The overall order of reaction gives an indication of how changing the concentration of the reactants will change the speed of the reaction. For higher orders of reaction, changing the concentration of the reactants results in large changes in the rate of reaction. For lower orders of reaction, the rate of reaction is less sensitive to changes of the concentration.

The order of reaction is found experimentally by changing the concentration of reactants and observing the change in the rate of reaction. For example, if doubling the concentration of a reactant doubles the rate of reaction, the reaction is a first-order reaction for that reactant. If the rate increases by a factor of four, or the doubling of the concentration squared, the reaction is second order. For several reactants taking part in a reaction, the overall order of reaction is the sum of the orders of the individual orders of reaction.

The rate of a reaction is related to the concentration of a reactant by the rate constant, represented by the letter k. The rate constant changes when parameters such as the temperature change, but if only the concentration changes, the rate constant remains fixed. For a reaction at constant temperature and pressure, the rate equals the rate constant times the concentration of each of the reactants to the power of the order of each reactant.

The general formula is the following:

Rate of reaction = kA^xB^yC^z..., where A, B, C... are the concentrations of each reactant and x, y, z... are the orders of the individual reactions.

The overall order of reaction is x + y + z +.... For example, for three first-order reactions of three reactants, the overall order of reaction is three. For two second-order reactions of two reactants, the overall order of the reaction is four.

The iodine clock reaction rate is easy to measure because the solution in the reaction container turns blue when the reaction is complete. The time it takes to turn blue is proportional to the rate of the reaction. For example, if doubling the concentration of one of the reactants makes the solution turn blue in half the time, the rate of reaction has doubled.

In one variation of the iodine clock, the concentrations of the iodine, bromate and hydrogen reactants can be changed and the times for the solution to turn blue can be observed. When the concentrations of the iodine and bromate are doubled, the reaction time is reduced to half in each case. This shows that the rates of the reaction double and that these two reactants take part in first-order reactions. When the hydrogen concentration is doubled, the reaction time decreases by a factor of four, meaning the rate of reaction quadruples and the hydrogen reaction is second order. This version of the iodine clock therefore has an overall order of reaction of four.

Other orders of reaction include a zero-order reaction for which changing the concentration makes no difference. Decomposition reactions such as the decomposition of nitrous oxide are often zero-order reactions because the substance decomposes independently of its concentration.

Reactions with other overall orders of reaction include first-, second- and third-order reactions. In first-order reactions, a first-order reaction for one reactant takes place with one or more reactants that have zero-order reactions. During a second-order reaction, two reactants with first-order reactions take place, or a reactant with a second-order reaction combines with one of more zero-order reactants. Similarly a third-order reaction can have a combination of reactants whose orders add up to three. In each case, the order indicates how much the reaction will speed up or slow down when the concentrations of the reactants are changed.