Is Matter Around Us Pure

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.

Mixture

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

1. Variable composition: The constituents of a mixture are present in any ratio. Example: A mixture of sand and salt can be in a ratio of 1:2 or 5:6, etc, by weight.
2. Physical change: The mixture is a result of physical change. The constituents of a mixture do not bind each other by chemical bonds. Example: In air the main constituents, i.e., oxygen, nitrogen and carbon dioxide, do not bind each other with chemical bonds.
   
3. No specific properties: The properties of a mixture are the average of the properties of its constituents.
   Example: The properties of air are midway between properties of nitrogen and oxygen.
4. Homogeneity: Most of the mixtures are heterogeneous, i.e., their constituents are not
   spread evenly throughout. However, some mixtures are homogeneous. Example: In the mixture of iron and sulphur, at some places iron is more and at some places sulphur is more.
   
5. Separation: Generally, the constituents of mixture can be separated by employing
   suitable physical means. Example: Iron can be separated from the mixture of iron and sulphur with the help of a magnet.
6. Energy changes: No energy is released or absorbed during the formation of a mixture. Example: On mixing iron and sulphur, heat energy is neither absorbed nor evolved.