerful scientific ideas in chemical sciences.
Chemical sciences big ideas
The chemical sciences involve the study of the composition and properties of substances. This involves classifying substances, exploring physical changes (changes of state or dissolving), and how chemical changes result in the production of new substances.
This can be broken into two core concepts:
- The chemical and physical properties of substances are determined by their structure at a range of scales.
- Substances change and new substances are produced by rearranging atoms; these changes involve energy transfer and transformation.
More information on alternative conceptions held by students can be found on the Chemical science conceptions page in the Pedagogical Toolbox.
Chemical and physical properties
The chemical and physical properties of substances are determined by their structure at a range of scales.
Changing substances
Substances change and new substances are produced by rearranging atoms; these changes involve energy transfer and transformation.
Biological sciences big ideas
The biological sciences involve the study of the diverse range of living things, their interdependence on each other, and their interactions in the environment.
This can be broken into three core concepts:
- The form and features of living things are related to the functions that their body systems perform.
- Biological systems are interdependent and interact with each other and their environment.
- A diverse range of living things have evolved on Earth over hundreds of millions of years; this process is ongoing.
More information on alternative conceptions held by students can be found on the Biological science conceptions page in the Pedagogical Toolbox.
Form and features of living things
The form and features of living things are related to the functions that their body systems perform.
Biological systems are interdependent
Biological systems are interdependent and interact with each other and their environment.
Diverse range of living things
A diverse range of living things have evolved on Earth over hundreds of millions of years; this process is ongoing.
Energy
Energy can be transferred and transformed from one form to another and is conserved within systems.
Energy
When something changes, energy is involved. Energy is abstract but you can often detect it through the effect it has on your body: you can see patterns of light, you can feel the warmth created by heat energy, though you cannot see or feel magnetic energy. Energy from the Sun drives the growth of plants and the development of rainstorms, while energy from chemical reactions gives life to animals and is important to modern industry.
Some important characteristics of energy include:
- Energy exists in different forms, such as light, sound, heat, electricity and movement.
- Energy can be transformed (changed) from one form to another, for example, kicking a ball transforms chemical energy in our bodies to movement energy in the ball.
- Energy can be transferred from one location to another, for example, electrical energy moves along wires from a power station to our houses.
- Energy can be changed into other forms but it cannot be created or destroyed.
- Energy can be stored in many ways. Batteries and fossil fuels are stores of chemical energy.
On this page:
- Browse these tabs to learn about powerful scientific ideas in physical sciences.
Chemical energy
Chemical energy is energy stored in atoms and molecules. Chemical energy can be released in a chemical reaction, often in the form of heat. Examples of stored chemical energy include matches, batteries, petroleum, natural gas and dry wood. As each of these burns, they release chemical energy which is converted to thermal energy (heat) and light energy.
For example, a match has chemical energy stored in it. When the match is struck, it burns and the chemicals in it produces heat energy and light energy.
On this page:
- Browse these tabs to learn about powerful scientific ideas in physical sciences.
Heat energy
Heat is a form of energy created by the movement of molecules in an object. All matter is made up of atoms and molecules (groupings of atoms). The atoms and molecules of a material are always moving. Even objects which are very cold have some heat energy because their atoms and molecules are still moving.
When molecules get more energy in them than they had before, they move faster, and we call that heat. Things are described as ‘hot’ if their molecules are moving quickly and ‘cold’ if their molecules are moving more slowly. Temperature is a way of measuring how fast the molecules are moving.
Heat itself isn’t a ‘thing’ but rather a process of energy transfer. When two materials at different temperatures are placed in contact with each other, heat passes from one to the other until their temperatures are the same. For example, when you hold a cup of hot coffee, heat flows from the cup to your hand—the hot thing warms up a cooler thing by the transfer of heat or heat energy. This heat transfer is known as conduction.
How quickly heat is transferred between the two materials depends on several variables. The more surface contact between the materials, the faster the transfer. Different materials also ‘heat up’ at different speeds. When heat travels easily through a material it is known as a heat conductor, and when heat travels slowly the material is known as a heat insulator.
A metal spoon at room temperature is the same temperature as the air. It might feel colder than a wooden or plastic spoon because metal conducts heat so much better. Because your hand is hotter than room temperature, heat is conducted away from it, and a metal spoon ‘cools’ your hand faster. Heat flow depends strongly on the geometry and type of materials. For example, graphene, which is composed of just a single layer of carbon atoms and is the thinnest material in the world, is known for its fast heat transfer properties.
Heat has its origins in other forms of energy:
- The Sun changes nuclear energy into light and heat energy.
- A fire and living cells can change chemical energy into heat energy.
