Which law states that the volume of a gas is directly proportional to its temperature at constant pressure?

The law that defines the direct relationship between the volume of a gas and its temperature at constant pressure is indeed Charles's Law. This law states that as the temperature of a gas increases, its volume also increases, provided that the pressure remains unchanged. This relationship can be expressed mathematically as V/T = k, where V represents volume, T represents temperature in Kelvin, and k is a constant.

Charles's Law is especially important because it demonstrates the importance of temperature in determining the behavior of gases. For example, when a gas is heated, its molecules gain energy and move more vigorously, leading to an increase in volume if the pressure is kept constant. This concept plays a crucial role in understanding behaviors of gases in various scientific and practical applications such as hot air balloons or engines.

Avogadro's Law relates to the volume of gas and the number of moles at a constant temperature and pressure, while Boyle's Law addresses the inverse relationship between pressure and volume of a gas when temperature is held constant. Gay-Lussac's Law relates pressure and temperature but does not consider volume; thus, these laws describe different behaviors and conditions of gases.