What does thermal expansion refer to?

Thermal expansion refers to the phenomenon where an increase in temperature causes materials to expand, resulting in an increase in their length, volume, or surface area. This response is a fundamental concept in thermodynamics, grounded in the behavior of molecules within a substance. As the temperature rises, the energy within the molecules increases, causing them to move more vigorously and occupy more space.

This principle is essential in various fields, including materials science and engineering, where it impacts the design and functionality of structures and materials. Understanding thermal expansion is particularly relevant in fire investigations, as it can affect how different materials behave when exposed to intense heat, potentially leading to changes in structural integrity or the characteristics of burning materials.

The other options reflect different physical phenomena: the change in a material's state from solid to liquid pertains to phase changes, the decrease in temperature over time relates to cooling processes, and the alteration of chemical composition due to heat involves chemical reactions rather than physical changes related to temperature alone.