For some reactions including nuclear reactions, the rate of the slowest step depends on just one molecule reacting in the slow step. In this case the rate of reaction is directly proportional to the concentration of the reacting substance.

In many other reactions, two molecules of reactants must collide before reaction can take place. Not all collisions result in reactions, but collision is required for all reactions. In either gaseous or liquid solutions, the probability of molecules colliding is proportional to the concentration of the molecules. In the slowest step of a reaction in solution, the rate of that step is proportional to the concentrations of the molecules colliding.