The Colour That Tastes For You

You own an excellent acid detector. It is in your mouth, it works in under a second, and it has never once given you a wrong answer.

Bite a lemon: sour. That is an acid, and your tongue knew before you did.

Now taste a pinch of baking soda: bitter, and if you rub a little between your fingers it feels oddly soapy. That is a base, and again your tongue got there instantly.

So the problem is solved. Except that it is not, and here is why.

The lemon in your hand contains citric acid. The bottle in a school laboratory marked sulfuric acid is also an acid — the same family, the same sourness in principle. Taste that one and you will not be reporting your findings, because it would burn through the inside of your mouth before you could spit.

The detector works perfectly and you must never use it. That is a genuinely awkward position to be in, and chemistry's answer to it is one of the neatest tricks in the whole subject:

Build a substance that does the tasting for you, and reports back in colour.

That is what an indicator is. A strip of litmus paper dipped into a liquid does exactly what your tongue would have done, and it is perfectly happy to do it inside concentrated acid. You get the answer; the paper takes the risk.

And once you can safely tell an acid from a base, one more thing becomes interesting: what happens when you mix them. The answer is that they cancel — and it is why your grandmother's indigestion remedy works.

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

• recognise an acid and a base by their properties, without ever tasting anything
• use litmus and a natural indicator, and say which colour means what
• explain why neutral is a third answer and not just another word for water
• explain neutralisation — acid + base → salt + water — and give three places it is used