The four factors
There are four main factors that affect the rate of a reaction. The next four sections examine each of them in turn.
1. Surface area of the reactants
How finely a solid reactant has been divided up.
2. The temperature the reaction runs at
How hot the reacting mixture is.
3. Concentration of the reactants
How concentrated a solution is. For gaseous reactants, the pressure.
4. The presence of a catalyst
Whether a substance is present that speeds the reaction up without being used up.
The order of the four is not arbitrary — the first three are about how the reactants themselves are prepared, the fourth about something added from outside. But the explanation of all four is the same one: collisions.