(General Science) CHEMISTRY - Solubility & Separation of Mixtures


General Science Note for IAS Exam


The concentration of a solution is defined as the amount of solute present in a given amount of the solution. A solution in which some more solute can be dissolved without increasing its temperature is called an unsaturated solution. Whereas a solution in which no more solute can be dissolved at that temperature is called a saturated solution. The maximum quantity of a solute that can be dissolved in 100 grams of a solvent at a particular temperature is known as the solubility of the solute in that solvent at that temperature.

The solubility of a solid in a liquid generally increases on increasing the temperature, and decrease on decreasing the temperature. It remains unaffected by changes in pressure. The solubility of a gas in a liquid generally decreased on increasing the temperature, and increased on decreasing the temperature. In contrast, it increase on increasing the pressure, and decrease on decreasing the pressure. For example, when water is heated, air dissolved in water comes out in the form of tiny bubbles. This shows that solubility of air (gas) in water (liquid) decrease with increase in temperature. When a soda water bottle is opened, the pressure decreased and carbon dioxide gas dissolved in water escapes producing a fizz. This shows that solubility of a gas in a liquid decreases on decreasing the pressure.

Separation of Mixtures

Many procedures have been developed to separate mixtures into their component. The method which is used for this purpose depends upon the nature of the components present in the mixture.

1. A mixture of two solids cab be separated by one of the following methods:

(a) Use of suitable solvent: A mixture of sugar and sand can be separated by adding water as the solvent which dissolve sugar but not sand. Filtration of the solution leaves sand the filter paper. Evaporation of water from the filtrate gives sugar.

(b) Sublimation: The process of sublimation is used to separate the component which sublimes on heating from the one which does not. Thus, naphthalene, which sublimes, can be easily separated from sodium chloride by this method.

(c) Use of magnet: Iron is attracted by a magnet. Therefore, it can be separated from other components of a mixture with the help of a magnet. In factories, scrap iron is separated from a heap of waste material with the help of electromagnets fitted to a crane.

2. A mixture of a solid and a liquid can be separated by one of the following methods:

(a) Filtration: Filtration is used to separate insoluble substance from a liquid, e.g., a mixture of sand and water can be separated by filtration. Different kinds of filters can be used. e.g., filter paper, wire-mesh, cotton, muslin cloth or a layer of sand. Used tea leaves are separated from prepared tea by filtration, using a tea strainer. Drinking water is filtered using water filters.

(b) Centrifugation: The method of centrifugation is used to separate suspended particles from a liquid. The mixture is separated by rotating it at high speed in a centrifuge. This process is used in dairies to separate cream from milk.

(c) Evaporation: A solid substance dissolved in a solvent can be separated by the process of evaporation. The dissolved substance is left as a solid residue after the solvent has evaporated. The solvent itself cannot be recovered by this method. Common salt is obtained from sea water by evaporation. Sea water. Trapped in shallow lakes called lagoons, is subjected to the heat of the sun. Water evaporation leaving behind salt as a solid. If any impurities are present in the dissolved solid, they would still be present after it’s recovered after its recovery by evaporation.

(d) Crystallisation: When a hot, concentrated solution of a substance is allowed to cool slowly, crystals of pure solid are formed, while impurities remain dissolved in the solvent. This process is called Crystallisation. The crystals can be separated by filtration. An impure sample of compound, like copper sulphate or alum, can be purified by crystallization.

(e) Chromatography: Two or more dissolved solids present in a solution in very small amounts can be separated and identified by chromatography. Though these substances are soluble in the same solvent, yet their solubilities may different. The components of the mixture dissolved in a solvent, move to different extents on an adsorbent material (filter paper, silica gel, etc.) and thus get separated.

There are many types of chromatography. In Paper Chromatography, a special kind of filter paper is used, whereas Thin Layer Chromatography (TLC) involves the use of a glass plate coated with silica gel. In Column Chromatography a glass column packed with adsorbent material is used. Gas-Liquid Chromatography (GLC) is very powerful technique for the rapid analysis of mixtures containing volatile components. High Performance Liquid Chromatography (HPLC) is an important method for compound which are non-volatile or thermally unstable. It is used to separate isomers.

(f) Distillation: In order to recover both the solute and the solvent from a solution, the process of distillation has to be used. It is a process is which evaporation and condensation go on side by side. When the solution is heated, the solvent evaporates, leaving behind the solid. Vapours of the solvent are condensed to give the liquid, which is called the ‘distillate’. Pure water is obtained from tap water by distillation. Distillation is used to obtain drinking water from sea water in hot and arid countries, which get little or no rain.

3. A mixture of two or more liquids can be separated by one of the following methods:

(a) Fraction Distillation: Liquids which mix together in all proportions to produce a single layer are called miscible liquids. Two or more miscible liquids can be separated by fractional distillation using a fractionating column. The distillate is collected in fractions, boiling at different temperatures. Mixture of miscible liquids, like alcohol-water or acetone-water mixture can be separated by this method. Fractional distillation is used to separate crude petroleum into useful fractions, like kerosene, petrol, etc.

(b) Use of separating Funnel: Liquids which do not mix with other and form separate layers are called immiscible liquids. A separating funnel is used to separate two immiscible liquids. The heavier liquid forms the lower layer. Mixture oil and water, petrol and water. Water and chloroform, ether and water can be separated by this method.


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