Chemistry REVISION OF PHYSICAL AND CHEMICAL CHANGES & ELEMENTS, MIXTURES AND COMPOUNDS FOR NDA Part-3
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Mixtures :

A mixture is a substance which consists of two or more elements or compounds which are not chemically combined together. e.g. air, food, wood, milk, ink, brine, lime water, glass, paints, soil, kerosene etc.

There are two types of mixtures, which are as follows :

(a) Homogeneous Mixture or Solution : A homogeneous mixture has same composition throughout its mass. There are no visible boundaries of separation between the various constituents. e.g. solution of sugar in water, alloy, solution of salt in water, a mixture of alcohol and water etc.

(b) Heterogeneous Mixture : A heterogeneous mixture does not have a uniform composition throughout its mass. It has visible boundaries of separation between the various components. e.g. gun powder, solution of `K_2Cr_2O_7` in water, solution of `CaCO_3` in water, solution of oil in water, soil, etc.

`text(Note) :` A mixture which boils at a definite temperature and distills over without any change in composition is called azeotropic mixtures. e.g. ethanol-water, acetone-benzene, benzene-chloroform etc.

Properties of a Mixture :

Following are the main properties of a mixture :

• A mixture can be separated into its components by physical methods like filtration, evaporation, sublimation, distillation, magnet etc.
• A mixture shows the properties of all the constituents present in it. Energy is usually neither given out nor absorbed during the preparation of a mixture.
• A mixture does not have a definite melting point, boiling point etc. (except azeotropic mixture).
• The composition of a mixture is variable, i.e. the constituents can he present in any proportion by weight.

Methods for Separation of Mixture :

General methods used for the separation of a mixture are as follows :


(i) By separatory funnel : This method is used to separate a mixture of two immiscible liquids. e.g. mixture of oil and water, extraction of iron from its ore.

(ii) Crystallisation : This method is based on the difference in the solubility of the various compound dissolved in a common solvent. e.g. a mixture of `KNO_3` and `NaCl` can be separated by this process.

(iii) Distillation : This method is used for the purification of liquids which boils without decomposition and contain non-volatile impurities. e.g. pure water can be obtained from sea water by distillation, mixture of ether and toluene, benzene and aniline, etc.

(iv) Fractional distillation : This process is used to separate a mixture of two or more miscible liquids which have their boiling points within the limit of 10 to 15 K. e.g. petrol, diesel, kerosene.

(v) Distillation under reduced pressure : This process is used for those liquids which decomposes below their boiling point under reduced pressure, liquid boils at low temperature. Hence, the temperature for decomposition is not reached. Thus, this process is known as vacuum distillation, e.g. glycerol can be separated by this method. It is also used for the concentration of sugar cane juice in sugar industry.

(vi) Steam distillation : It is used for the separation and purification of liquids which are appreciably volatile in steam from non-volatile components of mixture, e.g. o-nitrophenol and p-nitrophenol are separated by this method. Aniline is also purified by this method.

(vii) Sublimation : When a solid changes directly from solid to gaseous state on heating, the process is known as sublimation. This process is used to separate such mixtures that contain a sublimable volatile component from a non-sublimable impurity. This process is generally used for the separation of naphthalene, anthracene, benzoic acid, camphor, `NH_4Cl`, iodine etc. Iodine (`I_2`) is the only halogen which sublimes. `NH_4 Cl` sublimes due its decomposition.

(viii) Evaporation : It is the process by which soluble solids can be obtained from their solution by allowing the solvent to vaporise, e.g. salt can be obtained from salt solution by evaporation.

(ix) Chromatography : It is a modern method proposed by T. Swett in 1903. This method is based on the difference in the rates at which the components of a mixture are adsorbed on a suitable adsorbent.

This method has been used

(a) to separate o- and p-nitrophenol,
(b) to separate blue and red dyes,
(c) to separate plant pigments and other natural products.

(x) Atmolysis : It is used for separating the mixture of gases. This method is based on the difference in their rates of diffusion.

`text()^(235)UF_6` and `text()^(238)UF_6` can be separated by this method.

(xi) Dialysis : The process of separating the colloidal particles from those of crystalloids by diffusion of mixture through a parchment or animal membrane is known as dialysis e.g. A ferric hydroxide sol can be purified by this method.

 
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