Diffusion

A glass of hot water. And a bag of tea. Look what happens now. The warm water extracts flavours and other molecules from the tea leaves, and they float out into the water, making it darker in colour. If we wait long enough, the entire glass is filled with equally strong tea.

But why does the tea spread out so evenly in the water, without even stirring? Well, the water sort of stirs itself, all the time. This, is diffusion. Diffusion works because the water molecules move around continuously, in random directions. And when water molecules bump into the dissolved tea, the tea particles are pushed around.

A bit at a time, particle by particle, the water stirs the tea. Diffusion causes the particles to spread out, from areas where there are many of them in the same place -- higher concentration -- to areas where there are fewer -- lower concentration. The energy that the water uses to push - and hence stir - with, is heat. Since heat is just tiny movements - on a particle level - in random directions, It doesn't even have to be particularly hot for diffusion to occur. Wait until the tea has cooled down, and pour some more water in... ...

carefully, so you don't stir it, and look! It takes a moment, but diffusion spreads the tea here too! Diffusion doesn't just happen in liquids. It works in gases too. Say you're in a lift with your little brother.

And happen to... ... well, fart. It doesn't take long before the molecules in the air, moving around in all directions, have knocked into the fart molecules, enough times for them to travel all the way to your little brother's nose. Diffusion is useful for a lot, not just stirring tea or keeping a lift to yourself. In nature, all living things use diffusion every day.

It's with the help of diffusion that oxygen gets from your lungs into your blood. And the nutrients from the food you eat are transferred from your intestines into the blood with the help of diffusion. The engine driving diffusion, is tiny but rapid movement at the molecular level. Exactly the same tiny movement that we experience every day as... heat.

Diffusion is fast in gases, where molecules can move freely and quickly. Diffusion is slower in liquids, since the molecules there move slower. In solid materials the molecules are fixed, and vibrate only in place, so there diffusion won't happen at all. Diffusion. That's particles... ...

that spread - from areas with higher concentration to areas with lower concentration - by bumping into each other.