Fast reactions are those reactions that occur instantaneously. A fast reaction depends upon the reaction rates. These reactions are so fast that they occur as soon as the reactants are bought together. Generally, these reactions involve ionic species and thus known as ionic reactions. These reactions take about 10-14 to 10-16 seconds for completion. It is very hard to determine the rates of these reactions. It is also defined as a reaction with a half-life of milliseconds or less; such reactions occur so rapidly that special experimental techniques are required to observe their rate. The rate of a reaction is defined in terms of the rates with which the products are formed and the reactants are consumed. For chemical systems it is usual to deal with the concentrations of substances, which is defined as the amount of substance per unit volume. The reaction rate can then be defined as the concentration of a substance that is consumed or produced in unit time. The half-life of a reactant is defined as the time that it takes for half of the initial amount to undergo reaction. The half-life is independent of the initial amount. The disadvantage of fast reactions is that the time that it takes to mix reactants or to change the temperature of the system may be significant in comparison with the half-life so that the initial time cannot be measured accurately and the other is that the time it takes to measure the amounts of substances may be comparable with the half-life of the reaction.