The half-life of a reactant is the time taken for the concentration of a reactant to reduce by half. If the order is zero with respect to a reactant, the concentration of the reactant does not affect the rate of reaction. A zero-order reaction has a decreasing half-life. If the order is one with respect to a reactant, the rate of reaction doubles if the concentration of the reactant doubles. The rate is directly proportional to the concentration. A first-order reaction has a constant half-life. If the order is two with respect to a reactant, the rate is proportional to the square of the concentration of the reactant. A second-order reaction has an increasing half-life.