Solids keep their shape and volume. Liquids keep their volume but take the shape of their container, while gases spread to fill the available space. Use this Year 9 lesson to explain why with particle models, a video, a recap game and a worksheet you can complete online or print.
Allow about 30 minutes for the lesson and one practice choice. Pause or finish another day if you need.
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Solids keep their shape and volume. Liquids keep their volume but take the shape of their container. Gases spread to fill the space available. The particle model helps explain these differences.
The block is solid. The water is liquid. The air inside the balloon is a gas.What you’ll learn
Compare particle arrangement and movement in solids, liquids and gases.
Explain shape, volume, flow and compression using the particle model.
Correct the idea that solid particles do not move.
Keep particle size and count unchanged in each model.
1 · CONNECT THE MODEL TO THE PROPERTY
Same idea, three different states
A solid can keep its shape because its particles stay close together in fixed positions. They still vibrate. In a liquid, particles stay close together but can move past one another, so it flows. Gas particles are far apart and move in all directions, so a gas fills the available space.
Solid: close particles vibrate about fixed positions. The solid keeps its own shape and volume.
Liquid: close particles move past one another. The liquid keeps its volume and takes the shape of its container.
Gas: widely spaced particles move in all directions. The gas fills its container and can be compressed.
The twelve dots stand for particles. Their size and number stay the same. Movement stops after a short comparison. Solids can have different particle arrangements. The pictures are not to scale.
2 · EXPLAIN WHY A GAS CAN BE COMPRESSED
Same gas, less room
Hold the amount of gas fixed and keep temperature unchanged. In a smaller volume, the same particles are closer together, so the gaps decrease. Their size and count do not change.
Density is mass divided by volume. It increases because the same mass occupies less volume. Pressure rises because particles reach the container walls more often. The fixed temperature keeps average particle speed the same for this comparison.
Need a hint?
Keep temperature the same in both pictures. Look at the amount of gas, the volume and the gaps.
Compress the same gas
Same gas, starting volume. Watch the gaps between the particles.
Eight particles throughout. Particle size stays the same. Model not to scale.
3 · CHECK YOUR EXPLANATION
Sam says, “Particles in a solid do not move.”
Correct Sam’s claim. Explain how a solid can keep its shape even though its particles move.
Show the answer
Solid particles vibrate about fixed positions. They do not move past one another as liquid particles do, so the solid keeps its shape. Fixed positions do not mean the particles are motionless.
Go further: density and pressure
You can finish the main lesson without these extra investigations. Use them when you are ready to connect the particle model to density and warming.
EXTEND · DENSITY
Use mass divided by volume
Sample A has mass 24 g and volume 8 cm³. Its density is 24 g ÷ 8 cm³ = 3 g/cm³. Sample B has 30 g in 30 cm³, so its density is 1 g/cm³. A is denser.
Density compares mass with volume. A denser material is not automatically a solid; different substances can have different densities.
EXTEND · WARM A GAS
Fixed volume, faster particles
Now keep the same amount of gas in a sealed rigid container, so its volume stays fixed. Warming increases average kinetic energy; particles move faster on average. Their collisions with the walls become more frequent and more forceful, so force per unit area and pressure rise.
The density stays the same because mass and volume stay the same. Particles do not grow when warmed.
Need a hint?
Keep the rigid container and amount of gas fixed. What changes about particle movement and wall collisions?
Warm a gas in a rigid container
Same gas, fixed volume. Watch the speed of the particles.
Eight particles throughout. Particle size stays the same. Model not to scale.Look closer at a wall collisionThe particle moves toward the wall and pushes the wall away from the gas. The wall pushes back on the particle in the opposite direction. Pressure is force per area from many collisions.
Start with tasks 1 and 2: compare the three states, then explain gas compression. Tasks 3 to 6 extend the model to density and warming. You can leave those for another time.
