The purpose of categorizing a chemical reaction is to help predict the product or products of the reaction. In general, a chemical reaction is a process of molecular rearrangements, in which atoms change partners. Atoms are not destroyed or created. Total mass is conserved, that is, the mass of all of the reactants before the reaction must equal the mass of all of the products after the reaction, although the physical or chemical states of individual substances may change. This mass-conservation requirement explains why all chemical reactions must be properly balanced to be quantitatively valid and useful. Energy changes also accompany reactions— energy is either liberated or absorbed—since existing chemical bonds are broken and new bonds are formed.

With the letters A, B, C, and D used to represent simple elements or polyatomic ions, the five categories of reactions are as follows:

• Combination or synthesis

• Decomposition

• Single replacement/displacement

• Double replacement/displacement

• Combustion (complete)

The notation (g), (l), (s), or (aq) immediately following a reactant or product is often employed to designate the chemical state of the substance.

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