According to Boyle's Law, the relationship between pressure and volume of a gas when temperature is constant is inversely proportional. This means that as the pressure on a gas increases, its volume decreases, and vice versa. The mathematical expression of Boyle's Law is P1V1 = P2V2, where P1 and V1 represent the initial pressure and volume, and P2 and V2 represent the final pressure and volume.
