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2.2.1 Thermal expansion of solids, liquids and gases

Syllabus
0625–2026–2027
Topic
2.2.1
Level
—

Describe expansion and contraction in every state

At constant pressure, most solids, liquids and gases expand when heated and contract when cooled: their dimensions or volume change with temperature.

State When heated at constant pressure When cooled
solid length, area and volume increase slightly; any hole in the solid also becomes larger length, area, volume and holes become smaller
liquid volume increases, usually more than the containing solid volume decreases
gas volume increases substantially if its boundary can move volume decreases

Expansion does not add matter: the mass stays constant. Because the same mass occupies a larger volume, density decreases on heating; contraction at constant mass increases density.

The particles themselves do not get larger. ‘Gas expands when heated’ here assumes constant pressure and space for the gas boundary to move. In a sealed rigid container the volume cannot expand, so heating raises the pressure instead.

Use thermal expansion and allow for its consequences

Thermal expansion is useful when a changing size produces a measurement or movement, but unwanted expansion must be allowed for so structures do not bend, buckle or crack.

Situation Expansion or contraction Design response or useful effect
railway tracks and bridges long sections expand in hot weather leave expansion gaps or joints so the sections can lengthen without buckling
overhead cables metal contracts in cold weather install the cable with slack so contraction does not create a dangerously large tension
tight metal lid on a glass jar for the same warming, the metal lid expands more than the glass neck the lid opening becomes relatively larger, making the lid easier to remove
liquid-in-glass thermometer the liquid expands more than the glass bulb liquid rises along a narrow uniform capillary; a narrower tube gives a larger rise per degree
thermostat with a bimetallic strip its two bonded metals expand by different amounts the strip bends and can open or close an electrical contact

For each example, identify what changes temperature, which part expands or contracts, and what would happen without the design feature. A gap is useful because the material expands into it; the gap itself is not the expanding object.

Do not explain every heating effect as expansion: melting and simply becoming hotter are different processes. The application must depend on a change in dimensions or volume.

Explain why gases expand most and solids least

For the same temperature rise at constant pressure, the usual order of volume expansion is gas > liquid > solid.

State Particle arrangement and forces Effect of heating Relative expansion
solid particles are close and held strongly in fixed positions vibrations become more energetic and average separation increases only slightly least
liquid particles are close but can move past one another; attractions are weaker than in a solid faster motion produces a larger increase in average separation intermediate
gas particles are already far apart and attractions are negligible faster particles spread much farther apart while the gas volume grows to keep pressure constant greatest

Heating increases particle kinetic energy. The size change comes from an increase in average separation, not from particles swelling. Stronger constraints in a solid resist separation most; the weakly constrained gas changes volume most.

Compare equal temperature rises under the same constant-pressure condition. The statement is an order of magnitudes, not a claim that every solid, liquid or gas has one identical expansion value.