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In the day to day life, there are times when one needs to separate a useful substance from a mixture. This is done by using various methods of separation of substance.
Mixture: A substance which is composed of two or more substances; in which each component retains its unique property; is called mixture. Air is a mixture of gases. Water which we drink is a mixture of pure water and many other substances.
Pure Substance: A substance is called pure when each particle of the substance has the same unique property. For example; distilled water is pure water as each drop of it contains nothing but water.
Need for separation of substances: Taking out useful substances from a mixture is usually the main reason for separation of substances. Sometimes, we also need to separate substances when we need to use different components in different ways.
Separating solid from a mixture of solids
Handpicking- It is the simplest method of separation of substances. This method is used only when unwanted material is in small quantity. Moreover, shape, size, or colour of the unwanted material is different from that of the useful materials. For example; pebbles, broken grains and insects are separated from rice, wheat and pulses; by handpicking.
Threshing: Threshing is used for separating seeds from the harvested stalks.
  • Manual Threshing: When the quantity is small, threshing is done manually. Small bundles of the harvested stalks are thrashed on a hard surface. This helps in separating the grains. 
  • Threshing by Animals: For larger quantities, threshing is done in the traditional way; by using animals. For this, stalks are spread around a pole. Several bullocks are tied to the pole and are made to walk over the harvested stalks. Trampling by hooves of the animals helps in separating grains. 
  • Threshing Machine: Now-a-days, threshing machines are used for the purpose. It can be powered by either a diesel engine or an electric motor. It helps in saving time and labour.
Sieving- When the size of particle it too small to be picked by hand or when the quantity is too large, sieving is used for separating substances. A sieve; having holes of proper size is used. The bigger particles are retained by the sieve whereas the smaller ones pass through it. For example; this method is used for separating bran from flour, sand from gravel, pearls of different sizes, etc.
Winnowing-The process of separation of lighter particles from heavier particles; with the help of wind; is called winnowing. This method is used for separating grains from husk. Farmers drop the mixture of wheat and husk from a height. The husk is carried by the wind and form a heap at a small distance away. The wheat grains being heavier fall vertically to form a separate heap.
Combine Harvester: Modern farming involves use of a machine; called combine harvester. This machine harvests, threshes and winnows in a single operation. Combine harvesters are used in developed countries; like the USA. In India, such machines can be seen in the farms of Punjab.
These methods are usually used in combination. They are used for separating soluble and insoluble solids from a mixture of solids and liquid.
Sedimentation: Insoluble particles settle down at the bottom and the process is called sedimentation. For example; muddy water contains soil and sand in water. Soil and sand; being insoluble in water; settle down at bottom if water is allowed to stand for some time.
Decantation: This process is used after sedimentation. The upper layer; which contains water is slowly poured out from the container. It leaves the sediment behind.
Filtration: This method is used for separating fine insoluble solid particles from the liquid. In this process, the mixture is passed through a filter. The solid particles do not pass through the filter and clear liquid is collected.
Evaporation: The process of conversion of water into its vapour is called evaporation.
Condensation: The process of conversation of water vapour into its liquid called condensation.
Evaporation and condensation are used for separating a soluble solid from water. For example; salt can be separated from a solution of salt and water; by using the combination of evaporation and condensation.
Salt is prepared from sea water by evaporation and condensation. Sea water is collected in shallow pits and allowed to evaporate. The water evaporates and crystals of salt are obtained in the pits. The salt is then sent to factories for further purification.
Saturated Solution: When a solution of a liquid and solid has enough of the solid and it cannot take any more solid; the solution is called saturated solution. On the other hand, when more solid can be dissolved in the given solution; it is called unsaturated solution.
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By Nikunj Savani

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