--- title: "Why can you squash air but not water?" subtitle: "Year 7 Chemistry · Particle theory · Lesson 3" --- # Why can you squash air but not water? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Seal the end of a syringe full of air and you can push the plunger a long way in. Do the same with water and it barely moves. Particle theory explains why, and the same explanation works for density, runny honey and smells crossing a room. :::teacher-note **In one sentence:** Students use particle theory to explain physical properties (compressibility, density, viscosity), naming the arrangement, motion or attraction behind each (AC9S7U05, node 3, a hinge node for all of AC9S7U06). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **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?** ☐ Wax ☐ Water ☐ They all move the same amount ☐ Air My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Only the air squashes. Its particles are far apart, with empty space to move into. Wax and water particles are already close together. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: use the particles :::keep-together ### The water syringe **Why does the plunger stop in the water syringe?** ☐ The water particles are too hard to squash ☐ The water particles are already close together, so there is almost no space to push them into ☐ The water particles stop moving when pushed ☐ Water is heavier than air ::: :::teacher-note **Answer:** 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. ::: :::keep-together ### Density **Why is air much less dense than water?** ☐ Air particles are smaller ☐ Air particles are far apart, so there are far fewer in each cm³ ☐ Each air particle is lighter than a water particle ☐ Air has no mass ::: :::teacher-note **Answer:** 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. ::: ## Mix it up :::keep-together **Looking back at the particle model: a drop of water is heated until it becomes steam. What happens to the water particles?** ☐ They disappear ☐ They get bigger ☐ They break into smaller pieces ☐ They stay the same size, but move faster and spread far apart ::: :::teacher-note **Answer:** 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. ::: :::typical ## Elaborate: three substances :::keep-together **Honey, water and air. Which particle feature best explains each observation?** Groups: Arrangement: how close and ordered the particles are, Motion: how fast the particles move, Attraction: how strongly particles pull on each other - Honey pours much more slowly than water.: {{field-15}} - Air can be squashed but water can't.: {{field-15}} - Water is about 800 times denser than air.: {{field-15}} - The smell of honey spreads through a whole room.: {{field-15}} - Warm honey pours faster than cold honey.: {{field-15}} ::: :::teacher-note **Answer:** 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. ::: ::: :::wider ## Elaborate: float or sink Each window is 1 cm³. Iron particles are packed tightly and each one is heavier. Foam is mostly trapped air, so a window of foam holds very few particles. :::keep-together ### Foam **Will a block of foam float on water?** ☐ Floats ☐ Sinks ::: :::teacher-note **Answer:** Floats Foam is mostly trapped air: less mass in each cm³ than water, so it floats. ::: :::keep-together ### Iron **Will a block of iron float on water?** ☐ Floats ☐ Sinks ::: :::teacher-note **Answer:** Sinks Iron packs far more mass into each cm³ than water, so it sinks. ::: ::: :::wider ## Elaborate: diffusion Two colours of the same substance start on opposite sides of a barrier. When it lifts, they mix: this is **diffusion**. Predict first: will the gas or the liquid mix faster? Then time both. :::keep-together ### Why? **Why did the gas mix faster?** ☐ Gas particles are smaller ☐ Gas particles move fast and have lots of space to move into ☐ The liquid particles stop moving ::: :::teacher-note **Answer:** 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. ::: ::: :::wider ## Elaborate: mystery substance X :::keep-together ### Squashing **Can X be squashed easily?** ☐ Yes, easily ☐ No, hardly at all ::: :::teacher-note **Answer:** No, hardly at all Its particles are already close together. ::: :::keep-together ### Flowing **Does X flow?** ☐ No, it holds its shape ☐ Yes, easily like water ☐ Yes, but slowly ::: :::teacher-note **Answer:** Yes, but slowly No fixed pattern, so it flows; strong attraction makes it slow. ::: :::keep-together ### Warming **If you warm X, how does its flow change?** ☐ It flows slower ☐ It flows faster ☐ No change ::: :::teacher-note **Answer:** 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. ::: ::: ## Evaluate: look back Which syringe did you predict would move furthest, and why? Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Gases can be squashed because their particles are far apart, with lots of empty space between them. - Solids and liquids hardly squash at all: their particles are already close together. - Gases are much less dense because there are far fewer particles in each cm³. The same substance as a solid or a liquid has a similar density: its particles are close together in both. - The particles themselves never shrink or squash. Only the spaces between them change. ## Exit ticket :::keep-together **Why is steam much less dense than the same mass of liquid water?** {{lines-3}} ::: :::teacher-note **Answer:** It is the same particles, but in steam they are far apart, so the same mass takes up much more space. :::