New stuff from old

About this video

Leave an iron nail, or a bicycle chain, out in the rain for a winter and it grows a flaky, orange-brown crust. The crust is not dirt and not paint. It is rust, and it wasn't there before. New stuff has been made.

Chemists call this a chemical reaction: substances meet and turn into different substances. The substances you start with are the reactants, and the new ones you end up with are the products. For rust, the reactants are the iron, a gas in the air called oxygen, and water, and the product is rust. We write it with an arrow, read as "turn into": iron + oxygen + water → rust. Rust is nothing like iron. Iron is hard, grey and strong; rust is orange-brown, crumbly and weak. Rusting is slow, but other reactions take less than a second.

You can't watch new stuff being made, but you can often see the signs: bubbles of a gas that wasn't there before, like baking soda fizzing in vinegar; a new colour, like a cut apple turning brown; heat or light given off, like a candle flame; a new smell, like milk gone sour; or a solid appearing in a clear liquid.

A sign is a clue, not a proof. Water boiling in a pan makes bubbles, but it is still water, and a light bulb gives off light without making anything new. The real test is whether new stuff has been made.

Transcript

1. 0:00An iron nail left out in the rain grows an orange crust. Not dirt, not paint. Rust, which wasn't there before.
2. 0:09Iron, oxygen from the air and water are the reactants. They turn into a new product, rust. Rust is nothing like iron.
3. 0:20Look for the signs. Bubbles, a new colour, heat or light, a new smell, or a solid appearing.
4. 0:32But boiling water bubbles, and it is still water. A sign is a clue. The test is new stuff.
5. 0:41In a reaction, old stuff turns into new stuff.