Still the same stuff?

About this video

Drop an ice cube into juice and it melts. Freeze the water again and you get your ice cube back. Crack an egg into a hot pan and the clear, runny part turns white and firm, and nothing will ever turn it raw again. Both changed. What is the difference?

When ice melts it turns from a solid into a liquid, but it is still water. Tearing paper, crushing a biscuit or dissolving sugar are the same kind of change: the stuff changes its shape, its size or its state, but it stays the same stuff. That is a physical change, and many physical changes are easy to undo.

The egg is different. Heat has turned the egg white into a different substance, with a different colour, feel and taste. That is a chemical change: something new is made. Toasting bread, baking a cake, a nail going rusty and a match burning are all chemical changes, and most are very hard or impossible to undo. Torn paper is still paper; burnt paper has become black ash and smoke, which are new substances.

"Can I undo it?" is a useful clue, but only a clue. A broken glass is a physical change, because it is still glass, even though you can't easily mend it. The real test is one question: is it still the same stuff, or has something new been made?

Transcript

1. 0:00Melted ice freezes back into ice. A fried egg never turns raw again. Both changed. What is the difference?
2. 0:09Tear paper, and it is still paper. Burn it, and you get black ash and smoke. Something new.
3. 0:19Melting, tearing and dissolving keep the same stuff. Toast, rust and cake are new stuff.
4. 0:31A broken glass is hard to undo, but it is still glass. Undoing is only a clue.
5. 0:39Ask one question. Is it still the same stuff?