A compound is a pure substance that contains two or more elements chemically combined in fixed proportions by mass. Compounds have different chemical properties from their constituent elements. Some of the characteristics of a compound are:

1. Composition: A compound has a definite and constant composition. The elements present in a compound are always present in the same proportion by mass.

2. Chemical Properties: The chemical properties of a compound are different from those of its constituent elements. For example, water (H2O) is a compound that is composed of hydrogen and oxygen. Hydrogen and oxygen are both flammable gases, but water is a liquid that is not flammable.

3. Physical Properties: The physical properties of a compound are also different from those of its constituent elements. For example, water (H2O) is a liquid at room temperature, while hydrogen and oxygen are both gases.

A compound cannot undergo a decomposition reaction. A decomposition reaction is a chemical reaction in which a compound breaks down into its constituent elements. For example, water (H2O) can undergo a decomposition reaction to produce hydrogen and oxygen.