- Some electrical devices can change electrical energy into heat energy.
- Kinetic (motion) energy can be changed into heat energy through friction.
These are all examples of primary sources of heat. We might produce heat as a side product of any work we might do. Secondary sources of heat do not produce heat themselves but are previously heated by a primary heat source. If then moved to a cold environment they exchange heat with objects around them until all materials reach the same temperature.
On this page:
- Browse these tabs to learn about powerful scientific ideas in physical sciences.
Sound energy
Sound is produced by tiny, rapid vibrations in the particles that make up materials. This can be air, liquids or solids, but not in a vacuum or outer space, which have no particles. When the vibrations reach our ear, our brain translates the signal into what we call sound.
We define sound by what we can hear. Generally, human ears interpret sounds caused by an object vibrating between 20 and 20,000 hertz, abbreviated as Hz. 1 Hz means one vibration per second. Anything vibrating faster is called an ultrasonic vibration and is a higher pitch. Dogs have ears that can hear vibrations that are ultrasonic; this is why they can hear a dog whistle and humans cannot. Elephants communicate using subsonic vibrations below 20 hertz and, therefore, humans are unable to hear them.
Sound is produced when the energy of vibrating objects is carried away by pressure waves.
- Blowing into a recorder makes the air vibrate.
- Drawing a bow over a violin string makes the string vibrate.
- Hitting a drum makes its skin vibrate.
A vibrating object pushes the air, liquid or solid around it and then pulls away. These alternations of pushing and pulling create pressure waves in material. These are not up and down waves like those in water. The diagram below illustrates how waves of pressure travel through a slinky, a coil-shaped toy made from a ribbon of material.
Diagram of pressure waves in a slinky
If the end of the slinky is quickly moved backward and forwards, a set of pulses travels along it. For each pulse, sections of coil are compressed and then stretched apart more than normal. The overall effect is the movement of a wave pattern along the slinky. While each coil moves backwards and forwards, the slinky as a whole does not move. In sound waves, the air molecules move backwards and forwards in a similar way. This occurs much more rapidly (and with much smaller-sized vibrations) than in a slinky. Because the air molecules are moving they have movement (or kinetic) energy.
A sound wave is a series of high-pressure and low-pressure zones. An object vibrating produces pressure waves in all of the materials around it. The sound energy spreads out in all directions from the source, unless the source is designed to send sound in a particular direction, like a loudspeaker or the horn of a trumpet.
Sound pressure makes a thin membrane—the eardrum—within the ear vibrate, and connected nerves then carry the signal to the brain, where it is interpreted as sound. The ear also hears the pressure waves travelling through water as sound, which is why we can hear with our head underwater. Pressing our ear to a solid, for example, a door, allows us to directly hear the pressure waves travelling through the solid.
The pressure waves that we call sound are a form of energy. All sound comes from another form of energy. For example, when we hit a drum the movement energy of our arm is transferred to the drum, which starts vibrating. The vibrations transfer some of the drum’s movement energy into sound energy in the air. Sound can be absorbed by materials if the energy contained in the pressure waves is transformed into different forms of energy. For example, sound energy could be transformed into heat energy when it hits sound-absorbing curtains.
Sound cannot exist without something to travel through. If you put an alarm clock in a vacuum the pieces will still be vibrating, but since there is nothing around them to transmit the vibrations, no sound waves can be produced.
Sound bounces off objects and therefore can produce echoes. In the mountains, the waves of pressure travelling through the air, generated by a shout, can travel to a cliff face opposite, bounce off and travel back through the air to the shouter’s ears. The further away the object that reflects the sound, the longer it takes to get back to the ears. Sound travels slowly compared with light, which is why we see a flash of lightning before hearing the thunder.
On this page:
- Browse these tabs to learn about powerful scientific ideas in physical sciences.
Light energy
Energy cannot be created or destroyed; therefore, light has its origins in other forms of energy. Primary light sources are things that change another form of energy into light energy. For example:
- The Sun changes nuclear energy into light energy.
- A fire, glow-worms and glow sticks change chemical energy into light energy.
- Light bulbs, lightning and computer screens change electrical energy into light energy.
Secondary light sources are things that reflect energy from a primary light source. For example, the Moon is a secondary light source that reflects light from the Sun.
The Sun is the Earth’s primary source of energy, emitting a broad spectrum of electromagnetic radiation, including sunlight. What we call light is a specific type of energy called ‘visible light’— the light that humans can see.
We know of nothing in our universe which travels faster than light. It races towards us through the vacuum of space at about 300,000 km per second. It takes a fraction of a second for light to cross Australia, about eight minutes for light from the Sun to reach the Earth, over four years for light to reach the next nearest star and 100,000 years for light to get from one side of the Milky Way Galaxy to the other.
