Charles’s law states that if a given quantity of gas is held at a constant pressure, its volume is directly proportional to the absolute temperature. Think of it this way. As the temperature of the gas increases, the gas molecules will begin to move around more quickly and hit the walls of their container with more force—thus the volume will increase. Keep in mind that you must use only the Kelvin temperature scale when working with temperature in all gas law formulas! Here’s the expression of Charles’s law that you should memorize:

Try using Charles’s law to solve the following problem.

A sample of gas at 15ºC and 1 atm has a volume of 2.50 L. What volume will this gas occupy at 30ºC and 1 atm?

The pressure remains the same, while the volume and temperature change—this is the hallmark of a Charles’s law question.

This makes sense—the temperature is increasing slightly, so the volume should increase slightly. Be careful of questions like this—it’s tempting to just use the Celsius temperature, but you must first convert to Kelvin temperature (by adding 273) to get the correct relationships!