Need and Method of Separation

As we know that many naturally occurring as well as man-made mixtures are desirable. But there are many mixtures which are undesirable and cannot be used as such. The constituents of mixtures need to be separated for the following reasons. 

(1) To remove the undesirable constituents  

For example, we need to separate impurities from ordinary water to make it potable (safe for drinking). We must remove husk, stones and dirt from food grains before we cook them.

(2) To obtain desirable substances  

For example, we obtain common salt from sea water, which contains many substances including salt. Similarly, we separate petrol, kerosene and diesel from petroleum (crude oil).

(3) To obtain highly pure substances  

For example, from ordinary water, we obtain highly pure samples of water for medicinal purposes.

1.0Methods of Separation

Methods of separation are based on the properties of materials present in a mixture. The constituents of a mixture retain their properties, so they can be separated easily. Some of them are

2.0Methods to Separate A Solid From Solid-Solid Mixture, 

Hand-Picking

Separating rotten grapes by hand

Principle- It is based on the fact that, one of the components differs in shape, size, and colour.

Method- If a constituent of a solid mixture is big and visibly different, it can be separated by hand-picking. 

For example, stones can be separated from rice and wheat or the seeds of different varieties of pulse get mixed up, they are separated by hand-picking. Plucking rotten grapes from a bunch is nothing but hand-picking. For picking, forceps can also be used. 

However, hand-picking is useful only if you need to separate a small amount of constituents 

Threshing

Principle- The process used to separate grains from the stalks is called threshing. 

Method- Bundles of paddy (rice) or wheat stalks are kept in the fields after harvesting the crop.

These stalks are dried in the sun before the grain is separated from them. These grains cannot be separated from stalks by hand picking. The farmers make small bundles of these stalks and then hit it against the hard wall of mud or hard substances. In doing so the grain separates from the stalks.

These days there are mechanical threshers which hammer the stalks and separate grains from them.

Winnowing

Principle - This method is based on the difference in the weight of the components of a mixture of solids. Winnowing

Method- Lighter solids are separated from heavier ones by wind is called winnowing. Farmers use this method to separate chaff (the covering of grain) from grain. A mixture of chaff and grain is made to fall from a height. The lighter chaff drifts away and the heavier grain falls vertically on the ground. Wind helps in winnowing by blowing away the chaff more easily.

If there is no wind, the winnower moves slowly through a length of about five metres while letting the mixture fall. The movement of the winnower stirs the air, which helps the chaff drift. If some chaff falls on the grain, it can be blown away.

You can mimic winnowing. Take some roasted groundnuts on your palm and peel the nuts. 

Blow air at them. The nuts will remain on your palm, while the outer covering will be blown.

Sieving

Principle- This method of separation is based on the difference in the size of the components of a mixture of solids. 

Method- If a solid mixture is stirred or shaken on a mesh, particles smaller than the holes in the mesh fall and the bigger ones remain on the mesh. Thus, particles are separated on the basis of their size. 

This process is called sieving and the mesh (usually fitted into a frame) is called a sieve. Sieve is a shallow vessel having small holes at its bottom. The size of the holes of a sieve to be used, depends on the size of the particles of the material, to be separated from the mixture.

Small and fine sieves are used in the kitchen to separate bran and other impurities from flour. Coarse particles of flour are called wheat bran.

The impurities remain on the sieve and flour passes through. Bigger sieves are used at construction sites to separate stones from sand. 

Sieving of Sand

  Unshifted Shifted

Magnetic Separation

Principle- It is based on the property of a magnet to attract magnetic substances towards it. Method- Iron is attracted by a magnet. So, if you move a magnet through a mixture of iron filings and sulphur, the iron particles cling to the magnet. 

[[Sulphur is not attracted by a magnet and so the particles of sulphur are left behind. 

This is how a mixture of iron and sulphur is separated. Such a method, in which a magnet is used to separate the constituents of a mixture, is called magnetic separation.

Scrap iron is picked up by huge magnets in scrapyards.

Huge magnets, held by cranes, are used to pick up scrap iron in the scrapyard.

Sublimation

Principle : This method is used in the separation of such solid-solid mixtures where one of the components sublimes on heating. 

