What Is Boyle's Law in Simple Words?

Boyle's law is a gas law. It dictates a pressure-volume relationship. The law states the pressure of an ideal gas is inversely proportional to its volume if the temperature and amount of the gas remain constant.

In other words, when the pressure of an ideal gas increases, its volume decreases, and vice versa. This is true as long as the temperature and amount of the gas remain constant.

Mathematically, we can express the law as:

or

Where, P is the absolute pressure of the gas, V is the volume, and k is a constant of proportionality.

Suppose an ideal gas is at the pressure P₁ and volume V₁. Now, the gas undergoes expansion/contraction such that the temperature and amount of gas remain constant. After the change, the new pressure and volume of the gas are P₂ and V₂.

Using the Boyle law, P₁V₁ = k and P₂V₂ = k. Thus, P₁V₁ = P₂V₂. This expression is very useful to calculate unknown pressure or volume if the other three variables are known.

Let's take an example. The initial pressure and volume of an ideal gas are 1.0 atm and 1.0 m³. The gas undergoes expansion, and its new volume is 2.0 m³. Thus, P₁, V₁, and V₂ are known to us. But, P₂ is not known, which we can calculate as:

Therefore, the new pressure P₂, after the expansion, is 0.50 atm.

In the example, the volume changes from 1.0 m³ to 2.0 m³, i.e., it doubles. As a consequence, the pressure changes from 1.0 atm to 0.50 atm, i.e., it halves. Thus, an increase in the volume causes the pressure to decrease. This is the Boyle law. When one increases, the other has to decrease, and vice versa.

Let's say, the same gas expands to V₃ = 3 m³ instead of V₂. We can calculate the corresponding pressure P₃ as:

Similarly, for every value of V, we can calculate the corresponding value of P. The table below lists volumes and its corresponding pressures.

Pressure versus volume data

P, atm	V, m3
1.0	1.0
0.50	2.0
0.33	3.0
0.25	4.0
0.20	5.0
0.17	6.0
0.14	7.0

We can plot the above data in a graph to get the graphical representation of Boyle's law. In the figure below, the pressure is on the y-axis, and the volume is on the x-axis. The curve in the graph is called the PV curve. It's an inverse proportional relationship—i.e., as the volume increases, the pressure decreases.

Associated articles

• Real-life examples of Boyle's law
• Graphs of Boyle's law
• The equation of Boyle's Law