What does the thermal coefficient of expansion refer to?

The thermal coefficient of expansion refers to the tendency of matter to change its volume in response to changes in temperature. Specifically, it quantifies how much a material will expand per unit length (or volume) for each degree of temperature increase.

When considering the choices provided, the general increase in the volume of a gas as its temperature increases aligns with the concept of thermal expansion. Gases expand significantly as they are heated due to an increase in the kinetic energy of their molecules, which move more vigorously and push apart from each other, leading to an increase in volume. This phenomenon is often described in relation to gases following the ideal gas law, where increasing temperature at constant pressure results in increased volume.

The other options do not accurately reflect the primary aspect of the thermal coefficient of expansion. Decreased pressure with increased temperature typically refers more to gas behavior under changing conditions rather than a specific measurement of thermal expansion. The change in density of liquid with pressure is more related to compressibility, not thermal expansion. Finally, volume displacement of solids when heated is not as directly representative of the complexities involved in thermal expansion compared to how gases react to temperature changes.