As a result of changes in temperature, matter changes in volume. This concept is known as thermal expansion. As a substance heats up, the particles within the matter move about, becoming more active as the temperature rises. This activity causes the particles to need additional space between each other, enlarging the size of a material. The equation to describe this concept is known as the coefficient of thermal expansion and can be defined by the fact that the degree of expansion is divided by the change of temperature in a material.
The use of certain solid materials in construction and engineering is a direct result of this thermal expansion. The need for objects to retain their shape without major distortion results in the common practice of using metals and plastics. For example, a hammer needs to retain its shape as temperatures rise to prevent impairment. While this principle is generally true based on the type of material used, certain structures, such as crystals, can posses different coefficients of thermal expansion depending on the shape.
Likewise with the concept of negative thermal expansion, conditions such as extreme cold cause materials to contract for the exact opposite reasons as regular thermal expansion. Often other materials with different coefficients are needed in conjunction with these solids. One example would be a window, which needs rubber fittings to prevent the expansion and contraction of the metal frame.
The harder the material, the lower the thermal expansion. Liquids will expand more readily than solids. This is caused by the fact that as the energy bond between the particles increases, the thermal expansion decreases in these substances. In addition, certain materials change size due to the absorption or desorption of water or solvents.
A type of expansion known as anisotropic expansion can occur within certain types of materials. This means that the solid will not expand exactly alike in each direction. Any material with a number of layers, such as graphite, usually expands more readily in a perpendicular fashion with the layers rather than vertically against the layers. This can be beneficial for designers when attempting to make a fitting that would need to expand in only one direction when exposed to temperature changes.