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. Okay, so how balloons expand? Balloons expand because you blow in air. In other words, you blow in oxygen molecules, water vapor, you blow in carbon dioxide, you blow in nitrogen molecules, and so on.