Key Stage 3 science · Physics – Matter

KS3 physical changes, density and diffusion: a guide for parents

Key Stage 3 pupils use the particle model to explain what happens in melting, freezing, evaporation, sublimation, condensation and dissolving, and why the mass is conserved even when a substance seems to disappear. They compare solids, liquids and gases, including their densities, and use Brownian motion and diffusion as evidence that particles are always moving. They also learn how to tell a physical change from a chemical one.

Physical changes, density and diffusion, explained

In a physical change, no new substance is made. Melting, freezing, evaporation (a liquid turning into a gas below its boiling point), condensation, sublimation (a solid turning directly into a gas, as dry ice and iodine do) and dissolving are all physical changes. The particles are rearranged or spread out, but ice, liquid water and steam are all made of the same water particles. Most physical changes can be reversed by heating, cooling or evaporating off a solvent. Pupils learn that mass is conserved: the mass before always equals the mass after, because no particles are created or destroyed. A sugar cube stirred into tea seems to vanish, but the tea is heavier by exactly the mass of the sugar.

The particle model describes the three states. In a solid the particles are packed closely in a fixed pattern and vibrate in place. In a liquid they are still close together but move past each other. In a gas they are far apart and move quickly in all directions. This explains why gases are easy to compress and solids are not. Density is the mass in a given volume, calculated as density = mass ÷ volume and usually given in g/cm³ or kg/m³. Solids and liquids are much denser than gases because their particles are close, and water has a density of about 1 g/cm³. An object floats if it is less dense than the liquid. Ice floats on water because freezing makes water expand by about 9%, so ice is less dense than liquid water, which is unusual among substances.

Brownian motion is the random jiggling of tiny visible particles, such as specks of smoke in air seen under a microscope. They are knocked about by collisions with the air’s invisible, fast-moving molecules, which hit from different sides with unbalanced force. It was important historical evidence that matter is made of moving particles. Diffusion follows from this random motion. Particles of one substance spread through another, gases or liquids, moving from where there are more of them (a higher concentration) to where there are fewer, until they are spread evenly. Nothing needs to stir them. It is why the smell of baking drifts through the house, and why a drop of ink spreads in water. It happens faster in gases than liquids and faster when it is warmer.

Chemical changes differ because new substances form. Clues include a colour change, a gas given off, a temperature change or light, and a solid appearing in a liquid. Burning, rusting and baking a cake are chemical changes, and they are generally difficult to reverse. A physical change leaves the substance the same and is generally easy to undo. The test to teach is: has a new substance with different properties been made? The line is not perfectly clear-cut, and teachers choose clear examples at this stage. Mass is conserved in chemical changes too, but pupils are less likely to see it, because gases are often lost or gained.

Words your child will hear

Conservation of mass: The total mass stays the same during a change, because no particles are lost or made.
Sublimation: A solid turning directly into a gas without becoming a liquid first.
Density: How much mass is packed into a given volume. Density = mass ÷ volume.
Brownian motion: The random jiggling of small visible particles caused by collisions with fast-moving molecules.
Diffusion: The spreading out of particles from a region of higher concentration to lower concentration.
Dissolving: When a solid (solute) mixes into a liquid (solvent) to make a solution, with its particles spread throughout.
Reversible: Able to be undone, so the original material can be recovered. Most physical changes are reversible.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about conservation of material and of mass, and reversibility, in melting, freezing, evaporation, sublimation, condensation, dissolving
Pupils should be taught about similarities and differences, including density differences, between solids, liquids and gases
Pupils should be taught about Brownian motion in gases
Pupils should be taught about diffusion in liquids and gases driven by differences in concentration
Pupils should be taught about the difference between chemical and physical changes

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

A worked example

50 g of salt is stirred into 200 g of warm water until it has all dissolved. (a) What is the mass of the solution? (b) The solution is left until all the water has evaporated. What mass of salt is left? (c) Is this a physical or a chemical change?

Use conservation of mass. Nothing has been removed or added, so the total mass is the mass of salt plus the mass of water.
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50 + 200 = 250 g
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Explain the missing salt. The salt has not vanished: its particles are still there, spread out among the water particles, too small to see. They still have mass.
Now evaporate the water. The water particles escape into the air as vapour, but the salt particles do not evaporate at this temperature, so they stay behind as salt crystals. Provided none is spilt, 50 g remains.
Note what happens to the rest of the mass. If the solution is in an open dish on a balance, the reading (not counting the dish) falls from 250 g to 50 g because 200 g of water has gone into the air, not out of existence. If all the vapour could be collected and condensed, it would have a mass of 200 g.
Decide the type of change. No new substance is made: the crystals left behind are still salt, with the same properties as before, and the change can be reversed. Dissolving and evaporation are both physical changes.

The solution has a mass of 250 g, 50 g of salt is left, and both stages are physical changes.

Common mistakes and how to help

Thinking sugar or salt disappears when it dissolves

You can no longer see it, and children often say it has “melted” or “gone”.

What helps: Weigh the water and sugar before and after. The total is unchanged. Evaporate a little salt solution on a saucer on a warm windowsill and the crystals come back.

Believing heavy objects sink and light ones float

In everyday life a pebble sinks and a feather floats, so mass seems to decide it.

What helps: It is density that matters. A large wooden log is heavy but floats, and a steel paper clip dropped into water is light but sinks. Compare equal volumes, and use density = mass ÷ volume to explain it.

Thinking Brownian motion means the smoke particles are moving by themselves

The specks dart around randomly like tiny living things.

What helps: They are being struck from different sides by many tiny, invisible air molecules, and the hits do not balance out. The movement is evidence for the moving particles we cannot see.

Believing that if a change cannot be reversed easily, or looks dramatic, it must be chemical

Teachers stress irreversibility, so children use it as the only rule.

What helps: Use the key question: is a new substance formed? Smashing glass is hard to undo but is physical. Ask about the properties before and after.

How to help at home

Weigh a solution. Using kitchen scales, weigh a cup of warm tap water and a spoonful of sugar or salt separately, then weigh the cup again after stirring them together. Compare the totals. Do not use boiling water.
Watch diffusion. Add a drop of food colouring to a glass of cold water and another to a glass of warm tap water, without stirring. Compare how fast the colour spreads and explain the difference using particles.
Chase a smell. Peel an orange at one end of a room while everyone else sits spaced along it. Note when each person first smells it. This is diffusion in a gas.
Sink or float? Collect household objects, predict whether each will sink or float in a washing-up bowl of water, then test. Find the density of a lump of modelling clay by weighing it and measuring its volume by how much it raises the water in a measuring jug.
Freeze a bottle. Fill a plastic bottle about nine-tenths full of water, cap it and leave it in the freezer. The ice takes up more room than the water did. Use plastic only, never glass, and supervise it.

Leads on to

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