Separation of Substances

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Materials which contain two or more substances in any proportion are called mixtures.
• A mixture may be a solid, liquid or gas.
• The various substances which are present in a mixture are called its components or constituents.

For example, in a mixture containing pulse and wheat, are called the constituents of this mixture. Each constituent of a mixture retains its properties.

Classification of Mixtures
All mixtures can be divided broadly into two categories– Homogeneous mixtures and Heterogeneous mixtures.
Homogeneous Mixtures
A mixture is homogeneous when its various components are uniformly spread throughout the mixture and each part of the mixture has its own properties. In general, you say that a mixture in which the particles are present and
cannot be seen with our naked eyes is called a homogeneous mixture.
A homogeneous mixture involving a solid and a liquid is commonly referred to as a solution. Some homogeneous mixtures are:
• solutions of sugar and salt in water.
• cold drinks and syrups etc. which are a mixture of sugar, colour, flavouring agents in water.
• air which is a mixture of gases like oxygen, carbon dioxide, nitrogen etc.
Heterogeneous Mixtures
Heterogeneous mixtures are not uniform in appearance and composition. In general, you say that a mixture in which the particles of the substance are present and can be seen by our naked eye is a heterogeneous mixture. Some heterogeneous mixtures are:
• a mixture of iron filing and sulphur
• dust in the air
• chalk in water
• a mixture of stone and rice
• a mixture of stone and clay
• a mixture of wood and plastic

Pure Substances and their Properties
Pure substances: The materials that are made of only single kind of atoms or elements are called pure substances.
A pure substance has the following properties.
• It has a fixed melting point (M.P), boiling point (B.P) and density.
• A pure substance has its characteristic taste, colour and odour.
• It has a definite composition.
• It is homogeneous in nature. It means that it has the same composition throughout.
For example, a packet of salt will have the same composition of salt throughout the packets.
Pure substances are of two types
(a) Elements: These are pure substances made of the same kind of atoms: For example oxygen, copper, gold, chlorine
(b) Compounds: These are pure substance made of two or more elements combined together in a fixed ratio. For example, carbon dioxide, water sugar, common salt, etc. are all compounds.


Impure Substances and their properties
A substance in which some other substances are also present in smaller amounts is called impure substance. Impure substances contain some impurities. An impure substance is actually a mixture.

Reasons for Separating Mixtures into Their Components
Most substances that occur in nature may contain
• one or more useful substances.
• one useful substance and one or more harmful or unwanted (undesirable) substances.

Need for Separation of Substances
1. To remove undesirable components

2. To remove a harmful and unuseful component

3. To recover useful components

4. To obtain a pure sample of substances



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