Expansion, an increase in the volume of a substance, usually due to the addition of heat to the substance. The heat causes the molecules of the substance to move farther apart, making the substance occupy more space. A loss of heat causes the opposite effect, known as contraction.

Although most substances expand when heated, not all expand at the same rate. Aluminum, for example, expands twice as much as iron when both are heated the same amount. Rubber and water are two common substances that differ from most others in their response to heat. Rubber contracts when heated. Water loses volume when its temperature rises from 0° C. to 4° C. (32° F. to 39° F.). It expands when its temperature rises above 4° C. (39° F.). It also expands when it freezes.

Expansion of Gases

Under equal pressures, all gases expand at the same rate. A gas expands by the same proportion as the temperature rises, provided external pressure remains the same. The effect of heat on the expansion of gases is stated in Charles' Law. A gas occupies 1/273.15 more space for each 1° C. rise in temperature and, conversely, 1/273.15 less space for each 1° C. drop in temperature. This fact enabled scientists to determine absolute zero (-273.15° C.), the temperature at which all molecules theoretically stop moving.

A gas can be made to expand without the addition of outside heat if the pressure confining the gas is reduced. This principle is stated in Boyle's Law. Energy for the expansion is drawn from the expanding gas itself, thus causing a lowering of temperature in the gas.

Steam engines and turbines, rockets, and internal combustion engines are powered by the expansion of gases. Bread rises in baking because heat expands the carbon dioxide gas it contains.

Expansion of Liquids

Unlike gases, liquids expand at different rates, depending on their composition. Liquids also expand by different amounts at different temperatures. How much a volume of a given liquid will expand as its temperature rises from one degree to the next can be determined by experiment or—in the case of many common liquids—by consulting tables. The expansion of liquids by heat (and loss of volume when heat is reduced) is used in thermometers. The expansion of freezing water can cause rock and masonry to crack, and will also crack plumbing pipes and engine blocks; precautions against their freezing are therefore important.

Expansion of Solids

Solids vary greatly in their response to heat. Tables showing the coefficients of expansion, the percentages by which various solids increase in length, area, and volume, have been prepared for engineers. Engineers need this information in order to take precautions against expansion and contraction. Railway rails will buckle on hot days if there is insufficient space between them. It may be necessary to place one end of a bridge on rollers to allow for expansion. The property of metals to expand with heat is put to use in thermostats.