Changes in state
You can follow the changes in state for a gas from one point to another in a p/V diagram. It should really need three axes for the variables p, V and T. With a change in state you move along a curve on the surface in space that is then formed. However, you usually consider the projection of the curve in one of the three planes, usually the p/V plane. Primarily a distinction is made between five different changes in state: Isochoric process (constant volume), isobaric process (constant pressure), isothermic process (constant temperature) isentropic process (without heat exchange with the surroundings) and polytropic process (where the heat exchange with the surroundings is stated through a simple mathematical function).