Diffusion

Diffusion is the random movement of particles from a region of higher concentration to a region of lower concentration. It is commonly seen in gases, because of the fast, erratic and random movement of gas particles (which you already know from Particle Theory).

A really simple demonstration is if you take two gas jars full of two different gases. and sit one on top of each other, but have a glass plate separating the two jars. One jar is filled with orange coloured bromine gas and the other is just filled with air. See diagram below...

Bromine air particles are considerably heavier than those gas particles found in air and that gas jar is placed on the bottom on purpose. Some people would guess that the heavier bromine particles would just stay at the bottom because, well, they are heavier!

But as time goes on, the orange colour starts moving upwards. What you cannot really see is that the 'colourless' colour also starts moving downwards!

This can be explained using diffusion... originally the concentration of bromine is very high in the bottom jar and zero in the top jar... so over time the bromine particles DIFFUSE from the higher concentration to the lower concentration.

AT THE SAME TIME, the air particles diffuse from the top jar (high concentration of air) to the bottom jar (low concentration of air).

Eventually, there is an even mix of air and bromine particles and the whole container looks slightly orange.

Another brilliant way this can be demonstrated is using a 'diffusion tube'.

In one end of the tube is inserted some cotton wool which is dipped in concentrated hydrochloric acid. This cotton wool released HCl gas down the tube.

At the other end of the tube some cotton wool is inserted which has been dipped in concentrated ammonia. This cotton wool releases NH₃ (ammonia) gas down the tube.

IT IS REALLY IMPORTANT THAT THE TWO ARE INSERTED AT THE SAME TIME TO MAKE THIS FAIR AND SEE SOME INTERESTING OBSERVATIONS....

The two gases 'diffuse' down the air tube (moving from a region of higher to lower concentration) and eventually will meet each other. Now this does not sound interesting... however when NH₃ gas meets HCl gas, a chemical reaction takes place which produces a solid! This forms as a white 'smoke ring'.

You would probably expect the ring to form in the middle of the tube, because you'd think the two gases diffuse down at the same speed and meet in the middle. But this experiment shows something very cool.... the speed at which a gas diffuses is related to the mass of its particles!

Because the HCl particles have a high mass than the NH₃ particles, they diffuse slower down the tube. So the point at which they meet each other is closer to the HCl cotton wool side than the NH₃ cotton wool side! This means the 'smoke ring' of ammonium chloride doesn't form in the middle... and it proves that the speed of diffusion is based on the mass of the particles moving.