Particles Do Not Get Bigger When Heated

What students typically write

“When a solid is heated, the particles expand and take up more space, so the solid gets bigger.”

This appears in examiner reports almost every series. Students transfer their everyday experience of objects expanding when heated and assume the particles themselves are growing. Some write “particles swell” or “particles get larger”, which is the same error in different words.

Why it is wrong

Particles are atoms, ions or molecules. They do not change size when temperature changes. What changes is how fast they move and how far apart they are from each other.

When a substance is heated, energy is transferred to the particles as kinetic energy. In a solid, the particles vibrate faster and with greater amplitude about their fixed positions, pushing neighbouring particles slightly further away. In a liquid, the particles move faster and slide past each other with more energy. In a gas, the particles move faster in all directions and the average separation increases.

The macroscopic expansion you observe (a metal rail getting longer, mercury rising in a thermometer) is caused entirely by increased spacing between particles, not by any change in the particles themselves.

What the mark scheme actually wants

A typical 3-mark question asks: “Explain, using the kinetic particle model, why a gas expands when heated at constant pressure.”

The marking points are:

1. Particles gain kinetic energy / move faster (1 mark)
2. Particles collide with the walls with more force and more frequently (1 mark)
3. Particles move further apart / the average distance between particles increases (1 mark)

Notice that none of the marking points mention particles changing size. Any answer that says “particles expand” or “particles get bigger” scores zero for that marking point and may contradict another scoring idea.

Worked example: wrong vs right

Question: A balloon filled with air is placed in warm water. Explain why the balloon gets bigger. [3]

Wrong answer: “The air particles expand when heated and take up more room, so the balloon inflates.” This scores 0/3. It states the particles expand (wrong), offers no kinetic energy idea, and does not mention spacing or collisions.

Right answer: “The air particles gain kinetic energy and move faster (1). They collide with the walls of the balloon more frequently and with more force (1), pushing the walls outward. The average distance between particles increases, so the gas occupies a larger volume (1).” This scores 3/3. Each sentence matches one marking point.

How to avoid this mistake

Train yourself to write the phrase “particles gain energy and move faster” as your opening line for any kinetic theory explanation. This immediately puts your answer at the particle level and steers you away from macroscopic language like “expands” or “gets bigger”.

Then ask yourself: what happens to the spaces between particles? The answer is always that the spaces increase. The particles themselves are unchanged.

A useful self-check: if your answer would still make sense with the word “particles” replaced by “marbles”, you are probably describing macroscopic behaviour. Marbles do not gain kinetic energy from heating. Particles do.

For more detail on kinetic particle theory and the exact phrasing examiners accept, see the kinetic particle theory subtopic page. The solids, liquids and gases page covers particle arrangement diagrams, where the same error appears when students draw larger circles for the “heated” state.