How You Know Something Reacted
- Chemistry
- Ages 8–11 · Grades 3–5
- Hands-on experiment
What you’ll find out
- A chemical reaction makes a new substance.
- Bubbles, colour change, heat and a new solid are the usual signs.
- Melting and dissolving change a substance without making a new one.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Four things to watch for
How can you tell that a reaction has happened? You cannot see the atoms rearranging, so you have to watch for clues instead, and there are four good ones. Bubbles, which mean a gas is being made. A colour change that does not go back on its own. Heat, so that the container feels warmer or colder than before. And a new solid appearing in a liquid that was clear. Any one of these is worth a closer look, and two or three together mean something new has almost certainly been made.
Changed is not the same as reacted
Be careful, though: not every change is a reaction. Ice melting into water looks like a dramatic change, but it is still water the whole time, and freezing it brings the ice straight back. Sugar dissolving looks even more dramatic — the sugar seems to vanish — yet drying off the water returns it exactly. Nothing new was made in either case. A reaction is different because it does not simply run backwards. Burn a sheet of paper and you cannot unburn it; the ash will never become paper again. That is the real test.
Curriculum links: NGSS 5-PS1-4