When a substance is heated, its particles gain energy and move further apart on average, so the substance expands. The particles do not grow. This idea explains gaps in railway tracks, the liquid rising in a thermometer and the bending of a bimetallic strip.
It is the first lesson in thermal processes, and the picture it builds is used by every later lesson in the topic.
What happens to the particles on heating?
In a solid, particles vibrate about fixed positions. Heating gives them more energy, so they vibrate with larger movements. The average distance between neighbours grows slightly, and the solid gets a little bigger in every direction.
In a liquid, particles slide past each other. More energy means faster movement and slightly greater spacing.
In a gas, particles move freely and far apart. More energy means faster movement, and if the gas is free to expand it fills more space.
Order of expansion for the same temperature rise: gases expand most, liquids less, solids least. The forces between particles are weakest in a gas and strongest in a solid.
How to write a full explanation
- State the change: the substance is heated, so the particles gain energy.
- Describe the motion: they vibrate (solid) or move (liquid or gas) more vigorously.
- State the spacing: the average separation between particles increases.
- State the result: the volume increases. The number of particles and the mass are unchanged, so the density decreases.
Worked example
A liquid-in-glass thermometer has a thin tube filled with coloured alcohol. Explain why the liquid rises when the bulb is placed in warm water. (Invented question.)
Step 1: the warm water transfers energy to the alcohol in the bulb, so the alcohol particles gain energy.
Step 2: the particles move faster and the average distance between them increases.
Step 3: the alcohol takes up more volume. The bulb has a fixed size, so the extra liquid is pushed up the narrow tube.
Step 4: a narrow tube means a small extra volume produces a long movement along the scale, which makes the change easy to see.
The mistake to watch for
Mistaken answer: “The alcohol particles expand, so the liquid rises.”
This says the particles change size. They do not. It would also score poorly because it gives no reason for the change.
The correction is to write that the particles move further apart on average, because they have more energy. Whenever you catch yourself writing “the particles expand”, replace it with “the spacing between particles increases”.
Check yourself
1. A steel bridge has small gaps between sections. Explain why they are needed on a hot day.
Show answer
On a hot day the steel particles gain energy and vibrate more, so their average separation increases and each section gets longer. The gaps give room for this expansion, so the bridge does not buckle.
2. A syringe holds 50 cm³ of air and the plunger is free to move. Would the air expand by a larger or smaller fraction than a solid block of the same size for the same temperature rise? Give a reason.
Show answer
Larger. The air particles are far apart with very weak forces between them, so extra energy lets them spread out much more than the closely bonded particles in a solid.
3. A bimetallic strip is made of brass joined to iron. Brass expands more than iron. On heating, which way does the strip bend?
Show answer
It bends towards the iron. The brass becomes longer than the iron, so the brass must be on the outside of the curve.
Where this leads next
Expansion leads to a drop in density, which is the key to comparing conduction, convection and radiation. You can also practise reading precise values with the bounds and rounding explainer before you start measurement-based thermal questions.
Writing “the particles expand” instead of “the spacing increases” is a small slip that costs marks. A teacher in online one-to-one Physics tuition can catch it as you write and drill the right wording.