Kinetic Theory Of Gases: Assumptions

Gases are made up of molecules. What are molecules? Molecules are the smallest unit which behaves the same as the sample, i.e. they have the same chemical properties as of the sample. The kinetic theory of gases has developed a model which explains the behavior of molecules, which should further explain the behavior of an ideal gas. Do you know what an ideal gas is?
We have learned that the pressure (P), volume (V), and temperature (T) of gases at low temperature follow the equation:

\(PV\) = \(nRT\)


n = number of moles in the gas
R = gas constant having value \(8.314 JK^{-1}mol^{-1}\)

Now, any gas which follows this equation is called ideal gas. But there are certain assumptions that we consider for describing ideal gas behavior.

kinetic Theory

Assumptions of Kinetic Theory of Gases

  • All gases are made up of molecules which are constantly and persistently moving in random directions.
  • The separation between the molecules is much greater than the size of molecules.
  • When a gas sample is kept in a container, the molecules of the sample do not exert any force on the walls of the container during the collision.
  • The time interval of collision between two molecules, and between a molecule and the wall is considered to be very small.
  • All the collisions between molecules and even between molecules and wall are considered to be elastic.
  • All the molecules in a certain gas sample obey Newton’s laws of motion.
  • If a gas sample is left for a sufficient time, it eventually comes to a steady state. The density of molecules and the distribution of molecules are independent of position, distance and time.

There is a lot more to be learned about kinetic theory of gases. Explore more on physics formulas and physics calculators at BYJU’S

Practise This Question

Imagine a gravity free laboratory, in outer space, where you are asked to study the motions of a large number of marbles kept in a transparent cubical jar, as you give the jar quick, small jerks in random directions. You will observe, that after giving a sufficient number of jerks, the marbles start moving rather chaotically, i.e., each marble bounces off other marbles every now and then, and takes a random trajectory between two points due to these collisions. You seem to have given some kinetic energy to each marble, when the jar was being shaken. What happens when you release the jar suddenly, leaving it suspended in space? Assume all collisions between the marbles to be perfectly elastic.