6.2.1 Diffusion of Gases
It is observed that when a sample of a gas is set free
in one part of the closed container, then its molecules
very quickly spread throughout the container. In some
cases we can smell a gas as it diffuses throughout the
closed room. For example molecule of perfume spread
throughout room or smell of H2S gas spreads in
The spreading of a substance through medium like air or
liquid is called diffusion. The rate of diffusion
of a substance depends upon its molar mass or density.
A Scottish chemist, Thomas Graham in 1846 studied the
rate of diffusion of different gases and formulated
Graham's law. This law states that, the rate of
diffusion of a gas is inversely proportional to the
square root of its molar mass or density.
In other words, lighter gases can diffuse faster than
heavier gases. For example hydrogen gas diffuses four
times faster than oxygen gas at similar conditions.
6.2.2 Diffusion of Liquids
Liquid is intermediate between gaseous and solid states.
Like gases liquid molecules are able to move and thus
flow and diffuse. The rate of movement of liquid
molecules is smaller than gases; hence they diffuse
slower than gases. For example add two or three drops of
blue ink in 200mls of water in a beaker. It is seen that
blue colour of ink spreads
slowly in water and the
whole water becomes bluish after some time. It means
that diffusion also occurs in liquids but the rate of
diffusion in liquids is slower than gases.
6.3 BROWNIAN MOVEMENT
This property first of all observed by British Botanist,
Robert Brown in 1827, during the movement of pollen
grains in water by microscope.
If a drop of liquid with particles suspended in it, is
observed under a microscope, it is seen that the
particles are not at rest but they are moving in all
directions in zigzag motion. This motion is called as
Brownian movement after the name, who observed it. "A
continuous, rapid, zigzag motion of suspended particles
through the medium is called Brownian motion."
Example: Mix some powdered sulphur in water and stir it,
after stiring filter the suspended sulphur some of the
sulphur particles are very small and they can pass
through the pores of filter paper into filterate. Now
put a drop of this filterate on a slide and examine it
under high powered microscope. It is observed that
sulphur particles perform rapid random zigzag motion
through the medium and this motion is called Brownian