During heat transfer, the energy that is stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bond. As a result, solids typically expand in response to heating and contract on cooling; this response to temperature change is expressed as its coefficient of thermal expansion.

The thermal expansion coefficient is a thermodynamic property of a substance. It relates the change in temperature to the change in a material's linear dimensions. It is the fractional change in length per degree of temperature change.

Most solids expand when heated. The reason for this is that this gives atoms more room to bounce about with the large amount of kinetic energy they have at high temperatures. Thermal expansion is a relatively small effect which is approximately linear in the absolute temperature.

Most materials are subject to thermal expansion: a tendency to expand when heated, and to contract when cooled. For this reason, bridges are built with metal expansion joints, so that they can expand and contract without causing faults in the overall structure of the bridge. Other machines and structures likewise have built-in protection against the hazards of thermal expansion. But thermal expansion can also be advantageous, making possible the workings of thermometers and thermostats.