The Wrong Question

Take a sheet of paper and tear it into forty pieces. Take another and set fire to it.

Both are ruined. Neither can be put back. If somebody asked you which one was a chemical change, you might reasonably say both — after all, the usual test you have been taught is "can you undo it?", and you cannot undo either.

But only one of them is chemical, and the torn one is not.

Look at what you are holding. After tearing, you have forty pieces of paper. Smaller, yes; messier, certainly; but every single piece is still paper, with all of paper's properties. After burning, you have ash, smoke and gas — and ash is not paper. It is grey, crumbly, will not write, will not burn again. Something genuinely different exists now that did not exist before.

That is the whole distinction, and it explains why "can you undo it?" is the wrong question. It gets the right answer often enough to feel reliable and then fails you exactly when a question is testing whether you understand.

The right question is: is there a new substance now?

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

• tell a physical change from a chemical one using the test that actually works
• list the signs that a chemical change has happened, and say why one sign alone proves nothing
• explain why breaking a glass is physical and rusting is chemical
• say where the paper went when it burned