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I n our daily life, there are many instances when we notice a substance being separated from a mixture of materials. Tea leaves are separated from the liquid with a strainer, while preparing tea. Grain is separated from stalks, while harvesting. Milk or curd is churned to separate the butter. As we learned in Chapter 3, we gin cotton to separate its seeds from the fibre
Perhaps you might have eaten salted daliya or poha. If you found that it had chillies in it, you may have carefully taken them out before eating. Suppose you are given a basket containing mangoes and guavas and asked to separate them. What would you do? Pick out one kind and place them in a separate container, right? Seems easy, but what if the materials than mango or guava? Imagine you are given a glass of sand with salt mixed in it. Impossible, even to think of separating salt from this mixture by picking out grains of sand by hand!
The process with its purpose and the way separated components are used? We see that, before we use a substance, we need to separate harmful or non-useful substances that may be mixed with it. Sometimes, we separate even useful components if we need to use them separately. The substances to be separated may be particles of different sizes or materials. These may be in any three states of matter i.e., solid, liquid or gas. So, how do we separate substances mixed together if they have so many different properties?
METHODS OF SEPARATION: We will discuss some simple methods of separating substances that are mixed together. You may come across some of these methods being used in day to day activities.
Homogeneous mixtures :Mixture in which the particles of the substance present cannot be seen are called homogeneous mixtures. For example, solution of sugar and water, air, cold drinks.
Heterogeneous mixtures: Mixture in which particles of the substances present can be seen easily are called heterogeneous mixtures. For example, water in oil, dust in air.
Solution: When a soluble substance is dissolved completely in a liquid (say sugar in water). A homogeneous mixture is formed. It is known as solution.
Saturated solution: A solution in which no more soluble substance can be dissolved at room temperature is called saturated solution.
Need for separation: We carry out the separation of the components of a mixture or an impure substance with the following purposes :
(i) To remove the unuseful or harmful component.
(ii) To obtain the useful component.
(iii) To remove impurities for getting a pure sample.
Principle of separation
1. The substances present in a mixture retain their original properties like particle size, density, melting point, boiling point, volatility, etc.
2. We use the difference in anyone of these properties in the components of a mixture to separate them.
Methods of separation: Hand picking, winnowing, sieving, magnetic separation, sedimentation, decantation, loading, filtration, evaporation, sublimation, distillation, churning, etc. are some common methods of separation.
Handpicking: This method is used for separating small particles of dirt, stone, husk etc. from the grains of wheat, rice, pulses, etc.
Sieving: You might have seen your mother use a sieve to separate impurities and bran from flour before using it. The separation techniques of sieving are mainly used in a flour mill to separate impurities from wheat before grinding it. Sieving removes impurities like stones, stalk, and husk that still remain after threshing and winnowing. The sieve only removes particles of impurities that are larger than the pores in the sieve. Therefore, we can design the sieve according to our need. (i) Sieving is used when two component of a mixture have different particle size. (ii) Sieving allows the fine particles to pass through the holes of the sieve, while the bigger particles remain on the sieve. For examples, sieving of wheat flour, sieving of sand at construction sites.
Winnowing: Winnowing can be used to separate lighter and heavier components of a mixture. For example : to separate husk from grain with the help of air. Winnowing is used to separate lighter components of the mixture from heavier components by the wind. These separation techniques are commonly used by farmers to separate lighter husk particles from the heavier seeds.The lighter particles are carried away by the wind. The heavier seeds form a heap near the platform of winnowing. We can use either natural wind or blow air while winnowing.
Threshing: The process that is used to separate grain from stalks is threshing.
Decantation: It is the transfer of clean liquid from one vessel to other vessel without disturbing the settled (sedimented) particles.
Loading: It is the process of faster sedimentation by suspending alum to a liquid.
Filtration: Filtration is used to separate solid particles from liquid by passing the mixture through a filter paper. Filtration is the process of separating suspended solid matter from a liquid, by causing the latter to pass through the pores of some substance, called a filter. The liquid which has passed through the filter is called filtrate. The filter may be paper, cloth, cotton-wool, asbestos, slag or glass wool, unglazed earthenware, sand, or any other porous material.
Evaporation: It is the process of removing water (or moisture) from a mixture either by heating on flame or direct sunlight. For example : salt from sea water is obtained by this method.
Condensation: The process of conversion of water vapour into its liquid form is called condensation.The process of pouring out a clear liquid from a vessel (after sedimentation) without disturbing the sediments (heavy insoluble settled particles) is called decantation.
The above method has two disadvantages.
(a) It cannot be used for the miscible liquids that dissolve in one another. For example, petrol mixed in kerosene oil or salt or sugar solution in water.
(b) During the process of decantation, a small quantity still remains unseparated thus it gets wasted.
Churning (or centrifugation): It is the process of separation of the lighter particles of a suspended solid from a liquid. For example : to obtain butter from the curd or milk.
Crystallisation: The process of crystallization is used for obtaining pure crystallive substance from impure sample.
Sedimentation: It is the process of settling of heavy solid particles in a mixture at the bottom of the vessel. This process is based on the densities of different components of a mixture. The process of setting down of heavy insoluble particles in a mixture of water and insoluble substances is called sedimentation.
Coagulation or Loading
Coagulation is the process of improving the settling property of solids by addition of specific chemicals. When solid particles present in a mixture are not heavy enough to settle easily, some chemicals can be added to the mixture to enable the solids to settle. For example, when alum is added to dirty water, it attaches itself to the dirt particles and makes several dirt particles stick to each other. This makes them heavier and helps them to settle down. Alum is said to be a coagulating agent.
Distillation is a process of separating the component of substances from a liquid mixture by selecting evaporation and condensation.
Saturated Solutions:A solution in which no more substance can be dissolved at a given temperature is called a saturated solution.
Characteristics of a Solution:
• A solution is a homogeneous mixture of two or more substances.
• The particles of solute in a solution cannot be seen by naked eye.
• A solution is stable.
• The solute from a solution cannot be separated by filtration (or mechanically).
Solute: A substance dissolved in another substance, water (or any other solvent) is called solute.