Clear Is Not Pure

Put two glasses of water on the table. Into one, stir three spoons of salt until every grain has vanished.

Now look at them. Really look. Same clear liquid, same colourless liquid, same everything. There is no line, no cloud, no swirl. Whichever way you tilt them, they are twins.

But one of those glasses holds one substance and the other holds two.

Someone will say: taste them. Fair enough — for salt, that works. But it only works because you already knew salt was a possibility, and it only works for things with a taste, and a scientist cannot go around putting unknown liquids in their mouth. We need a test that works when you know nothing, and looking is not it.

Here is what makes this worth a whole lesson. "Clear" feels like it means "pure", and it does not. Clear tells you one real thing — that nothing is floating in there — and it tells you nothing whatsoever about whether something is dissolved in there. Those are two different questions, and the eye can only answer the first.

By the end of this lesson you'll be able to:

• say what makes a substance pure, and why pure things have sharp, exact properties
• name the parts of a solution — the solvent and the solute — and tell a solution from a suspension
• explain what "clear" really rules out, and what it cannot rule out
• describe the one test that separates pure water from salt water