For a reaction to occur, the particles that are reacting must collide with each other. Only some of all the collisions that take place cause a chemical change to happen. These are called 'successful' collisions. The greater the number of 'successful' collisions, the faster the rate of a reaction. This is called the 'collision theory'.

There are four factors that affect the rate (speed) of a chemical reaction:

• temperature
• concentration
• particle size
• use of a catalyst