The power to which the concentration of a component in a chemical reaction is raised. It provides an indication of the mechanism of the reaction and is determined experimentally. Most reactions are usually first or second order. In the reaction $A+B\to products$ the rate may be related to the concentration of component A by $-r_A=k{\left[A\right]}^X$. If X = 1 then the reaction is first order with respect to A and k is the velocity or rate constant. If X = 2 then the reaction is second order with respect to A. If the rate is related to the concentration of component B, then the order of reaction with respect to B is Y. The (p. 266) overall rate equation is $-r_A=k{\left[A\right]}^X{\left[B\right]}^Y$ and the overall order of the reaction is X+Y. The order of reaction can be fractional. If the rate of a reaction is independent of the concentration of a particular reactant, then the reaction is zero-order with respect to that reactant. The order of reaction cannot be deduced from a balanced chemical reaction equation.