A mixture is a material containing two or more elements or compounds in any proportion and the components of which can be separated by simple mechanical means.
Air is a mixture of oxygen, nitrogen, carbon dioxide, water vapour (moisture) and a small amount of other substances.
Gunpowder is a mixture of sulphur, nitre (potassium nitrate) and coal.
Steel is a mixture. It contains iron, carbon and a small amount of chromium, manganese, etc.
Properties of a mixture
(i) Homogeneity / Heterogeneity :
A mixture may be homogeneous or heterogeneous. A homogeneous mixture has a uniform composition throughout its mass. There are no visible and sharp boundaries between the various constituents of a homogeneous mixture. For example, sugar solution is a mixture of sugar in water. It is a homogeneous mixture. Any portion of this solution contains the same proportion of sugar and water. In this solution, we do not have any visible boundary of separation between sugar and water particles.
A mixture of two miscible liquids i.e. the liquids which mix into one mother completely, is also homogeneous. For example, when we mix water with alcohol, we get a homogeneous mixture of alcohol and water. Air is a homogeneous mixture of oxygen, nitrogen, carbon dioxide, etc.
A heterogeneous mixture does not have a uniform composition throughout its mass. There are visible and sharp boundaries between the various constituents of a mixture. For example, a mixture of sand and common salt is heterogeneous. The compositions of different parts of this mixture have different proportions of sand and common salt. When viewed through a magnifying glass, the sand and salt particles will be clearly seen separated by visible boundaries.
A mixture of two immiscible liquids is also heterogeneous. When oil and water are mixed together, they are separated into two layers, one above the other. This is because oil and water are immiscible.
(ii) Separation :
The constituents of a mixture can be separated by physical methods like filtration, evaporation, sublimation, magnetic effect, etc. Take a mixture of iron filings and sulphur. When a magnet is put in the mixture, the iron particles get attracted to the magnet, stick to it and are thus separated from the mixture. But sulphur is not at all attracted to the magnet. Further, when we add carbon disulphide to the mixture, sulphur dissolves in carbon disulphide. Iron particles remain unaffected. On filtration, iron is obtained as a residue.
(iii) Energy :
In the preparation of a mixture, energy is usually neither given out nor absorbed. When we mix sugar and sand together, there is no energy change, i.e., energy is neither evolved nor absorbed.
(iv) Composition :
The composition of a mixture is not fixed. It varies from one part of the mixture to another. The constituents of the mixture may be present in any proportion by weight.
(v) Melting & Boiling point :
A mixture has no definite melting point, boiling point, etc.
(vi) Chemical reaction :
In the formation of a mixture, no chemical reaction occurs. The constituents of the mixture do not undergo any change in composition.