Why can you squash air but not water?

Classroom resources

Load these into Waypoints Prepare: choose Load URL and paste the copied address, or download the file and import it. In Prepare, the worksheet's Teacher edition shows the answers; the student copy hides them.

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• Worksheet Download Copy address for Prepare Print the student copy; the Teacher edition has the answers.

Students will be able to

• I can explain why gases can be squashed and liquids and solids can't.
• I can explain why a gas is less dense than a liquid using particle spacing.
• I can name the particle feature (arrangement, motion or attraction) behind a property.

Timing

Engage and Explore 10 min · Explain 8 min · Evaluate 3 min · Typical adds 5 min, Wider about 15 min

Before class

A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider.

This is the hinge into pure substances and mixtures: every separation technique in AC9S7U06 is chosen by explaining property differences at the particle level. Students who can only label diagrams stall there.

Running the lesson

1. Engage: ask a student who chose water or wax to say why. Listen for 'the particles squash'.
2. Explore: the window count is the density idea. Point out that air's count is so low it is often 0 or 1.
3. Explain: insist on a particle feature in every sentence of the written explanation.
4. Follow up with real sealed syringes of air and water if you have them.

The hard part (on purpose)

Explaining with arrangement, motion and attraction, not the state's name. When a student says 'because it's a gas', ask what the gas particles are doing.

When students get stuck

Answers to the checks

1. Engage: three syringes: Three sealed syringes hold a solid plug of wax, water, and air. You push each plunger as hard as you can. Which plunger moves in the furthest?
   Only the air squashes. Its particles are far apart, with empty space to move into. Wax and water particles are already close together.
2. Explore: push the plunger, count the particles: Push the plunger all the way for wax, water and air, and record the window count for each.
   
3. The water syringe: Why does the plunger stop in the water syringe?
   The water particles are already close together, so there is almost no space to push them into
   The particles stay the same size; there is just no empty space left.
4. Density: Why is air much less dense than water?
   Air particles are far apart, so there are far fewer in each cm³
   Fewer particles in each cm³ means less mass in each cm³: lower density.
5. Mix it up: Looking back at the particle model: a drop of water is heated until it becomes steam. What happens to the water particles?
   They stay the same size, but move faster and spread far apart
   Ice, water and steam are the same particles. Only their arrangement and motion change, so any squashing comes from the space between them.
6. Elaborate: three substances: Honey, water and air. Which particle feature best explains each observation?
   Honey pours much more slowly than water. → Attraction: how strongly particles pull on each other; Air can be squashed but water can't. → Arrangement: how close and ordered the particles are; Water is about 800 times denser than air. → Arrangement: how close and ordered the particles are; The smell of honey spreads through a whole room. → Motion: how fast the particles move; Warm honey pours faster than cold honey. → Motion: how fast the particles move
   Honey and water have the same arrangement but behave differently: attraction makes the difference.
7. Foam: Will a block of foam float on water?
   Floats
   Foam is mostly trapped air: less mass in each cm³ than water, so it floats.
8. Iron: Will a block of iron float on water?
   Sinks
   Iron packs far more mass into each cm³ than water, so it sinks.
9. Time both: Time the mixing in the gas and in the liquid.
   
10. Why?: Why did the gas mix faster?
    Gas particles move fast and have lots of space to move into
    In the liquid, particles keep bumping into close neighbours, so they spread slowly.
11. Squashing: Can X be squashed easily?
    No, hardly at all
    Its particles are already close together.
12. Flowing: Does X flow?
    Yes, but slowly
    No fixed pattern, so it flows; strong attraction makes it slow.
13. Warming: If you warm X, how does its flow change?
    It flows faster
    X is glycerol, a syrupy liquid used in soap and cough syrup. It barely compresses, pours roughly a thousand times more slowly than water at room temperature, and runs much more freely when warm.

Solutions

One possible solution for each task. Other correct programs are fine.

Elaborate: float or sink › Foam

Open task: any program that does what the step asks.

