When the system's number of molecules and temperature are held constant , the volume of the gas is inversely proportional to the external pressure. As the external pressure increases the volume of the gas drops. This happens because when the volume decreases there are more collisions per unit area in a given unit of time with the walls of the container which increases the internal pressure. There are also more collision per unit volume in a given unit of time with the other gas molecules. Since the temperature is kept constant, the molecules are moving with the same average speed. Compare the system before and after a decrease in volume.

To further explore the relationship between pressure, volume, moles of gas and temperature look at the Gas Law MoLE. Be sure to print out a copy of the Gas Law Activity, or look at it in your Chemistry Activity Manual.