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Separating Mixtures

Matter around us is available not in pure form. Matter contains a mixture of many elements or compounds. In gaseous phase the atmosphere (air) is a mixture of many gases like Nitrogen, oxygen, sulfur di oxide, nitrogen oxides, oxides of carbon, ozone and traces of inert gases along with water vapor. In liquid state the water contains many dissolved salts of metals like Na,K,Mg,Zn,Al etc, Even organic liquid like petroleum is a mixture of many compounds. In solid state both metals and their compounds are in a mixed state.

In order to get any substance in its pure form one has to follow certain processes of purification. Such techniques which improve the quality of material is called separation techniques. There are many methods and techniques by which Isolation, extraction and purification can be done depending on the nature of the substance. Broadly they can be classified as physical separation techniques and chemical separation techniques.


Physical Separation Techniques

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Physical separation techniques depend on the physical properties of the substance. These physical properties include physical state, solubility, boiling point, melting point, density, specific gravity, electrical and magnetic properties. Some of the physical separation techniques are listed below.
  1. Washing: Insoluble solids like noble metals and compounds like silica can be washed with water until all the other impurities leave them. Washing can also be done with organic solvents when there is a mixture with soluble impurities and insoluble compound.
  2. Decanting: When the densities of two liquids that are immiscible in one another are to be separated this method can be used. Separating funnel helps to separately collect the tow liquids. In case of solids the lighter solids can be separated by decanting it in water medium when both of the solids are not soluble. By blowing air also the separation can be done with very light and heavy solid mixtures.
  3. Distillation: Mixtures of liquids can be separated when their boiling points are different by this technique. The mixture is slowly heated in a distillation flask. The component having the low boiling point vaporizes first. The vapors are collected by condensing them in Leibig's condenser. Mixtures containing more than two liquids can also be separated by this technique but the difference in boiling points must be at least 20ÂșC between each of them. Distillation can also be used to separate dissolved salts from their aqueous solutions. Sea water is distilled by the Sun rays to form clouds which rain fresh water without salts. Mixture of gases are separated by reducing their temperature and converting all the gases in to liquid state. By gradational increasing of temperatures each liquified gas gets converted into gaseous state and can be separated.
  4. Fractional distillation: When the difference in the boiling points are nearby to each other and not much, a detailed distillation is carried out which is called fractional distillation. It is done in a column called fractionating column. Fractionating column allows the condensation of different solvents at different temperatures and sending back the mixture fraction in to the flask. Petroleum distillation is done in the fractionating column in to various components over a wide range of temperatures. Melting point differences can also be utilized in the same way as boiling point in separation of mixtures. Icebergs are formed which are solidified fresh water is based on the depression of freezing point phenomenon.
  5. Solubility: Different compounds have different amounts of solubility at a certain temperature. When no more solid is soluble in solvent at a particular temperature it is called its saturation point at that temperature. In a mixture if one component is more soluble and the other less at a given temperature, they can be separated by this technique.
  6. Electrical and Magnetic separation: Electrolytes and non electrolytes, magnetic and non magnetic substances can be separated by this technique of separation using either electrical field or magnetic field.

Chemical Separation Techniques

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In a mixture of salts dissolved in a solvent requires to be separated, one of the salts can be converted into its insoluble salt. For example a mixture solution of calcium chloride and sodium chloride solutions can be separated by adding a solution of sodium carbonate just enough to precipitate calcium as insoluble calcium carbonate. This is a chemical separation technique. 

In the case of metallurgical processes chemical separation techniques are extensively used. Sulphide ores are roasted to oxides and then reduced to the metals. Example: In the metallurgy of Iron Iron ore is roasted in a blast furnace and then reduced to metallic iron by carbon. 

Noble metals like Gold and Silver are converted in to their soluble cyanide complexes which are later recovered after separation from insoluble impurities.
Organic compounds are separated by converting them in to their derivatives by adding reagents and recovered after removal of impurities.

Methods of Separating Mixtures

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Separating mixtures that have more similar properties require special techniques. Adsorption and desorption properties of the components of a mixture can be separated. Activated carbon removing colors and odors from organic compounds, butter absorbing the perfume of roses iodine getting adsorbed over starch are some examples.

The other special technique for separating mixtures is the chromatography which uses the flow of the liquid and effusion of gases in to consideration. Applying distribution law is one more special separating technique. When there is a mixture of substances, adding a third component as solvent added to it in which one of the components is more soluble. For example Iodine is soluble in water to an extent.

However iodine is more soluble in carbon tetra chloride. Carbon tetrachloride is immiscible with water. Iodine is more soluble in carbon tetra chloride and hence migrates from aqueous layer to organic layer and thus it can be separated.

Membrane Separation

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Membrane separation involves the separation through semi permeable membrane. Semipermeable membrane is a membrane which allows only the solvent particles to flow through and not allowing the solute particles to pass through. When the solution is in colloidal form the membrane separation is most useful. Sweat formation through skin and kidney function are the examples of membrane separation.
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