Review the definition of physical weathering introduced in the previous lesson: when physical forces break down rock into smaller pieces.

Discuss the physical forces that break down rocks that students explored in their investigations—abrasion (which occurs when rocks rub and tumble against each other), breaking of particles and smoothing edges of rocks.
 

Ask students to identify how water appears in the environment, prompting them to think about it in all its states.For example:

• Liquid: creek, river, lake, puddle, ocean.
• Solid: ice, snow (loosely held ice crystals).
• Gas: water vapour caused through evaporation.

Note: In Year 3 students would have learned about how adding and removing heat from liquids changes its state. They also will have learned about the water cycle in Year 4, and may have already learned about the three states of matter—solid, liquid and gas—earlier in Year 5. Their level of understanding of these phenomena will impact their responses. The key focus relevant here is water moving between a solid and liquid state through the adding and removal of heat energy.

Pose the question: Can freezing and thawing water cause weathering to rocks?

Discuss the definition of the term ‘thawing’ if required.

Ask students if they have ever seen what happens when a full bottle of water is left in the freezer. View the image on the What broke the bottle? Resource sheet telling students that the bottle had been left in the freezer overnight. Students compare this to previous experiences and suggest reasons why the bottle broke.

If not already offered by the students, share your own thinking: As the water froze, it expanded and broke the bottle.

Ask students how they might devise a fair test to investigate if water does indeed expand as it freezes.

If required, prompt students by showing them the equipment available for the investigation. For example, show them a syringe of coloured water (with a small amount of Blu-Tac used to seal the end) and ask them how we might use it to see if water expands when it is frozen.

Pose the broad question: What things might affect the volume of water?

Using a demonstration copy of the Variables grid Resource sheet, identify and record the thing to be measured during the investigation—that is, the volume of water in the syringe—and place that in the centre of the grid, marked with (M). Brainstorm other variables in the surrounding columns/rows. Add or remove additional columns/rows as required. For example,

Note: In some investigations it is appropriate to allow teams to select the variable they wish to change, and teams might select different variables. However, in this case, the goal of the investigation is to show that water expands as it freezes. This is achieved by changing the water from a liquid state to a solid state through freezing, so all teams should investigate this same variable.

Model how to use the question stem to write a question for the investigation: What happens to the volume of water when we change the state of the water from liquid to solid?

Discuss how the investigation will be conducted and data collected and recorded.

Optional: Discuss fair testing principles, including why it is essential to only change one variable and leave everything else the same: to ensure that we find out the impact changing a specific variable has on the outcome. If multiple variables are changed there is no way of knowing which one made the impact and to what degree.

In collaborative teams, students plan their investigation and record their results using the Freeze-thaw Investigation Planner Resource Sheet.

Ideally this investigation should take place over two lessons to allow the water time to freeze. You might organise this by:

• Completing page 1 of the investigation planner and the liquid water observations and measurements on page 2 in one lesson.
• Placing the water-filled syringes into the freezer.
• In a follow-up lesson the next day or week, complete the solid water observations, results and claims (remainder of page 2 and page 3).
• Leave the freeze-thaw weathering diagram to be complete AFTER the integrate step is complete.

Teams share the result of their investigations.

Using the QCER framework as a guide, each team makes a claim to answer the question they were investigating: What happens to the volume of water when we change the state of the water from liquid to solid? For example, Water expands and takes up more volume when it changes from a liquid to a solid. Teams support their claim with evidence from their investigation, for example The water in our syringe increased from 5ml as a liquid to 6ml when it froze solid.

Compare to see if each team got the same results and made the same/similar claims. Discuss why this happened, any results that differed, and why they might have differed. Ask students if they would now consider the prediction "water expands when it is frozen" to be shown as true, and what they think broke the glass bottle from the What broke the bottle? Resource sheet.

Using the Think-Pair-Share strategy, ask students to consider how the force created by water physically expanding when it freezes would contribute to weathering.

Show the video Freeze-Thaw Weathering Explained (2:48) so students can generalise what they have learned about water expansion and freeze-thaw weathering of rocks.

Students complete their freeze-thaw weathering diagram on the Freeze-thaw investigation planner Resource sheet.

Discuss the conditions required for freeze-thaw weathering to occur: rain or snow and temperature change that repeatedly goes above and below 0°C. Determine if it is likely for freeze-thaw weathering to occur in your location and why/why not.

Optional: Explore summer images of Blue Lake at Mt Kosciuszko, NSW. Zoom in to see signs of freeze-thaw weathering on the large rock faces and rocky debris on the ground.

Reflect on the lesson

You might:

• add new words and images to the word wall or glossary.
• add to the W and H sections of the TWLH chart.
• ask students to articulate the forces involved when the water volume expands when it freezes.
• discuss how the learning from this lesson relates to freeze-thaw weathering of rocks.
• discuss how students were thinking and acting like scientists during this lesson. Focus on recording, explaining, evaluating data and using scientific diagrams.