What happens to materials during thermal expansion?

During thermal expansion, materials tend to expand when heated due to the increased kinetic energy of their particles. As temperature rises, the vibrations of the atoms or molecules within the material become more vigorous, causing them to move apart. This movement leads to an increase in the overall dimensions of the material—length, area, and volume.

For example, when metals are heated, they typically experience a noticeable increase in size, which is essential to consider in engineering and construction to avoid structural damage. Thus, when heated, materials expand, making this answer the most accurate reflection of the behavior of materials during thermal expansion.