Thermal Expansion of Solids, Liquids and Gases

This lesson explains Thermal Expansion of Solids, Liquids and Gases for Cambridge IGCSE Physics 0625. It separates the Core requirements from the additional Supplement work for Extended candidates. Pay close attention to equation choice, unit conversion and the final sense check. Read the explanation once, then attempt the worked method from a blank page.

Core: what is thermal expansion?

Most materials expand when their temperature rises and contract when their temperature falls. At constant pressure:

• solids expand a little
• liquids expand more than solids
• gases expand much more than liquids or solids

Core questions can ask you to describe the change, recognise an application or explain why an engineering feature allows for expansion. A detailed particle comparison of the three states belongs to the Supplement section below.

Core: applications and consequences

Common examples include:

• gaps or sliding joints in bridges
• expansion gaps in railway tracks
• flexible bends or joints in long pipes
• slack in overhead cables
• liquid-in-glass thermometers
• tight metal lids becoming easier to open after warming

For an application answer, identify both the temperature change and the consequence.

Example:

A bridge expands as its temperature rises. Expansion joints provide room for the sections to move, reducing the risk of buckling or damage.

For a thermometer:

The liquid expands when its temperature rises and moves up the narrow tube. The scale is calibrated so the liquid level indicates temperature.

Core original question

A steel bridge has gaps between neighbouring roadway sections.

(a) State what happens to each steel section on a hot day. [1]

It expands.

(b) Explain why the gaps are needed. [2]

The gaps give the sections room to expand. Without the gaps, neighbouring sections could push against each other and buckle or become damaged.

Original marking guidance

• one mark for expansion
• one mark for allowing space or movement
• one mark for a valid consequence such as buckling, cracking or damage

Supplement: why do solids, liquids and gases expand by different amounts?

Heating increases particle motion.

• Solid: particles vibrate more strongly about fixed positions. Strong forces and close packing limit the increase in average separation, so expansion is small.
• Liquid: particles move more energetically and are less rigidly arranged, so the average separation can increase more than in a solid.
• Gas: particles are already widely separated and move freely, so a gas shows the greatest expansion at constant pressure.

The relative order is:

$\text{solid} < \text{liquid} < \text{gas}$

The particles themselves do not become larger. The spacing or average separation changes.

Supplement original question

Equal volumes of a solid, liquid and gas are heated through the same temperature rise at constant pressure.

State the order of their volume increases, from smallest to largest, and explain the order. [4]

A complete answer is:

Solid, then liquid, then gas. In a solid, particles are closely packed and held in fixed positions by strong forces, so their average separation changes only slightly. Liquid particles are less rigidly arranged and can move further apart. Gas particles are widely separated and free to move, so the gas expands the most.

Common mistakes

• Calling the whole particle explanation Core. The observed expansion and applications are Core; the detailed relative particle explanation is Supplement.
• Writing that particles expand. The particles do not become larger; their arrangement or average separation changes.
• Confusing expansion with melting. Expansion is a size or volume change within the same state.
• Giving an application without its purpose. “Bridges have gaps” is incomplete without explaining that the gaps allow expansion and prevent damage.
• Reversing the expansion order. For the same temperature rise at constant pressure: solid, liquid, gas from least to greatest.
• Claiming mass increases. Heating does not add matter; volume changes while mass remains the same.

Exam technique

Read the command word and tier requirement carefully:

• State or describe: give the observed expansion or application.
• Explain: link the temperature change to the engineering consequence.
• Explain in terms of particles: for Supplement, compare motion, arrangement, forces and average separation.

A concise cause-and-effect answer is usually stronger than a long generic paragraph.

How this is examined

Core Papers 1 and 3 can test the relative observed expansion of states and everyday applications or consequences. Extended Papers 2 and 4 can also require the detailed particle explanation for why solids, liquids and gases expand by different amounts. Practical questions may place expansion in an apparatus context, but the syllabus does not prescribe a separate thermal-expansion calculation.