What are materials made of?
            2. When materials mix
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Photo of perfume testing
Picture 2.1 Take the top off some perfume and you'll soon start to smell it - thanks to diffusion.
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Diffusion
We can mix materials together in a number of ways: solutions, alloys and things called colloids. In this chapter, we'll start by seeing how some fluids mix themselves together by diffusion. We'll then go on to look at these different types of mixtures:
What is diffusion?
If you open a bottle of perfume at arm's length you can soon smell it. Perfume particles mingle with air particles and move through the air until they reach your nose. We call this diffusion.
Animation of diffusion
Picture 2.2 How diffusion works. The blue particles move and mix with the red particles. Note: the real particles are not coloured.

Photo sequence of bromine diffusion
Picture 2.3 Bromine diffusing into air. After:
15 mins, 30 mins & 45 mins.
Explaining diffusion
Diffusion is when two fluids mix together without any help from us. We can explain this using the idea that particles move.

Instead of the particles moving around and mingling with particles of the same material, they move around and mingle with particles of the other material as well.

If you pour some water into a glass followed by some cola drink, the colour of the cola spreads through the whole liquid. The cola particles, which cause the colour, mingle with the water particles until they are all mixed together.

If the particles themselves weren't moving, then the two liquids wouldn't be able to mix on their own. We would have to stir them.

An example of diffusion
We can see a gas diffusing using bromine. This is a brown liquid that vaporises easily. You can see some of the vapour in picture 2.3. It is still at the bottom of the tube.

However, as time passes, the vapour will diffuse up the tube. The bromine particles are moving around randomly. Gradually, although they bump into the air particles, they will make their way to the top. It takes about 45 minutes for the tube to be filled with brown bromine vapour.

If there were no air in tube, then the bromine particles would not be obstructed. You can watch a QuickTime movie of this. Click to choose the file size:

Question 8
Look at the statements below. Some of them are true and some are false. Decide which ones are true and which are false. Then choose the answer from the drop down list.
All gases are fluids

All fluids are gases

Diffusion relies on the fact that particles are moving

Diffusion only happens in gases
We wouldn't be able to smell without diffusion
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