A reaction involves burning. This reaction requires energy to start, but it releases more energy than is required to start it. Oxygen is always a reactant, and the products are usually carbon dioxide and water. A reaction occurs when a compound breaks down. This reaction has one reactant and two or more products. Energy, as from a battery, is usually needed to break the compound apart. A reaction occurs when two substances combine to form one substance. An example of this kind of reaction is rusting, in which iron reacts with oxygen. A reaction that involves the replacement of atoms is a reaction. An atom from one reactant replaces an atom in the other reactant. This kind of reaction has two reactants and two products. An example is the neutralization of an acid and an alkali.