Boyle's law

Law stating that the volume of a given mass of gas at a constant temperature is inversely proportional to its pressure. For example, if the pressure on a gas doubles, its volume will be reduced by a half, and vice versa. The law was discovered in 1662 by Irish physicist and chemist Robert Boyle. See also gas laws.

If a gas is compressed in a cylinder the volume of the gas decreases. The number of particles of gas in the cylinder remains the same. The particles collide with the walls of the cylinder more frequently, since they have less distance to travel between one collision and the next. The collisions result in an outward force on the walls of the cylinder – the pressure of the gas. The pressure of the gas increases as the frequency of the collisions increases. Therefore, as the volume decreases the pressure increases.

If the pressure is denoted by P and the volume by V, then P is proportional to 1/V.

Boyle's law can be investigated by using an apparatus consisting of a foot pump attached to a pressure gauge. This is attached to a glass tube (with a scale) containing an oil and trapped air. As more air is pumped into the apparatus from the foot pump and into the oil reservoir, the oil in the glass tube is forced in and the pressure on the trapped air gets higher. A series of pressure and volume readings are taken. It is found that multiplying pressure by volume for each of the readings produces the same result, to a high degree of accuracy. Therefore, P × V = constant. When P is plotted against 1/V on a graph, a straight line results. Readings from the experiment show that:

P₁ V₁ = P₂V₂

where P₁ and V₁ are the initial pressure and volume of a gas, and P₂ and V₂ are its final pressure and volume.