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What Is Boyle's Law in Simple Words?

22nd Mar 2020 @ 3 min read

Physical Chemistry

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:

Pressure inversely proportional to volume

or

PV=k, Boyle's law

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.

Atkins Physical Chemistry

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:

pressure is 0.50 atm

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:

pressure is 0.33 atm

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.

Boyle's law graph simplified
Boyle's law graph: the pressure decreases with an increase in the volume.

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Boyle Law Gas Law Robert Boyle

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