Chemically pure substance is a kind of matter that cannot be separated into other kinds of matter by any physical process. It has the same colour, taste, texture and composition at a given temperature and pressure.
Meaning of pure substance different for a common man than to a chemist
For a common man, pure substance means that it is not adulterated.
For example, for a common man, pure milk means the liquid material that is given by cow or buffalo and it is not mixed with water or other materials to thicken it.
But for a chemist, any milk is not a pure substance. Milk is a mixture of water, fats and proteins.
Purity of a given substance
A pure substance always has the same colour, taste or texture at a given temperature and pressure. Also it has a fixed melting point or boiling point at constant pressure. For example, pure water boils at 373K at 1 atmospheric pressure. But water containing some invisible dissolved substance boils at a temperature above 373K.
If two or more substances (elements, compounds or both) mixed together in any proportion, do not undergo any chemical change, but retain their characteristics, the resulting mass is called mixture.
Kinds of Mixtures
Heterogeneous Mixture: A mixture in which various constituents are not mixed uniformly is called heterogeneous mixture.
Examples: A mixture of sand, salt and sulphur is a heterogeneous mixture. Similarly, a handful of soil is a heterogeneous mixture.
Homogeneous mixture: A mixture in which different constituents are mixed uniformly is called homogeneous mixture.
Examples: Brass is an alloy of copper and zinc and is a homogeneousmixture. Similarly, all solutions are homogeneous mixtures.
Characteristics of a Mixture