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Diffusion in Gases

We have discussed that molecules in matter are always moving. The movement is very small in solids, relatively large in liquids and very large in gases. This movement of molecules gives rise to a phenomenon known as diffusion.   


Suppose two gases A and B are kept at the same pressure in two parts of a container separated by a partition. Molecules of gas A are in continuous random motion in its own compartment. Similarly, molecules of gas B are in continuous random motion in its own compartment. If we remove the partition, the molecules of gas A at the interface, due to their random motion, will begin to stray into gas B. Similarly, the molecules of gas B, due to their random motion, will begin to stray into gas A. As time passes, more and more molecules of gas A will continue to stray into gas B and vice versa. After some time, a stage will come when the whole mass of gas in the container will be a homogeneous mixture of molecules of gas A and gas B. This has happened because of diffusion. 
 



 

Diffusion

Diffusion may be defined as the process of intermixing of two or more materials placed in contact, due to the random movement of their respective molecules.
When we light an agarbathi in one corner of a room, its fragrance spreads to all parts of the room. If it spreads with the help of a moving fan in the room, then it is not diffusion. But if there is no circulation of artificial air in the room, then it is due to diffusion. The molecules of the agarbathi fumes will mix with the air present in the room due to their own random movement. Thus the fragrance spreads throughout the room by diffusion.

 


Activity:6
To show the diffusion between ammonia and hydrogen chloride gas

 

 

Diffusion of gases

Take a long glass tube of about 50 cm in length, stoppered at both ends. Make sure that the tube is perfectly dry.

Take out the stopper from one end and place a plug of cotton wool soaked in concentrated ammonium hydroxide, in the tube. Replace the stopper. Take out the stopper from the other side and place a plug of cotton wool soaked in concentrated hydrochloric acid in the tube. Replace the stopper.

 Leave the tube as it is for some time. Record your observations.
You will observe that a white deposit forms near the centre of the tube. 
The plugs of cotton wool soaked in ammonium hydroxide and hydrochloric acid liberate molecules of ammonia and hydrogen chloride gas, which diffuse and try to move towards the other end of the tube. They meet near the centre of the tube and form ammonium chloride which makes the white deposit on the tube.

Since the tube is stoppered at both ends, there is no circulation of air through the tube. Hence, the movement of ammonia and hydrogen chloride gas is only due to the random motion of their molecules. This makes it a case of pure diffusion.


 




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