A chemical combined with another chemical to for a product

Example:

A + B --> AB

One reactant breaking down into two or more atoms

Example:

AB --> A + B

An organic substance may contain C+H or C+H+O

Example:

organic substance + O2 --> CO2 + H2O

Example:

A + BC --> AC + B

Where A and B are metals (or H)

A must be more reactive than B 

(see "activity series")

Example:

AB + CD --> AD + CB