Diffusion

All these effects are due to diffusion.

Diffusion is the movement of one type of gas molecules through another. Perfume through air, manure through air, food through air, copper sulphate through water and so on.

Lets think about the diffusion of perfume. The molecules of air in the room and the molecules of perfume in the spray are all moving around quickly and randomly (in all directions). The molecules of perfume collide with other molecules of their own type and also with molecules air. It is very unlikely for any one perfume molecule to be able to move across the room without hitting another molecule of one type or the other but they will eventually get there after many collisions. This slow erratic progress is called diffusion. It is rather like a drunken man staggering across a field through a crowds of excited soccer supporters. They will eventually make it to the other side but it will take a long time.

Diffusion can be demonstrated in the laboratory by two or threes simple experiments.

Take a bottle of perfume, remove the stopper and then hold it a few centimetres away from your friend's nose – after a second or two (but not instantly) they will be able to smell the perfume although their nose is not touching the bottle. The perfume molecules have diffused through the air to their nose! (Make sure that they always sniff gently. You never know how unpleasant the smell may be!)

The next two use two dangerous chemicals and so should only be performed by your teacher.

The first of these experiments is really simple. Take a bottle of concentrated hydrochloric acid and a bottle of concentrated ammonia. (Figure 2) Put them one the bench about 20 cm apart and carefully take the stoppers out of the bottles.

Watch what happens.

After a minute or two you should see a white cloud drifting off the top of the hydrochloric acid bottle. Both the ammonia molecules and the hydrochloric acid molecules have diffused through the air but the ammonia molecules are lighter and so they will diffuse faster. Where the two types of molecules meet they react giving the white cloud.

To show this in a more precise way the apparatus shown in figure 3 can be used.

Two plugs of cotton wool are put into the ends of a horizontal glass tube. One is soaked with concentrated hydrochloric acid and the other with concentrated ammonia.

After a few minutes a white ring will form in the tube where the molecules of the two liquids have met. It will be closer to the end of the tube where the hydrochloric acid plug is.

THIS SHOULD ONLY BE DONE BY TEACHERS

The diffusion of bromine through air is also but since bromine is a hazardous chemical it should only be performed by your teacher in a fume cupboard. A small capsule of bromine is broken in a large glass tube, the brown colour of the bromine gas slowly diffuses through the air. If the experiement is repeated in an evacuated tube the colour reaches the other end in a fraction of the tube a second because there are no air molecules in the way.

THIS EXPERIMENT SHOULD BE UNDERTAKEN WITH GREAT CARE AND INDEED MAY NO LONGER BE CONSIDERED SAFE.

Diffusion in liquids can be seen in the experiment using a crystal of copper sulphate or potassium permanganate.

A crystal is put at the bottom of a gas jar or beaker which is then carefully filled with water and left to stand for a few days. After that time some of the molecules from the crystal will have dissolved and diffused through the water, (Figure 5).

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If you break a bottle of perfume at one side of the room the smell will eventually be smelt on the other side
The colour of a crystal of a copper sulphate in water moves through the water as the days pass
The smell from manure being spread on a field can be detected many metres away
Your evening meal may smell really good from the next room
Your girl friend's perfume may smell nice although you are standing half a metre away from her
People often cry when peeling onions because of the smell of the onions makes their eyes water

Figure 1

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Figure 2

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white ring

hydrochloric acid

ammonia

Hydrochloric acid molecules diffusing

Ammonia molecules diffusing

Figure 3

Figure 5

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Figure 4

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