What condition must be met for Charles' Law to apply when changing a gas's volume?

For Charles' Law to apply, it is essential that the pressure of the gas remains constant while observing changes in volume and temperature. Charles' Law states that the volume of a gas is directly proportional to its absolute temperature (measured in Kelvin) when the pressure is held constant. This means that if the temperature of the gas increases, the volume will also increase, provided that the pressure does not change. Therefore, the condition of maintaining constant pressure is fundamental in applying this law accurately to predict the behavior of the gas as temperature changes.

Other conditions, such as cooling the gas or changing the volume, are not necessary for Charles' Law to hold true. The law's relationship specifically emphasizes the interplay between volume and temperature under constant pressure, making that crucial for its application.