What is thermal expansion?

Thermal expansion refers to the change in shape or volume of matter when its temperature increases. As temperature rises, the energy of the particles within a substance increases, causing them to vibrate more vigorously. This heightened motion leads to an increase in the average distance between particles, resulting in the expansion of the material.

This phenomenon can be observed in solids, liquids, and gases, as they all expand when heated. For example, when heating a metal rod, it becomes longer and potentially wider, demonstrating thermal expansion in solids. In liquids, like water, thermal expansion can be seen when it is heated, causing the water level to rise. Gases also exhibit thermal expansion; as air is heated, it expands and increases in volume.

Other options do not accurately define thermal expansion: contraction relates to a decrease in size rather than an increase, changes in pressure stem from different principles, and the idea that thermal expansion is only seen in gases is inaccurate since it is applicable to all states of matter. Thus, the correct understanding acknowledges thermal expansion as a universal property of matter in response to increased temperature.