But now it doesn't do work by acting over some distance. It does work by expansion. So expansion work is called PD work, and you find the work done by multiplying the pressure when it's constant by the change in volume. So this equation only holds when the pressure is constant. When the pressure isn't constant, we have to use the integral. Okay, so there are many different examples that could be used to describe a chemical work, but for the most part, people use two examples, one with balloons and the other one with pistons.