Key Stage 3 science · Physics – Matter

KS3 temperature, particles and internal energy: a parent guide

This topic joins two ideas pupils already have: that matter is made of moving particles, and that energy can be stored and transferred. In Key Stage 3, children learn that raising the temperature of a material makes its particles move faster and, on average, sit slightly further apart, and that every material has internal energy stored by its particles. It explains expansion, cooling, and the puzzling flat sections on a heating graph.

Energy in matter, explained

Pupils start with the particle model they met earlier: the particles in a solid vibrate on the spot, in a liquid they slide around, and in a gas they fly about freely. Moving particles have kinetic energy, the energy of movement. Temperature tells us, roughly, how much kinetic energy the particles in a material have on average. When something is heated, its particles move faster, or vibrate more strongly; when it cools, they slow down. This is why gas in a warm room is moving faster than gas in a fridge, and why spilt colouring spreads faster through warm water than cold.

Moving faster has a second effect that pupils are expected to explain. Particles that move more vigorously push a little further apart on average, so most materials expand slightly as they warm up and contract as they cool. Solids expand only a tiny amount, liquids expand more, and gases expand the most. This is how liquid thermometers work, and why bridges have expansion gaps and overhead cables sag more in summer. The important wording is that the spacing between particles grows, not that the particles themselves swell up. Water has an unusual exception near freezing, which pupils meet in the particle model topic.

Internal energy is the next idea. Every particle in an object has kinetic energy, and the particles also store energy because of the forces between them, a bit like a stretched spring stores energy. The total of all this energy in an object is its internal energy. Heating an object raises its internal energy, and cooling lowers it. Because internal energy adds up over every particle, it depends on how much material there is as well as how hot it is. A bath of warm water has far more internal energy than a cup of boiling tea, even though the tea has the higher temperature, because there are so many more particles in the bath. This is the difference teachers want pupils to hold on to: temperature is how hot something is, while internal energy is the total energy stored.

The idea of stored energy explains a feature of changes of state that surprises many pupils. When ice is melting, or water is boiling, energy is still being supplied, yet a thermometer shows no rise in temperature. The energy is no longer making the particles move faster. Instead, it is being used to pull the particles apart from one another, which increases the energy stored between them. Once the change of state is complete, the extra energy goes back into speeding up the particles and the temperature starts to climb again. Less energy is needed to melt a solid than to boil the same amount of liquid, because in boiling the particles must be separated far more completely, and this shows up as a longer flat section on the graph.

In class, pupils use heating and cooling graphs, particle diagrams and short written explanations. They are asked to use correct vocabulary, for example “the particles gain kinetic energy” rather than “the heat gets inside”. They also learn that energy always moves from a warmer object to a cooler one. Nothing called “cold” flows into your hand from an ice cube; your hand loses energy to the ice. Energy is measured in joules, and in this topic pupils describe changes in internal energy rather than calculate them.

Words your child will hear

Kinetic energy: The energy an object has because it is moving. Faster particles have more of it.
Temperature: A measure of how hot something is. It is linked to the average kinetic energy of the particles.
Internal energy: The total energy stored in the particles of an object, from their movement and from the forces between them.
Expansion: A material taking up more space when it is warmed because its particles move slightly further apart.
Contraction: A material taking up less space when it is cooled.
Thermal energy transfer: Energy moving from a warmer place to a cooler place. Teachers prefer this to calling it “heat flowing”.
Joule: The unit used to measure energy, shown as J.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about changes with temperature in motion and spacing of particles
Pupils should be taught about internal energy stored in materials

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

A worked explanation

Ice at −10 °C is heated steadily until it has become steam above 100 °C, and the temperature is recorded every minute. The graph has two flat sections, one at 0 °C and a longer one at 100 °C. Explain the shape of the graph using particles and energy.

From −10 °C up to 0 °C the ice is a solid. The energy supplied makes the particles vibrate more strongly, so their kinetic energy rises and the temperature goes up.
At 0 °C the graph goes flat. The ice is melting. The energy is now being used to break the particles out of their fixed positions, so the energy stored by the particles increases but their average kinetic energy does not. The temperature stays at 0 °C until all the ice has melted.
From 0 °C to 100 °C the water is liquid. The energy goes back into making the particles move faster, so the temperature rises again.
At 100 °C the graph goes flat again. The water is boiling. The energy is used to pull the particles apart so they are far from each other. Separating them completely needs much more energy than loosening them, which is why this flat section is longer.
Above 100 °C the substance is steam. The energy again increases the kinetic energy of the particles, and the temperature rises.

The sloping sections are where the energy increases the particles’ kinetic energy, so temperature rises. The flat sections are changes of state, where the energy changes how the particles are arranged and the energy stored between them, so the temperature stays constant. The flat section at 100 °C is longer because boiling separates the particles far more than melting does.

Common mistakes and how to help

Thinking temperature and heat or internal energy are the same thing

In daily speech, “hot” and “lots of heat” mean the same. A thermometer reading seems to tell you how much energy is in something.

What helps: Compare a mug of tea and a bath of warm water. The tea is hotter, but the bath holds far more energy because it contains many more particles. Ask which could melt more ice, and why.

Believing the particles get bigger when something is heated

The whole material gets larger, so it seems natural that each piece must grow too.

What helps: Ask what is actually bigger. The particles stay the same size, but they vibrate or move more, so the average gap between them increases. A diagram with same-sized circles drawn further apart gets it right.

Thinking nothing is happening during the flat section of a heating graph

The thermometer does not move, so pupils assume that no energy is being supplied or that the heating has stopped.

What helps: Ask where the energy is going. It is still being supplied, but it is breaking particles apart from one another instead of making them move faster. The energy stored between them is going up.

Saying that cold or coldness moves into things

A cold drink feels as if the chill is passing into your hand, and everyday language, such as “let the cold in”, supports the idea.

What helps: Always describe it as energy moving from the warmer object to the cooler one. Your hand loses energy to the ice, so your hand cools while the ice warms up or melts.

How to help at home

Watch particles move with colour. Put a drop of food colouring into a glass of cold tap water and another into a glass of warm tap water, not hot, and watch how fast the colour spreads. Ask your child to explain the difference using particle movement. An adult should pour any warm water.
Balloon in the fridge. Put an empty plastic bottle with its lid off in the fridge for half an hour. Take it out, stretch a balloon over the neck straight away, and stand the bottle in a bowl of warm tap water. The balloon lifts and starts to fill as the air inside warms up and expands. It is a quick way to see how temperature changes the spacing of particles.
Which has more energy? Ask the question “which has more internal energy, a mug of hot tea or a bath of warm water?” and let your child argue for an answer. Having to separate temperature from the amount of material is exactly what the topic asks of them.
Spot expansion around you. Look for the expansion gap in a bridge, a metal jar lid that loosens after it is run under warm tap water, or cables sagging between poles. Ask what the particles are doing in each case, keeping to places where you are safely away from roads.

Leads on to

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