Which law states that volume is directly proportional to absolute temperature at constant pressure?

The law that states that volume is directly proportional to absolute temperature at constant pressure is Charles' Law. This law describes how gases tend to expand when heated, allowing you to understand the relationship between temperature and volume.

In practical terms, if you have a gas contained in a flexible balloon, as you increase the temperature of the gas while maintaining constant pressure (for example, by keeping the balloon at the same atmospheric conditions), the volume of the balloon will increase. This occurs because the molecules of the gas move faster at higher temperatures, leading to more forceful collisions against the walls of the balloon, resulting in an expansion of volume.

Charles' Law is often formulated mathematically as V/T = k, where V is the volume, T is the absolute temperature (measured in Kelvin), and k is a constant for a given amount of gas at constant pressure. This clear proportional relationship helps reinforce why an increase in temperature results in an increase in volume, as long as pressure remains unchanged.

Other laws mentioned, such as Avogadro's Law, Gay-Lussac's Law, and Boyle's Law, describe different gas behaviors involving variables like moles, pressure, and volume in various conditions but do not relate volume directly to absolute temperature under constant pressure