Gas laws

What are gas laws?

The gas laws or sometimes called fundamental gas laws or volumetric laws deal with the study of perfect gases and comprise three fairly simple laws:

They are formulated according to the behavior of three properties of gases: volume, pressure and absolute temperature.

In addition, they are also related to:

which describes that the volume of a gas is directly proportional to the number of gas particles (n).

The above four laws combined, constitute what is known as the ideal gas law:

p·V = n·RT

Gas law parameters

The parameters studied in the different gas laws are:

  • Pressure: is the amount of force applied to a surface. The SI unit of pressure is the pascal (Pa) but for the mathematical analysis of gas laws the unit of atmosphere (atm) is used; 1 atm is equal to 101325 Pa.
  • Volume: is the space occupied by a certain amount of mass and is expressed in liters (L).
  • Temperature: is the measure of the internal agitation of gas particles and is expressed in Kelvin (K) units. To convert centigrade to Kelvin, we only have to add 273.
  • Moles: is the amount of mass of the gas. It is represented by the letter n and its units are moles.


General gas law

In this fundamental law are united the laws of Charles, Gay-Lussac and Boyle-Mariotte. These laws deal with the variables V (volume), T (temperature) and P (pressure).

In all these laws there must be some interdependence which is where the combined gas law begins to exist.

The relationship between the pressure-volume product and the temperature of a system remains constant.

To represent it mathematically we can use the following formula:

p·V/T = cte

Here we see that P is the pressure, V is equal to the volume, T represents the absolute temperature measured in Kelvin and finally cte is the constant. The latter is represented by the units of energy divided by the temperature. This constant will then depend on the amount of gas used at the time.

Ideal gas law

The above four laws can be combined to form the ideal gas law, a single generalization of the behavior of gases known as the equation of state. This ideal gas law can be represented mathematically using the following equation:

p·V = n·R·T

where n is the number of mole-grams of a gas and R is called the universal gas constant.

Therefore, if we compress a gas, keeping its temperature constant, we will see that the pressure increases as the volume decreases. Similarly, if we make a gas expand at constant temperature, its pressure decreases as the volume increases.

This law belongs to what is known as the ideal gas equation of state.

An ideal gas is defined as one that is hypothetically formed by point particles which neither attract nor repel each other. Furthermore, for simplicity, when they collide, only elastic motions are considered where kinetic energy is conserved.