What a catalyst is
So far we have altered the reactants themselves — their shape, their temperature and their concentration. The fourth factor is different: here we add something new to the mixture.
A catalyst
A substance that increases the rate of a chemical reaction but is not used up in it is called a catalyst.
The definition has two halves, and both of them are essential.
1. It increases the rate
The reaction would happen without the catalyst too — but more slowly.
2. It is not used up
At the end of the reaction it is still there, unchanged. It is not a reactant, so it is not written on either side of the equation.
Both of these can be shown by experiment. That is what the next two pages do.