Gas: Pressure Volume
GAS PRESSURE DROPS AS VOLUME INCREASES: when the volume of a gas increases the particles become more spread out. A given area of the container therefore experiences fewer collisions and therefore a lower force. Effectively, the pressure drops as the volume increases. This is enshrined in Boyle's Law which states that the gas volume is inversely proportional to the pressure at a given temperature (see an animation of pressure increase with temperature increase). In this animation it is the container that enlarges, not the gas forcing the expansion.