Boyle's Law, named after Robert Boyle, states that the the volume and pressure of a confined gas at constant volume and temperature are inversely related. In English, when the volume of a gas is decreased, the pressure increases. This change is always proportional – that is, if the volume is doubled, the pressure is cut in half. This relationship can be stated mathematically:
This relationship can also be shown graphically:
Why are pressure and volume inversely related? The easiest way to understand this relationship is to picture a simple container whose volume can be easily changed. In this case we'll use a piston – a cylinder with one closed end and one end that can be moved back and forth, altering the volume of the gas inside. This is the same thing as a syringe that holds gas.
If the volume is decreased (if the piston is pushed in), two things happen.
The first is that the distance between the two ends decreases. This means that a gas particle moving from end to end will make the trip in less time and will, as a result, collide with the ends more often (more times in a given minute, for instance). As a result, there are more collisions with the walls and the force on the walls increases.
In addition, the piston has moved past some of the side walls, effectively cutting off that portion of the walls from the gas particles. So, the collisions that do happen, occur on a smaller area.
Remembering that pressure is defined as force/area, we can see that the pressure must increase.