A basic tenet of collision theory is that the reactants must collide in order to react and that the rate of reaction is related to the number of collisions per unit time. Any factor that increases the number of collisions, like more reactant particles in the reaction volume (i.e., a concentration increase), will increase the rate.

The effect of concentration is demonstrated in the figure above. In the top case, two molecules each of A and B give rise to four possible collisions. When another A molecule is added, the number of collisions increases to 6 (i.e., 2 x 3 not 2 + 3). Finally, another B increases the number to 9 (3 x 3).