According to Charles's Law, if the pressure of a gas stays constant, when the temperature increases, the volume __________.

The correct answer is "the volume increases".

In fact, Charle's law states that when the pressure of a gas stays constant, the ratio between the temperature and the volume of the gas is constant:
[tex] \frac{T}{V} =k[/tex]
where T is the gas temperature and V is its volume. We see from the formula that, if the temperature T increases, since the ratio must remain constant, the volume of the gas V increases as well.