On Earth, matter exists in one of three states: solid, liquid or gas. Matter in each state exhibits distinct characteristics. Gases, for example, do not have a fixed volume* or shape. As a result, gases respond to pressure changes by changing their volume. In other words, gases are compressible. In contrast, liquids and solids are not compressible: their volume does not change in response to changing pressure. This is the reason air-filled spaces in our ears “pop” during airplane takeoff and landings while the liquid filled spaces in our bodies do not. Boyle's Law describes the relationship between pressure and volume at constant temperature for a fixed mass* (number of molecules) of a gas.
To understand Boyle's law, it helps to visualize the behavior of gas particles (or molecules) in an enclosed space. In a closed container, individual molecules are constantly hitting and bouncing off the container walls. Each time a gas molecule bounces, it imparts a force on the wall.1 In a flexible container such as a balloon, the force of the molecules hitting the inside of the balloon hold the balloon inflated. The force of each impact is small, but the sheer number of collisions create enough force to keep the balloon open.2
Pressure in a closed container changes if
1) temperature changes 2) number of molecules increases or decreases 3) volume changes
Boyle’s Law deals with number 3; the relationship between volume and pressure when the other two remain constant.
According to Boyle’s Law, the amount a gas will compress is proportional to the pressure applied. Its mathematical expression is:
Where, P1 is the pressure of a quantity of gas with a volume of V1 and P2 is the pressure of the same quantity of gas when it has a volume of V2. This means that if nothing else changes, the volume of a given mass of gas is inversely proportional to pressure it is under. It is a linear relationship. If pressure on a gas doubles, its volume will decrease by 1/2. An alternative expression of the law is: