Thermal expansion

When a substance is heated, its dimensions generally increase. This is called thermal expansion. When it is cooled, it generally contracts: its dimensions decrease.

At constant pressure, solids usually expand the least, liquids more and gases the most for the same temperature rise. The amount of expansion also depends on the particular substance.

Thermal expansion and particles

Heating increases the energy of a substance’s particles. The resulting expansion depends on how the particles move:

• In a solid, particles vibrate more as they gain energy. Their average separation increases slightly, so the solid’s dimensions increase.
• In a liquid, the average separation between particles generally increases, so the liquid’s volume increases.
• In a gas, particles move faster and exert pressure on their surroundings through collisions. If the gas is free to expand while its pressure remains constant, its volume increases.

Expansion is due to an increase in the average separation of particles, not an increase in the size of the particles themselves.

Effects and uses of thermal expansion

Structures such as bridges have expansion joints or gaps that give their materials room to expand in warm weather. Without enough room, expansion can create excessive stress. Railway rails, for example, can buckle if their expansion is restricted.

A liquid-in-glass thermometer uses the expansion of a liquid to indicate temperature. As the liquid warms, its volume increases and it rises along a narrow tube. When it cools, it contracts and the level falls.

Overhead wires expand in warm weather. Because they become longer between their supports, they sag more.

Particle explanation of relative thermal expansion

When substances are heated, their particles move more vigorously. How much a substance expands depends on how its particles are arranged and how strongly their interactions restrict changes in their separation.

For the same temperature rise at constant pressure, solids generally expand least, liquids more and gases most. This comparison is based on fractional expansion: the change in a dimension or volume compared with its original value. It is not simply a comparison of the absolute increase in size.

Expansion of solids and liquids

In a solid, particles are closely packed and held around fixed positions. Heating makes them vibrate more. Their interactions restrict the increase in their average separation, so the solid generally has a small fractional expansion.

In a liquid, particles are also close together, but they can move past one another rather than remaining around fixed positions. Heating generally increases their average separation more than in a solid, giving the liquid a larger fractional expansion.

Expansion of gases

In a gas, particles are far apart and move freely. Heating makes them move faster. At constant pressure, the gas expands, increasing the space available to the particles so that their faster motion does not produce a sustained increase in pressure.

Because gas particles have much more space between them than particles in a solid or liquid, the fractional increase in a gas's volume is generally much larger for the same temperature rise.