A compound is a pure substance formed from two or more elements combined together in definite proportion by weight. Alternatively, a compound is a pure substance that can be decomposed by chemical action into two or more simpler substances. The chemical constitution of a compound is known and a chemical formula can be given to it. For example, lime is a compound of calcium and oxygen. The chemical formula is CaO. Similarly, water is a compound of hydrogen and oxygen. The chemical formula of water is H2O. Other examples are ammonia, carbon dioxide, chalk, hydrogen chloride, ammonium chloride, etc.
Properties of a compound
(i) Separation :
The constituent elements of a compound cannot be separated by mechanical or physical methods.
Iron sulphide is a compound of iron and sulphur. If we put a magnet close to the pieces of iron sulphide, iron present in iron sulphide does not get attracted towards the magnet because iron has no individual identity in iron sulphide. Further, sulphur dissolves in carbon disulphide. But if we add carbon disulphide to iron sulphide, sulphur present in iron sulphide does not dissolve in it, because sulphur has no individual property in iron sulphide. This clearly shows that the constituents (iron and sulphur) present in iron sulphide cannot be separated by physical methods. Thus, in general, we can say that the constituents of a compound cannot be separated by physical methods.
(ii) Different from elements :
The properties of a compound are entirely different from those of its constituent elements. Water is a compound made up from hydrogen and oxygen. But the properties of water are different from those of hydrogen and oxygen. Water is a liquid, while hydrogen and oxygen are gases. Similarly, sodium chloride is a compound. It is formed from the elements, sodium (solid and highly reactive) and chlorine (gaseous and pungent smelling). Sodium chloride (common salt) can be directly taken with food, but both sodium and chloride are poisonous as food material.
(iii) Proportion in weight :
In a compound, the constituent elements are present in a definite proportion by weight. For example, water (H2O) is a compound made up of two elements, hydrogen and oxygen. In water, hydrogen and oxygen are present in a fixed ratio of 1:8 by weight.
(iv) Melting point & boiling point :
A compound has a fixed melting point, a fixed boiling point, etc. For example, ice always melts at 0°C.
(v) homogeneity :
A compound is a homogeneous substance, i.e., a compound is such a substance which is the same throughout in its properties and composition. For example, iron sulphide is a compound of iron and sulphur. If we see iron sulphide through a microscope or a magnifying glass, particles of iron or sulphur cannot be separately identified.
Since compounds are formed by the chemical combination of elements, they are also called chemical compounds.