Compounds
A compound is a substance made up of two or more elements chemically combined in a fixed proportion by mass.
For Example
Water(H+O), Sodium Chloride(Na+Cl), Ammonium Chloride(N+H+Cl), Calcium Carbonate(Ca+C+0),Calcium Hydroxide(Ca+H+O) etc.
Compounds are further classified into acids, bases and salt.
Properties of Compounds
(1) They cannot be separated in its components by physical method.
(2) The properties of a compound are entirely different from those of its constituent elements.
(3) Formation of compound is a chemical change(energy is either given out or absorbed during the preparation).
(4) In a compound, constituents are present in a fixed proportion by mass.
(5) They have fixed melting and boiling point.
For Example
When a mixture of iron filling and sulphur powder is heated, a black compound called iron sulphide is formed.
(a) If a magnet is put in the iron sulphide compound, iron does not get separated from sulphur.
(b) When dilute sulphuric acid is added to iron sulphide compound, we get a colourless, foul smelling gas called hydrogen sulphide. No hydrogen gas is formed. This means that iron sulphide does not show the properties of iron present in it.
(c) Iron sulphide compound is prepared by heating together iron filling and sulphur.
(d) Iron sulphide compound is prepared by heating together 7 parts by mass of iron and 4 parts by mass of sulphur.
(e) Iron sulphide compound melts at a definite temperature.