Explanation:
if we fix the temperature, we are just left with PV = constant for the gas law. So, in this situation, if the volume is doubled, the pressure must go down by one-half. And vice-versa. The simplest illustration of this would be a cylinder with a plunger on one end: if you push the plunger in so that the volume of the cylinder is halved and the temperature remains constant, then the pressure will double.