The range of the electromagnetic spectrum
Electromagnetic radiation
In ordinary situations electromagnetic radiation, including visible light, manifests as a wave. Careful experiments have shown that at a deeper level, electromagnetic radiation also consists of particles, or point-like packets of energy, called photons. How photons conspire to produce an ordinary wave can only be understood using advanced quantum physics.
Our eyes cannot detect all the electromagnetic radiation from the sun—only the ‘visible light’ can be seen. Other types of electromagnetic radiation include x-rays, ultraviolet and infrared light and radio waves. We can feel the infrared light as heat while the ultraviolet light can cause eye damage, sunburn or even skin cancer. The Earth’s atmosphere filters out much of the ultraviolet ‘UV’ radiation.
Light (unlike sound) does not need material to travel through. In a vacuum, for example, in interstellar space, all forms of electromagnetic radiation travel at the same speed regardless of their wavelengths. This speed is universally referred to as the speed of light.
Scientists call the distance between the crests of the waves wavelength, which is measured in metres. Waves with a very short wavelength, for example, gamma rays, will have many crests pass by in one second and are said to have a high frequency. Waves with a long wavelength, for example, radio waves, will have a lower frequency because fewer waves will pass by in one second.
a. longer wavelength b. shorter wavelength
Visible light has a narrow range of wavelengths which are measured in micrometres. A micrometre is 1 millionth of a metre. These different wavelengths are evident as different colours. When all wavelengths of visible light are present, we see white light.
Shadows
We see objects only when light travels from the object to our eyes. These objects might be primary light sources that give out their own light, such as the Sun, an electric light or a candle, or they might be secondary light sources that reflect light to the eye. For example, the page of a book is a secondary source which you are able to read because it is reflecting the light from a primary light source (such as the sunlight coming through a window or an electric light). Once our eyes detect the light, information is transferred through the optic nerve to the brain, which then interprets the signals from the eye as an image of the object. Take away the light source or stop the light from reaching your eyes and you will no longer be able to see the object; it still exists, but you can no longer see it.
When a surface or object is coloured all the colours of the visible light are absorbed except for the colour of the object which is reflected. For example, a red object absorbs all colours except red, which is reflected. Thus, the object appears red. Black surfaces absorb all colours and reflect the least light. White surfaces reflect all colours and absorb the least light.
Ray diagrams use lines to show light travelling. The lines are straight because light travels in a straight line. Arrowheads show the direction of travel.
How we see
Shadows are formed because light does not go around objects to light up areas behind them. Light travels in approximately straight lines. Careful examination shows that near edges, there are changes in the light that can be attributed to the wave properties of light. The light appears to bend slightly as it passes near a sharp edge.
Shadows are dark shapes created when an object blocks out light. Materials that block out light are opaque, while materials that allow light to pass through them are transparent. The shape of a shadow is affected by:
- the shape of the object blocking the light
- how close the object is to the light source
- the position of the light source relative to the object, for example, whether it is above or at the same level as the object.
Casting shadows
When the position of the light source is right above the object, the object will cast a short shadow on the ground. As the light source moves downwards, the object will cast a longer shadow. This is why when the Sun is directly overhead, people and objects cast very small, short shadows. In the morning and afternoon when the Sun is low on the horizon, they cast very long shadows.
Reflection
When light from primary sources hits surfaces, it can be reflected, transmitted (let through) or absorbed (transformed into heat energy). Different surfaces reflect, transmit or absorb light in different ways. Some surfaces are very smooth and even. These reflect light in an ordered way and appear to be shiny. Other surfaces are more irregular. They reflect light in a scattered way and appear matt or dull. Some surfaces, for example, glass, transmit most light, reflecting very little, and thus appear transparent.
We can use these characteristics of light to manipulate it. Reflective surfaces can be used to direct light where we want it to go. A mirror is a very good reflective surface. It reflects light so well that we can see an image of an object placed in front of it. Light always reflects off a smooth surface at the same angle it strikes. Other surfaces will scatter the light in all directions because, at the microscopic level, the surface is uneven.
Refraction
Light travels at slightly different speeds through different mediums, such as water or air. When light travels from one medium to the other the change in speed can cause the light to change direction. This bending or refraction of the light can be seen when you view a pencil in water or the different colours as a rainbow. When light hits a piece of glass at an angle, the light changes direction (that is, it refracts). The change of direction is different for different colours. Consequently, when light hits a glass prism, the different colours can be seen because they are bent differently. The different bending is due to the different wavelengths interacting with the electrons in the glass differently. The result is that they appear to travel at different speeds.
