Diffusion, Effusion and Graham’s Law


According to Kinetic Molecular Theory, gaseous particles remain in a constant state of motion, moving at random speeds and in many different directions. Because of their kinetic energy at a temperature above absolute zero, all particles undergo diffusion.

Diffusion refers to the process of particles moving from an area of high concentration to an area of low concentration. The rate of this motion is a function of temperature, the viscosity of the medium, and the size (mass) of the particles. Diffusion leads to the gradual blending of materials, and ultimately, it forms a homogeneous mixture.

The dispersing of the scent of a rose or a fragrance is because of diffusion. When two gases diffuse into each other, they want to make their partial pressures exact everywhere. Suppose NO2, a brown colored gas, and O2, a colorless gas, are separated from each other by a partition. When the partition is eliminated, both diffuse into each other due to collisions and random motion.

A phase reaches when both gases create a uniform mixture and partial pressures of both are consistent throughout the mix.



The effusion of a gas is its motion through a very small opening into a region of low pressure. This spreading of particles is not due to collisions, but due to their tendency to escape one by one. In fact, the particles of a gas are regularly hitting the walls of the vessel. When a molecule approaches simply in front of the opening it enters the other portion of the vessel. This type of escape of particles is called an effusion.

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Graham’s Law of Diffusion

Thomas Graham (1805 -1869), an English scientist, discovered that the rate of diffusion or effusion of a gas is inversely proportional to the square root of its density at constant temperature and pressure.


The constant k is the same for all gases when they are all studied at the same temperature and pressure. Let us have two gases 1 and 2, having rates of diffusion as r1 and r2 and densities as d1 and d2 respectively.

According to Graham’s law


Divide the two equations and rearrange



Given that the density of a provided gas is directly proportional to its molecular mass. Graham’s law of diffusion can likewise be written as follows


Where M1 and M2 are the molar masses of gases.

Demonstration of Graham’s Law

This law can likewise be extremely quickly verified in the laboratory by observing and noting the rates of diffusion of 2 gases in a glass tube when they are enabled to move from opposite ends. 2 cotton plugs soaked in HCl and NH3solutions are introduced in the open ends of 100 cm long tube simultaneously. HCl molecules travel a distance of 40.5 cm while NH3 particles cover 59.5 cm in the very same duration.

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They produce dense white fumes of ammonium chloride at the point of junction. So,


1.46= 1.46

For this reason, the law is verified.