What is the relationship described by the equation P1V1 = P2V2?

The equation P1V1 = P2V2 describes the relationship between pressure and volume in a gas at constant temperature, known as Boyle's Law. According to this law, for a fixed amount of gas, if the temperature remains constant, the product of the pressure (P) and the volume (V) before a change (P1V1) will equal the product after the change (P2V2). This means that if the volume of the gas decreases, the pressure increases, and vice versa, as long as the temperature does not change. This inverse relationship between pressure and volume is a fundamental principle in physics that helps explain the behavior of gases in various conditions.

The other options do not specifically relate to the behavior of gases under the constraints of Boyle’s Law, making them incorrect in the context of this particular equation.