What is a Chemical Compound?

A compound is a pure substance that contains more than one element.  Water is a compound of hydrogen and oxygen.  The compounds methane, acetylene, and naphthalene all contain the elements carbon and hydrogen, in different proportions.  

Compounds have fixed compositions.  That is, a given compound always contains the same elements in the same percentages by mass.  A sample of pure water contains precisely 11.19% hydrogen and 88.81% oxygen.  In contrast, mixtures can vary in composition.  For example, a mixture of hydrogen and oxygen might contain 5, 10,  25, or 60%  hydrogen, along with 95, 90, 75, or 40% oxygen.

Characteristic properties of compounds:

1.  Elements in a compound are present in a definite proportion 

Example:  2 atoms of hydrogen + 1 atom of oxygen becomes 1 molecule of compound-water.

2.  Compounds have a definite set of properties

Elements of the compound do not retain their original properties

Example-Hydrogen(element{which is combustible and non-supporter of combustion}) + Oxygen(element{which is non-combustible and supporter of combustion}) becomes  Water(compound{which is non-combustible and non-supporter of combustion})

3.  Elements in a compound cannot be separated by physical methods.

The properties of compounds are very different from those of the elements they contain.  Ordinary table salt, sodium chloride, is a white, unreactive solid.  As you can guess from its name, it contains the two elements sodium and chlorine.  Sodium (Na) is a shiny, extremely reactive metal.  Chlorine (Cl) is a poisonous, greenish-yellow gas.  Clearly, when these two elements combine to form sodium chloride, a profound change takes place.