Here's why:
* Kinetic Energy and Temperature: The average kinetic energy of gas molecules is directly proportional to the absolute temperature (in Kelvin). This is a fundamental relationship in the kinetic theory of gases.
* Heating and Energy Transfer: When you heat a gas, you are adding energy to the system. This energy is absorbed by the gas molecules, causing them to move faster.
* Constant Pressure: Keeping the pressure constant means the volume of the gas can expand. As the molecules move faster and collide more frequently with the container walls, the pressure would naturally increase. However, the constant pressure condition allows the volume to expand, mitigating the pressure increase.
In summary: Heating a gas at constant pressure directly increases the average kinetic energy of its molecules. This increased kinetic energy is reflected in the higher temperature of the gas.
