A CHEMICAL REACTION TAKES PLACE whenever bonds between atoms are broken or made. In each case, atoms or groups of atoms rearrange, making new substances (products) from the original ones (reactants). Reactions happen naturally, or can be made to happen; they may take years, or only an instant. Some of the main types are shown here. A reaction usually involves a change in energy. In a burning reaction, for example, the making of new bonds between atoms releases energy as heat and light. This type of reaction, in which heat is given off, is an exothermic reaction. Many reactions, like burning, are irreversible, but some can take place in either direction, and are said to be reversible. Reactions can be used to form solids from solutions: in a double decomposition reaction, two compounds in solution break down and re-form into two new substances, often creating a precipitate (insoluble solid); in displacement, an element (e.g., copper) displaces another element (e.g., silver)
from a solution. The rate (speed) of a reaction is determined by many different factors, such as temperature, and the size and shape of the reactants. To describe and keep track of reactions, internationally recognized chemical symbols and equations are used. Reactions are also used in the laboratory to identify matter. An experiment with candle wax, for example, demonstrates that it contains carbon and hydrogen.