Elaborate: float or sink › Iron

Open task: any program that does what the step asks.

Elaborate: diffusion › Time both

Open task: any program that does what the step asks.

Elaborate: diffusion › Why?

Open task: any program that does what the step asks.

Elaborate: mystery substance X › Squashing

Open task: any program that does what the step asks.

Elaborate: mystery substance X › Flowing

Open task: any program that does what the step asks.

Elaborate: mystery substance X › Warming

Open task: any program that does what the step asks.

Stretch and support

Stretch: Why does a bike pump get warm when you pump it hard? Use particle motion.

Support: Give the sentence frame: 'X can/can't be squashed because its particles are … so there is … space between them.'

Coach prompt (optional)

For a student who has done the lesson and is still stuck. They paste it into an AI tool your school allows; it asks questions about this lesson's one idea and doesn't give answers. Students see it on the course recap page only through a link ending in ?coach=1: recap with coach prompts.

You are a patient science coach for a Year 7 student in Australia. Help me understand ONE idea only, from the lesson "Why can you squash air but not water?".

The idea: Explaining physical properties using particle theory: I can use particle theory to explain physical properties of substances including compressibility, viscosity and density.

What understanding looks like:
- Uses particle theory to explain at least 2 physical properties of a given substance
- Explanation references arrangement, motion or attraction — not just state labels
- Correctly links a named property to the relevant particle behaviour

Mistakes students often make here:
- A student says the air particles get squashed.
- A student says air is less dense because its particles are lighter.
- A student explains with state labels: 'because it's a gas'.

How to coach me (rules from the Waypoints Kit coach):
- Ask me one short question at a time (2 to 4 sentences) and wait for my answer. Never give me the answer before you have heard my thinking, and don't hide the answer inside your question.
- Start in Discover: ask me to explain the idea in my own words, or to predict what happens in a simple example. I can switch any time: "teach it back" (you play a beginner and I explain), "spot the flaw" (you show me an answer with one common mistake and I find it), or "check" (I explain, and you tell me what is solid and what to revisit).
- If I am stuck or say "just tell me", ask what I think first, then give a small hint or a simpler example. Explain only after I have had a real go.
- If I seem to be missing the idea that comes before this one (Particle arrangement, motion and attraction in each state), check it quickly, then come back.
- Stay in this idea: don't teach later ideas or add extra content. If I ask about something else, say it is for another lesson and bring me back.
- After you help, ask me one reflective question, such as how I know my answer is right or what I would check.
- Praise my effort and method, not my ability.
- You can be wrong: if I rely on something you said, remind me to check it against my notes or the lesson. If you don't know, say so.
- Use simple words.
- Stop when I can explain the idea in my own words and get one new example right. Then tell me to go back to the lesson "Why can you squash air but not water?".

Exit ticket

Why is steam much less dense than the same mass of liquid water?

It is the same particles, but in steam they are far apart, so the same mass takes up much more space.

What each step is for

• Engage: three syringes: Committing to a prediction exposes the idea that particles themselves squash or shrink.
• Explore: push the plunger, count the particles: The plunger tests compressibility; the fixed window turns density into something students can count.
• Explain: use the particles: Two properties, each explained by spacing, with distractors built on the classic misconceptions.
• Mix it up: Particles never change size: the same particles in a different arrangement explain the three states, and this lesson depends on it.
• Elaborate: three substances: Honey and water share an arrangement but not an attraction: comparing three substances makes students choose the feature.
• Elaborate: float or sink: Packing per cm³ links node 3's density to floating, a new context for the same idea.
• Elaborate: diffusion: Diffusion is motion plus space: the same model predicts which state mixes faster.
• Elaborate: mystery substance X: Predicting properties from a diagram, then checking real data, is the strongest test that students can use the model.

Where it leads

Next: AC9S7U06 node 1, pure substances and mixtures at the particle level.

In class: sealed syringes of air and water; a density column of honey, water and oil.