Three conditions speed a reaction up: warming it, crowding the molecules closer together, and breaking a solid into smaller pieces. Each one gives more collisions that work.
Heat
The molecules in a gas or a liquid move at a spread of speeds: a few slow ones, a lot of middling ones and a few very fast ones. When it warms up, they all speed up. They collide more often, and they hit harder, so many more collisions carry enough energy to climb the hill.
For many reactions, a rise of 10 °C roughly doubles the speed. The chemistry of food going off follows the same rule. Food spoils as bacteria and moulds grow on it, and as its own molecules react with oxygen. In a fridge at about 4 °C these reactions run several times slower than in a warm kitchen, so food keeps for days longer.
Crowding
Pack more molecules into the same space and they collide more often, so more collisions work every second. Chemists call how crowded a substance is its concentration. Air is about a fifth oxygen. In pure oxygen, a glowing wooden splint bursts back into flame, because oxygen molecules hit the hot wood about five times as often. Chemists use this as a quick test for oxygen.
Smaller pieces
A solid reacts only at its surface, where its particles meet the other reactant. Cut it into smaller pieces and more of it is surface. Thin twigs and wood shavings catch fire from a match, and a big log takes far longer to light.
| Each cube | Number of cubes | Total surface |
|---|---|---|
| 1 cm | 1 | 6 cm² |
| 1 mm | 1,000 | 60 cm² |
| 0.1 mm | 1 million | 600 cm² |
| 0.01 mm | 1 billion | 6,000 cm² |
Every time the pieces get ten times smaller, the surface grows ten times. Ground down to grains a hundredth of a millimetre across, the 1 cm cube has as much surface as the top of a small desk.