Faster Is Not More

Put four spoons of sugar in a glass of cold water and stir. It goes. Put in four more and stir. It goes. Keep going, spoon after spoon, and at some point something changes: you are stirring, and stirring, and the sugar at the bottom is just going round and round, refusing to disappear.

Almost everybody's first thought is stir harder. And almost everybody is wrong, because stirring is not what has run out.

The glass is full — not full of water, full of dissolved sugar. Water can only hold so much, and you have reached the limit. Stirring faster is like reading a full page faster: the speed changed, the page did not get bigger.

Now empty the glass, fill it with hot water, and try again. This time the same amount of sugar vanishes easily, and you can add far more before it starts piling up. Heat did what stirring could not.

That is the whole lesson, and it is one sentence: stirring changes how fast; heating changes how much. Both of them make dissolving quicker, so they feel like the same kind of help. Only one of them can move the limit.

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

• say what a saturated solution is, and why more stirring will not rescue it
• explain the difference between how fast something dissolves and how much can dissolve
• describe what heating does to the limit — and why salt is a famous exception
• read a solubility graph and say what happens when a hot saturated solution cools down