For a bimolecular reaction to be first order, the rate of the reaction must be directly proportional to the concentration of one reactant. This occurs in cases like a simple reaction between two molecules where the reaction rate is determined by the concentration of just one of the reactants. An example is the reaction between hydrogen and iodine to form hydrogen iodide, which is first-order with respect to hydrogen and iodine. The order and molecularity of elementary reactions, such as the reaction of two molecules of a reactant, will always be the same.