Chemical reaction. A process in which at least one new substance is produced as a result of chemical change.

Combination reaction. A chemical reaction in which a single product is produced from two or more reactants.

Decomposition reaction. A chemical reaction in which a single reactant is converted into two or more simpler substances (elements or compounds).

Single-replacement reaction. A chemical reaction in which an atom or a molecule replaces an atom or a group of atoms from a compound. Double-replacement reaction. A chemical reaction in which two substances exchange parts with one another and form two different substances.

Combustion reaction. A chemical reaction in which oxygen (usually from air) reacts with a substance with the evolution of heat and (usually) the presence of a flame.

Oxidation. The process whereby a reactant in a chemical reaction (1) gains one or more oxygen atoms, (2) loses one or more hydrogen atoms, or (3) loses one or more electrons.

Reduction. The process whereby a reactant in a chemical reaction (1) loses one or more oxygen atoms, (2) gains one or more hydrogen atoms, or (3) gains one or more electrons.

Redox reaction. A chemical reaction in which there is a transfer of electrons from one reactant to another reactant.

Nonredox reaction. A chemical reaction in which there is no transfer of electrons from one reactant to another reactant.

Oxidizing and Reducing Agents. An oxidizing agent causes oxidation by accepting electrons from another reactant; it itself is reduced. A reducing agent causes reduction by providing electrons for another reactant; it itself is oxidized.

Collision theory. Collision theory summarizes the conditions required for a chemical reaction to take place. The three basic tenets of collision theory are as follows: (1) Reactant molecules must collide with each other. (2) The collision must involve a certain minimum of energy. (3) In some cases, colliding molecules must be oriented in a specific way if reaction is to occur.

Exothermic and endothermic chemical reactions. An exothermic chemical reaction releases energy as the reaction occurs. An endothermic chemical reaction requires an input of energy as the reaction occurs.

Reaction rates. Reaction rate is the speed at which reactants are converted to products. Four factors affect the rates of all reactions: (1) the physical nature of the reactants, (2) reactant concentrations, (3) reaction temperature, and (4) the presence of catalysts.

Chemical equilibrium. Chemical equilibrium is the state wherein the rate of the forward reaction is equal to the rate of the reverse reaction. Equilibrium is indicated in chemical equations by writing half-headed arrows pointing in both directions between reactants and products.

Le Châtelier's principle. Le Châtelier's principle states that when a stress (change of conditions) is applied to a system in equilibrium, the system will readjust (change the position of the equilibrium) in the direction that best reduces the stress imposed on it. Stresses known to change an equilibrium position include (1) changes in amount of reactants and/or products, (2) changes in temperature, and (3) changes in pressure.