Thermal expansion occurs when a transfer of heat causes matter to change in volume in response to a change in temperature. When a substance heats up, the particles within it begin moving around, which creates space between the particles. As the space between the particles expands, the substance begins to increase in size.
According to Encyclopædia Britannica, when the material that is heating up is solid, thermal expansion usually causes a change in the length or thickness of the substance. Crystalline solids with the exact same structural configuration throughout expand uniformly in all dimensions. If the material is a liquid, its volume increases during thermal expansion. The forces that bond molecules and atoms are different for each substance, so the type of change that occurs in substances during thermal expansion always differs.
The opposite of thermal expansion is thermal contraction, which causes a substance to decrease in size as the space between its particles gets smaller. According to Penn State College of Engineering, environmental conditions can cause thermal expansion and contraction within a short time. Some substances experience negative thermal expansion as well. This occurs when a material contracts when heated rather than expands. Cubic zirconium tungstate exhibits negative thermal expansion when it is heated.