Boyle's Law


Later work has shown that Boyle's law is true only at relatively high temperatures (around room temperature and above) and low pressures (a few atmospheres or less).

Boyle's law states that at constant temperature, the volume of a sample of gas is inversely proportional to the pressure of the gas:

V =
a constant
P

or:

PV = a constant

Increasing the pressure on a sample of a gas at constant temperature decreases the volume of the gas.

Boyle's law can be used to calculate what the new volume of a sample of gas will be if the pressure is changed. It can also be used to calculate what change in pressure is needed to bring about a given change in volume (provided that the temperature does not change).

If a sample of a gas has a volume V1 at a pressure P1, and the same sample of gas has a volume V2 at a pressure V2, then:

P1V1 = a constant

and:

P2V2 = the same constant

providing that the temperature does not change. Since P1V1 and P2V2 are both equal to the same constant:

(Eq1)     P1V1 = P2V2

The relationship in Eq1 makes possible calculation of any one of the four variables P1, P2, V1, or V2, if the values of the other three are known. For example, dividing both sides of Eq1 by P2:

P1V1
V2
= P2

If the volume of a sample of gas is changed while the temperature is kept constant, the old pressure, P1 can be converted to the new pressure, P2 by multiplying the old pressure by (V1/V2).