The rate at which energy is carried by a light wave represents the intensity of the light. We perceive intensity as ‘brightness’. Theoretically, light waves from a source could travel forever. The intensity of light from the source will decrease rapidly as we increase our distance from the source because the light will spread out and will usually meet some material, for example, dust in the air. This will cause it to be reflected or scattered or absorbed, and thus the light might not be seen over a large distance.
In a vacuum, for example, interstellar space, all forms of electromagnetic radiation travel at the same speed regardless of their wavelengths. This speed is universally referred to as the speed of light.
On this page:
- Browse these tabs to learn about powerful scientific ideas in physical sciences.
Electrical energy
An electrical circuit is a complete path of conductive material through which electricity flows. Electricity will not flow in a circuit that is not complete, so switches are introduced into circuits to allow us to easily control when the electrical current flows. The wires in our homes carry the electricity to appliances, which use a certain amount of electrical energy. If the appliance malfunctions, and too much electrical energy flows through the circuit, the fuse boxes in our homes protect us by breaking the circuit before the excess energy causes damage.
Conducting electricity
All materials are made of atoms. Atoms have a central core composed of protons (that have a positive charge) and neutrons (that have no charge). This core is surrounded by a ’cloud‘ of very small, negatively charged particles called electrons. In some materials, particularly metals, the outermost electrons can leave the cloud around their core and jump to others. This occurs in all the atoms in a piece of metal, creating a cloud of randomly moving electrons shared by all the atoms. This makes metals able to conduct electricity and is also what gives metals their shimmer. The electrons in insulators cannot easily move away from their cores, which prevents them from conducting electricity.
Electrons travel easiest along conductors such as wires. A switch is simply a device that either connects two wires or keeps them apart. When a switch is ‘on’ the electrons have a path through the switch, when the switch is ‘off’ the path is broken and the wires are not connected.
Electrical circuits
The electrical circuit in a torch includes a battery (or batteries), connecting wires, a switch, and a light bulb. When the battery is inserted in the torch and the switch is turned on there is a complete path, or circuit, and the electrons can flow from the negative terminal of the battery around the circuit to the positive terminal.
When the switch is closed (the ‘ON’ position), the electrons in the circuit start moving in a single direction, rather than randomly as before. The electrons move from the negative terminal of the battery towards the positive terminal through the metal of the wires, switch and lamp filament.
The electrons themselves move very slowly, measured in centimetres per minute. However when you complete the circuit by turning on the switch the light bulb begins to glow almost immediately. This is because the wires are already full of electrons, and so it is like a bicycle chain connected to the pedals of a bicycle. When you put pressure on the pedal, the wheel turns at once even though the chain links near the pedal have not yet reached the wheel. The source of electrical energy creates an electric field across the whole circuit, pushing all the loose electrons in the circuit to start moving at once.
Batteries
When a battery’s positive and negative terminals are connected by the wires, switch, and lamp in a circuit, the chemicals within it react, transforming the chemical energy into electrical energy and causing the electrons to move around the circuit. If the terminals are connected to each other directly through a material with high conductivity, for example, a wire, the chemicals in the battery will react very quickly, causing a high flow of electrons through the circuit. The chemical energy is released very quickly, often causing the battery to become warm. The high flow of electrons may also cause the wire to become very warm and possibly even burn through.
Conductors and insulators
Some materials contain electrons that are held loosely by the atoms and are free to move throughout the material. This means that they can form part of an electrical circuit. They are called electrical conductors. Metals have many free electrons and are considered to be good conductors.
Materials such as glass or plastic have electrons that are not free to move. Therefore, they do not allow electrons to flow through them and are called insulators. Common insulators include plastics, rubber, wood and glass. A plastic insulator is used on electric switches and wires to prevent shock or electrical injury to a person using them.
The human body is a conductive material and people can be harmed if they become part of an electrical circuit with a voltage higher than about 40V. The danger depends on the amount of electrical energy (voltage) in the circuit. Electrical energy above a certain threshold will cause muscle contraction, meaning if a person touches a wire with the palm of their open hand, the hand snaps shut, and they cannot detach themselves from the current. Length of contact, amount of electrical energy and factors like sweat on skin determine the severity of symptoms, from burns to interfering with the functioning of the heart and the nervous system.
On this page:
- Browse these tabs to learn about powerful scientific ideas in physical sciences.
Kinetic energy (Motion energy)
All moving things have kinetic energy. It is energy possessed by an object due to its motion or movement. The heavier a thing is and the faster it moves the more kinetic energy it has.
On this page:
- Browse these tabs to learn about powerful scientific ideas in physical sciences.