<P> A reaction's kinetics can also be studied with a pressure jump approach . This involves making fast changes in pressure and observing the relaxation time of the return to equilibrium . </P> <P> Light provides necessary activation energy to the starting materials, therefore, most of the reactions becomes faster in the presence of light </P> <P> The experimental determination of reaction rates involves measuring how the concentrations of reactants or products change over time . For example, the concentration of a reactant can be measured by spectrophotometry at a wavelength where no other reactant or product in the system absorbs light . </P> <P> For reactions which take at least several minutes, it is possible to start the observations after the reactants have been mixed at the temperature of interest . </P>

The study of how quickly chemical reactions occur and the factors that affect this speed