Sorting them by speed
Take the nine changes from the last page and sort them into two groups, the ones that happen quickly and the ones that happen slowly, and a clear pattern appears. That is what assignment 17.1 asks for.
| Reactions that happen quickly | Reactions that happen slowly |
|---|---|
| Petrol vapour catching fire | Fruit ripening |
| A piece of zinc reacting with a dilute acid | Food being digested |
| A firecracker going off | Iron rusting |
| Sodium metal reacting with water | Yoghurt being made from milk |
| Firewood burning | — |
The sorting is a comparison rather than a strict measurement. Firewood burns more slowly than a firecracker goes off, yet set against iron rusting it is extremely fast.
What the table says
Different chemical reactions go at different speeds. That speed is a measurable property of the reaction, not an accident.