Â· A gas is composed of many very small particles known as atoms or molecules.

Â· All the molecules present in a gas have the same mass.

Â· These small atoms or molecules move continuously and the distance between them is great.

Â· All the atoms continuously collide with each other and with walls of the object. These collisions are considered as perfectly elastic in behavior.

Â· The theory of perfectly elastic collisions states that the particles are completely spherical in shape and elastic in behavior.

Â· Due to many molecules, the statistical treatment can be applied to them.

Â· Except during the collisions, the molecules do not exert any force on each other. It means that the interaction between them is negligible. Because of this, the quantum mechanical effects and the specific relativity of the molecules are zero.

Â· The successive time of collisions between a molecule and the wall of the object is so large that the elapsed collisions of the molecules are negligible.

Â· The average kinetic energy of gas (except the ideal gas) molecule depends on the thermodynamic (absolute) temperature of the gas. Because of this assumption, the average kinetic energy of gas can be directly calculated if the temperature of the gas is already given. It does not matter in the calculation that which gas is considered.

Â· If the macroscopic properties (volume, pressure, and temperature) of gases are given, then we can accurately calculate the microscopic properties (momentum, speed, and position) of the gas.

Â· On the other side, if the microscopic properties (momentum, speed, and position) of gases are given, then we can accurately calculate the macroscopic properties (volume, pressure, and temperature) of the gas.

Postulates of the kinetic theory of gases:

The number (amount) of particles in a gas is proportional to the pressure intensity.

According to the kinetic theory of gases, the molecules tend to collide frequently with the wall of the object they are kept in, and the collisions of the particles create pressure.

If a gas has a large number of particles, then more particles will collide with the wall of the object. Therefore, if the number of particles of the gas increases, then the intensity of pressure will also increase.

Figure: Movement of the molecules

1. Avogadroâ€™s Law: At constant pressure, the amount of gas is proportional to its volume.

If the number of particles is large, then the number of collisions and intensity of the pressure will also increase.

Therefore, if the pressure of the gas remains constant, then the amount of gas is proportional to its volume.

2. Boyleâ€™s Law: At constant temperature, the volume of the gas is inversely proportional to its pressure.

At constant temperature, if the volume is reduced, then the number of particles per unit area will increase and the kinetic energy of the particles will remain the same. The increase in the number of particles per unit area will increase the frequency of collisions (pressure of the gas).

Therefore, decreasing the volume of the container (gas) will increase the pressure.

3. Amontonâ€™s Law: At constant volume, the pressure of the gas is directly proportional to its temperature.

If the temperature of the gas increases, then its kinetic energy will also increase. The gas particles will move fast, and the number of collisions will increase, which will increase the pressure of the gas. Therefore, increasing the temperature of the gas will increase its pressure.