Diffusion In Gases And Liquids

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 laboratory etc.

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 motion.

 

 
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