Gases are one of the three forms of matter. The other two are solid and liquid. All three states of matter find their importance in our daily life. They have been studied extensively to study the phenomenon of expansion and compression under various conditions.

To study the behaviour of gases, it is important to understand the gases and their properties.

The basic three properties observed in gas molecules are:

1. Compressibility: Gas molecules show the property of being highly compressible. This is possible because gas molecules are very loosely packed. There is very little attraction between the gas molecules. The intermolecular force of attraction is very less. As the gas molecules are always in a state of random motion, there is a lot of intermolecular space present between each molecule. When a force is applied over an area, the compression happens easily because very few molecules are confined in a particular area.

1. Expandability: Gas Molecules are highly expandible because the gas molecules are in a state of random motion always. So, when gas molecules are heated, the molecules start moving in a random direction, and as a result, the gas expands.

1. Gas molecules occupy a larger space of the vessel. Gas molecules are very loosely packed. Unlike liquids and solids that have a definite shape and volume, gas molecules are scattered in a vessel. They take up the entire space of the vessel in which they are kept because they keep moving in a random motion.

Thermodynamic Behaviour of Gases

The behaviour of the gases has been studied with the help of three laws, namely Boyle’s Law, Charle’s law and Gay Lussac’s Law. As gases have unique behaviour than solids and liquids, they are studied under a different set of laws. They are

Boyle’s Law: 

Scientist Robert Boyle in 1662, studied the gases and the relation of their volume and pressure by keeping the temperature of the gas constant. It was found by him that by keeping the temperature of the gas constant, the product of the volume and the pressure came constantly.

Volume and Pressure share an inverse relationship when the temperature of the gas is kept constant.

Charle’s Law: 

Jacques Charles, in 1787 found that when an ideal gas’s pressure is kept constant, the volume and the temperature are found to be directly proportional to each other.

This assumption was made when the system was kept closed.

Gay Lussac’s Law: 

Joseph Louis Gay and Lussac in 1808 proposed the gay Lussac Law.

They said for a constant mass of volume at the constant volume, the pressure exerted on the sides of the container of the gas was found to be directly proportional to the absolute temperature of the gas. 

Avogadro’s Law: 

Avogadro’s law states that when the pressure and the temperature of an ideal gas were kept constant, the volume was observed to be directly proportional to the number of molecules that were present in the vessel.

The ideal gas equation for the ideal gas is given as 

PV=nRT where P stands for pressure, V for volume, n for no of moles of gas, R for gas constant and T for temperature.

The other laws for gases are: 

Henry’s law: Henry’s law talks about the partial pressure of gas molecules. It says that the amount of the gas that is dissolved in a liquid and the volume of that liquid was found directly proportional to the partial pressure of the gas when it was in equilibrium with the liquid.

Graham’s Law: Graham’s law states that the gas molecules diffuse inverse relation to the square root of the gas density when the temperature is kept constant.

Kinetic gas theory

The Kinetic theory of gases was put forward to study the behaviour of gases.

The following postulates were studied:

1. The gas molecules are always found to be moving in a random motion. This means that they can move in any direction of the container. They are found to be of the same size.
2. The size of the particles is assumed to be very small, and the distance between each molecule is found to be smaller.
3. As the molecules move in any direction, they are bound to collide with each other. The collision between the particles occurs, and they are found to be elastic.
4. The gas molecules occupy the larger space of the container as the particles are very loosely packed. 
5. The intermolecular force of attraction between the gas molecules was found to be negligible due to the large interatomic space between them. As a result, the gas molecules can move in any direction of their choice due to highly loose packing.

Conclusion:

Gas molecules are highly different from all the other types of particles. They are studied under different sections of laws. The behaviour of the gas molecules has been studied with the help of several gas laws.