What phenomenon describes the increase in size of materials when heated?

The phenomenon that describes the increase in size of materials when heated is known as thermal expansion. As materials are heated, their particles gain energy and vibrate more vigorously. This increased movement causes the particles to push against each other, leading to a change in distance between them and, consequently, an increase in the overall size of the material. This effect is observable in most materials, including solids, liquids, and gases, although the extent of expansion may vary based on the material's properties.

Understanding thermal expansion is crucial in various engineering applications, such as in the design of bridges, where expansion joints are incorporated to accommodate the changes in size due to temperature fluctuations. It is also important in engineering when selecting materials for components that will undergo variations in temperature, ensuring they can withstand the stresses caused by expansion and contraction.