Key Stage 3 science · Physics – Matter

KS3 particle model of matter: a guide for parents

In Key Stage 3, pupils move from simply describing solids, liquids and gases to explaining them. The particle model says that all matter is made of tiny particles, and that how those particles are arranged, how they move and how close together they are decide a material’s state. With this one idea pupils explain melting and boiling, why solids keep their shape, why materials differ in density and why ice, unusually, floats on water.

The particle model of matter, explained

The particle model pictures matter as a huge number of tiny particles, drawn as small circles. In a solid the particles are packed closely in a regular pattern and held together by strong forces. They cannot swap places, but they do vibrate about fixed positions, so a solid keeps both its shape and its volume. In a liquid the particles are still close together, often touching, but the pattern is irregular and the particles slide past one another. That is why a liquid flows and takes the shape of its container while keeping the same volume. In a gas the particles are far apart, move quickly in random directions and hardly attract one another. A gas spreads out to fill whatever space it is in, and it can be squashed because there is so much empty space to squash into.

Changes of state are explained by energy and movement. When a solid is heated, its particles gain energy and vibrate more strongly. At the melting point they have enough energy to break free from their fixed positions and the solid becomes a liquid. Freezing is the reverse. A liquid becomes a gas in two ways: evaporation happens at the surface of a liquid at any temperature, as the fastest particles escape, while boiling happens at the boiling point, when bubbles of vapour form throughout the liquid. Condensation is a gas turning back into a liquid as it cools. A few substances skip the liquid stage, which is called sublimation. Pupils learn that these are physical changes: the particles themselves stay the same and only their arrangement and movement change, so the total mass is the same before and after.

Density is how much mass is packed into a given volume, found by dividing mass by volume. The particle model explains why it varies: in a dense material there are more particles, or heavier ones, in every cubic centimetre. Because particles in a solid are usually closer together than in a liquid, and far closer than in a gas, most substances are densest as a solid and least dense as a gas. Pupils are expected to give this explanation in words and in particle diagrams, drawing the same number of particles in a smaller or larger space. Calculating density and studying diffusion are covered in a neighbouring topic, so here the focus is on explaining.

Water is the famous exception, and the curriculum calls it an anomaly. Ice is less dense than liquid water, which is why ice cubes float and why a full bottle of water can split in the freezer. As water freezes, its molecules lock into a regular open pattern that holds them slightly further apart on average than in the liquid, so the same mass takes up about 9% more space. One result is that lakes freeze from the top down, leaving liquid water underneath for fish. Other results are burst pipes and rocks cracked by frost. The deeper reason, the attraction between water molecules, is explored later, so at this stage pupils are asked to describe and use the pattern.

The last strand is the particles themselves. An atom is the smallest particle of an element, far too small to see even with a school microscope. A molecule is two or more atoms joined together: a water molecule is two hydrogen atoms and one oxygen atom (H₂O), and oxygen in the air is made of molecules with two oxygen atoms (O₂). Some substances, such as iron, are made of atoms that are not grouped into molecules. Teachers also point out where the model falls short: real particles are not hard coloured balls, and the gaps between particles are empty space, not air.

Words your child will hear

Particle: A very small piece of matter. In the model, atoms and molecules are the particles.
Atom: The smallest particle of an element, such as one atom of iron or of oxygen.
Molecule: Two or more atoms joined together, such as water (H₂O) or carbon dioxide (CO₂).
Melting point: The temperature at which a solid turns into a liquid. For pure water ice it is 0 °C.
Evaporation: A liquid turning into a gas from its surface, which can happen well below the boiling point.
Condensation: A gas cooling and turning into a liquid, like water droplets on a cold glass.
Density: How much mass there is in a certain volume of a material, found by mass ÷ volume.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about the differences in arrangements, in motion and in closeness of particles explaining changes of state, shape and density, the anomaly of ice-water transition
Pupils should be taught about atoms and molecules as particles

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked explanation

A full, sealed plastic bottle of water is left in a freezer and the sides bulge outwards. Use the particle model to explain this, and to explain why the ice floats when it is put in a glass of water.

Describe the liquid. In liquid water the molecules are close together in an irregular arrangement and they move around, sliding past each other.
Explain what freezing does. As the water cools, the molecules lose energy and move less. At 0 °C they stop sliding and settle into a regular pattern, vibrating about fixed positions.
Use the anomaly. Water’s regular pattern is open, so on average the molecules end up slightly further apart than they were in the liquid. The same molecules now take up about 9% more space.
Link to the bottle. The ice needs more room than the water did, so it pushes outwards on the plastic and the bottle bulges. The mass has not changed, because no molecules have been added or lost.
Link to floating. Density is mass ÷ volume. The same mass in a larger volume means a lower density, about 0.92 g/cm³ for ice compared with 1.00 g/cm³ for water, so ice floats with most of it under the surface.

Water molecules form an open, regular pattern when they freeze, so the ice takes up more space than the water and pushes the bottle outwards. The mass stays the same but the volume is larger, so the density is lower and ice floats. Most other substances do the opposite, and their solid sinks in their liquid.

Common mistakes and how to help

Thinking the particles themselves change when a substance is heated, for example that they melt or get bigger

Pupils see the whole material change, so they assume each tiny piece changes in the same way. The word “melt” is used for the lump, not for the particles.

What helps: Ask what is the same before and after. The particles are the same and the mass is the same. Only their energy, movement and arrangement change. Drawing particle diagrams with circles of the same size in every state helps.

Believing particles in a solid do not move at all

A solid looks perfectly still, and the model is often introduced with particles drawn in neat rows.

What helps: Explain that particles in a solid vibrate on the spot, and that this vibration grows with temperature until the solid melts. Without any movement, the idea of melting makes no sense.

Thinking there is air between the particles, or that bubbles in boiling water are air

Everyday life is surrounded by air, so it feels strange that anything could be empty. Bubbles in a pan look like air escaping.

What helps: Say that between the particles there is nothing at all. The bubbles in boiling water are water that has become a gas, called water vapour. A diagram with blank space between the circles is the correct picture.

Assuming a solid is always denser than its liquid

That is true for nearly every substance pupils meet, so it becomes a rule. Ice and water then seem to break it.

What helps: Treat water as the interesting exception and ask why it matters. A pupil who can explain why ice floats is showing that they understand density, not just remembering that solids are heavier.

How to help at home

Be the particles. Act out the three states in the garden or a clear room. Stand close together and wobble on the spot for a solid, shuffle past each other for a liquid, and spread out and walk briskly in straight lines for a gas. Ask what happens to your movement as the “temperature” goes up.
Watch ice float. Put an ice cube in a glass of water and look at how much sits above the surface. Then ask your teenager to explain, using particles, why it floats. Rest a small, loosely lidded plastic tub of water in the freezer, marking the starting level, and see how far the ice rises.
Spot condensation and evaporation. Notice droplets forming on the outside of a cold drink, a mirror misting up after a shower, or washing drying on a line. Ask where the water has come from or gone to, and what the particles are doing.
Draw it. Ask your child to sketch particle diagrams for ice, water and steam without looking at notes, with a sentence under each describing the movement and spacing. Checking that the circles are the same size and the gaps are empty shows how well the model is understood.

Leads on to

Key Stage 4: Density, changes of state and specific heat