Nothing In Between

Take a bottle of water and put it in the freezer. In the morning you have ice — hard, holding its own shape, sitting there like a stone. Now take that ice and boil it. It turns to steam and disappears into the kitchen.

Hard rock, running liquid, invisible cloud. Three completely different things.

Except that nothing was added and nothing was taken away. Not one drop. Whatever came out of the tap is still there in the ice, and still there in the steam. Weigh it if you like — a sealed bottle weighs exactly the same frozen as it does liquid.

So if nothing was added and nothing was removed, what changed?

The answer is the whole of chemistry's first idea, and it is this: everything is made of unimaginably tiny particles, and the three states are just three different ways for the same particles to be arranged. Ice, water and steam are the same particles standing in a line, milling about, and running loose.

This is the first lesson of chemistry, and it asks a different question from all the physics you have just done. Not how do things move — but what are things made of. By the end you'll be able to:

• describe a solid, a liquid and a gas by their particles, not just by how they look
• explain why a solid keeps its shape, a liquid finds a level, and a gas fills the room
• say what is in the space between the particles — and it is not what most people guess
• explain why a gas can be squashed and a liquid cannot