The Thing Water Will Not Dissolve

Here is a question with an annoying answer. Where would you go to find pure water?

Not a river — that has silt and salts and whatever the town upstream put in it. Not a well — that is water which has spent years soaking through rock, dissolving it. Not the sea, obviously.

Rain, then. Rain has just been distilled by the sun: evaporated off the ocean, condensed in a cloud, and dropped. That is exactly the process you learnt in lesson 20, done at the scale of a planet.

And rain is still not pure. On the way down, each drop dissolves carbon dioxide out of the air, which is why rain is very slightly acidic even where there is no pollution at all. It picks up dust. By the time it hits your roof it is already a solution.

Even the bottle of distilled water in a laboratory starts going impure the moment somebody opens the lid, because it immediately begins dissolving the air above it — and, slowly, the glass around it.

There is no pure water anywhere on Earth outside a sealed container. And the reason is not that the world is dirty. The reason is that water is extraordinarily good at dissolving things, which is the single most useful thing about it.

That one fact runs this whole lesson. It explains why water is in every living cell, why you cannot get it pure, why the water cycle is a distillery that still fails to produce pure water — and, upside down, it explains the one everyday thing water refuses to dissolve, which is why you can see oil floating on top of your curry.

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

• explain why water is such a good solvent and why that makes pure water impossible to find
• describe the water cycle, and say which two steps of it you already know by another name
• tell pure water from safe water, and say why boiling gives you one and not the other
• use density to say whether something floats or sinks, and explain why that only works for things that do not dissolve