Key Stage 3 science · Chemistry – The particulate nature of matter
KS3 states of matter and the particle model: a parent guide
The particle model says that all matter is made of tiny particles that are always in motion, and that how they are arranged and how fast they move explains the differences between solids, liquids and gases. In Key Stage 3 children use this model to explain properties such as shape and compressibility, how gas pressure arises, and what is happening when a substance melts, freezes, evaporates, boils or condenses.
States of matter and the particle model, explained
In primary school children learn that solids keep their shape, liquids flow and take the shape of their container, and gases spread out to fill the space. At Key Stage 3 they must explain why, and the answer is the particle model. In a solid the particles are packed close together in a regular pattern and vibrate on the spot, held by strong forces. In a liquid they are still close together, but they are not in a fixed pattern, and they can slide past one another. In a gas the particles are far apart, move quickly in all directions, and there are only weak forces between them. Between the particles there is empty space.
The model explains the everyday properties. Solids hold their shape because their particles cannot move out of position. Liquids flow because their particles can move past each other, but they are very hard to squash because there is almost no gap between them. Gases can be compressed, which means squashed into a smaller volume, because there is a lot of empty space. A gas also expands to fill any container since its particles move freely in all directions.
Gas pressure is the idea in this topic that is newest to most children. Gas particles move randomly and constantly collide with the walls of their container. Each collision gives a tiny push, and because there are huge numbers of particles, the pushes add up to a steady force on the wall. Pressure is that force on each unit of area. If you squash the same amount of gas into a smaller space, the particles hit the walls more often, so the pressure rises. If you heat a gas in a sealed container, the particles move faster and collide harder and more often, so the pressure also rises. This is why a sealed can can burst if it gets too hot.
Changes of state are explained in the same way. When a solid is heated, its particles gain energy and vibrate more strongly until they break free of their fixed positions, and the solid melts. In a liquid, the fastest particles at the surface can escape to become gas, which is evaporation, and boiling is when this happens throughout the liquid at its boiling point. Cooling reverses these changes: condensing and freezing. In each case the particles themselves do not change or grow, and the number of particles stays the same. Only the arrangement and the energy of movement change, which is why the mass stays the same too. While a pure substance is melting or boiling, its temperature stays the same even though it is still being heated, because the energy is being used to free the particles from each other. The same model is used elsewhere in the Key Stage 3 course to explain diffusion, where particles spread from a place where there are many of them to where there are fewer.
Words your child will hear
- Particle
- A tiny piece of matter. The particle model treats every substance as made of particles.
- Melting
- A solid turning into a liquid when its particles gain enough energy to move past each other.
- Evaporation
- A liquid turning to gas at its surface, at any temperature below boiling.
- Boiling
- Rapid change from liquid to gas throughout the liquid, at the boiling point.
- Condensation
- A gas turning to a liquid when it cools and its particles slow down.
- Compress
- To squash into a smaller volume, which is easy for a gas and almost impossible for a solid or liquid.
- Gas pressure
- The force of gas particles hitting the walls of their container, for each unit of area.
What the National Curriculum says
The programme of study for Key Stage 3 says:
- Pupils should be taught about the properties of the different states of matter (solid, liquid and gas) in terms of the particle model, including gas pressure
- Pupils should be taught about changes of state in terms of the particle model
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
Try it: explaining pressure in a syringe
A sealed syringe contains air. Using the particle model, explain why it gets harder to push the plunger in as the air is squashed.
- Describe the gas. The air is made of particles that are far apart and moving fast in random directions. They keep hitting the inside walls of the syringe and the plunger.
- Link collisions to pressure. Each collision pushes on the wall. Together the huge number of collisions makes a steady force on the walls and the plunger. This force on each unit of area is the gas pressure.
- Describe the change. Pushing the plunger in makes the volume smaller. The number of particles stays the same, but they are now squeezed into less space.
- Explain the effect. The temperature has not changed, so the particles move at the same speed, but they have less distance to travel between collisions, so they hit the plunger and the walls more often. More collisions per second means a bigger force on each unit of area, so the pressure rises.
- Connect to what you feel. The air pushes back harder on the plunger as it is squashed, so it feels harder to push in.
The same number of gas particles is squeezed into a smaller space, so they collide with the walls and plunger more often. The pressure rises, so the plunger is harder to push.
Common mistakes and how to help
Thinking the particles themselves get bigger when something is heated or melts
Things expand when heated, and it is easy to imagine each tiny part swelling.
What helps: The particles stay the same size. They move more, or move further apart, which makes the substance take up more space. A drawing with the same size circles in each state helps.
Saying that the particles in a solid do not move
Solids look still, so children assume their particles are still.
What helps: Solid particles vibrate on the spot. They vibrate more as the solid gets hotter, which is why it eventually melts. Acting this out with a family huddle shivering in a fixed spot makes the point.
Drawing air or other substances between the particles of a gas
It is hard to picture nothing at all, so children fill the gaps in with something.
What helps: Between the particles there is empty space, a vacuum. Ask what would be left if you removed all the particles of the gas.
Believing the white cloud above a kettle is steam, or that bubbles in boiling water are air
Everyday language calls the visible cloud “steam”, and bubbles look like air.
What helps: Steam itself, which is water as a gas, is invisible. The cloud is tiny drops of liquid water formed as the gas cools. The bubbles in boiling water are water vapour, not air.
How to help at home
- Act out the particles. Use a large room or the garden. Stand close together in a block and wobble on the spot for a solid, shuffle past each other for a liquid, and spread out and bounce in all directions for a gas. Then try heating the “solid” by wobbling harder until you melt.
- Watch ice melt. Put an ice cube in a dish on the table, and another in a bowl of warm tap water, and compare how fast they melt. Ask what is happening to the particles and where the energy comes from.
- Feel gas pressure. Use a bicycle pump or a needle-free plastic syringe, such as the kind sold for giving children medicine. Block the end firmly with a finger and push the plunger in gently. Feel the push-back as the air is squashed. Then fill the syringe with water, block it again over a sink and compare how far the plunger will go.
- Spot evaporation and condensation. Notice where washing dries, puddles disappear or a cold glass of water gets wet on the outside. Ask your child to explain each one using particles, and to say where the water in the droplets came from.
Related topics
Leads on to
- Key Stage 4: Bonding and properties of matter
- Key Stage 4: Carbon chemistry and carbon structures
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