Method : The mixture of common salt and ammonium chloride is taken in a China dish and placed on a tripod stand. The China dish is covered with an inverted glass funnel. A loose cotton plug is put in the upper open end of the funnel to prevent the ammonium chloride vapours from escaping into the atmosphere. The China dish is heated by using a low flame. On heating the mixture ammonium chloride changes into white vapour. These vapours rise up and get converted into solid ammonium chloride on coming in contact with the cold inner walls of the funnel.

When the mixture gives off no more white fumes, lift the funnel, scrap the fine white powder from its sides on a piece of paper. This is pure ammonium chloride. The residue left behind in the China dish is sodium chloride.

3.0Methods to Separate A Liquid or A Solid From Solid-Liquid Mixture

Sedimentation and Decantation

Principle- This method is used to separate solid-liquid mixture in which solid is heavier than liquid.

Method- Suppose you have a solid - liquid mixture in which the solid is heavier than the liquid, for example, a mixture of sand and water.

If you allow the mixture to stand for some time, the solid will settle at the bottom of the container and the liquid will remain over it. This process is called sedimentation, and the solid layer at the bottom is called the sediment. The liquid above the sediment is known as the supernatant liquid.

We can pour out the supernatant liquid carefully into another vessel, leaving behind the sediment. The process is called decantation.

Potassium alum is commonly used at our home by the name phitkariHowever, if you have a solid-liquid mixture containing a solid lighter than the liquid. 

Loading Loading helps sedimentation. You may have seen that water obtained from natural sources like wells and rivers is muddy, especially in the rainy season. Very fine, solid particles do not sink to the bottom they remain suspended in water. If the particles join some other particles, they become heavier and sink. We then say that the particles are loaded.

Suspended solid particles in water are loaded by the addition of alum.

Filtration

Principle- Filtration is a method used to separate fine particles of insoluble solids from a liquid.

Method- By decantation, a liquid is not completely separated from an insoluble solid. Filtration is a better method. The process of removing insoluble solid from a liquid by using a filter paper is called filtration.

The mixture is then poured on the filter paper. The supernatant liquid is slowly poured along a glass rod into the funnel. The solid collects on the filter paper (residue), whereas the liquid passes through. The clear liquid thus obtained is called the filtrate.

After decantation we observed that water in the second glass is still muddy or brown in colour. In a piece of cloth, small holes or pores remain in between the woven threads. These pores in a cloth can be used as a filter. A filter paper is one such filter that has very fine paper in it. Solid particles in the muddy water do not pass through it and remain on the filter.

You can easily separate a mixture of sand and water or chalk and water by filtration.

Churning 

Principle- The method of separating finely suspended particles in a liquid, by whirling the liquid at a very high speed is called centrifugation. 

Method-Fruit salad has cream in it. But do you know how cream is obtained? It is obtained by churning milk. When milk is churned, cream is separated from it. As cream is lighter than milk, it floats over the liquid. Churning is done either manually or by using an electric churner. In a washing machine also, the dirt separated from the cloth by a similar action.

Application of Churning

• It is employed in diagnostic laboratories in testing urine and blood samples.
• It is employed in blood banks to separate different constituents of blood.
• It is used in drying machines to squeeze out water from the wet clothes.

When solid is soluble in the liquid

Components of a solid-liquid mixture, containing a solid that is soluble the liquid, can be separated by the process of evaporation or distillation. If only the solid is required, then evaporation can be used. If both the components of the mixture are required, then the technique chosen is distillation,

Evaporation 

Principle- Evaporation involves heating the mixture so that the liquid evaporates leaving behind the solid solute.

Method-A solid can be recovered from its solution by evaporating the solvent. A solution of salt in water, when heated on a flame for some time, leaves a residue of the salt.By improved, but similar processes, distilled water is prepared for laboratory and medicinal purposes. Then the salt begins to crystallise. The salt is further purified and mixed with a chemical called sodium iodate. This mixture is iodised table salt. Sodium iodate contains iodine. The lack of iodine in our diet might cause the disease goitre.

when one constituent of a solid mixture is soluble in a solvent (water) and the other is not.

Method

Take a mixture of salt and sand in a beaker.  Keep this mixture in beaker and add water into this, leave the beaker aside for some time. Now, we need to separate salt and water from the decanted liquid. Transfer this liquid to a kettle and close its lid. Heat the kettle for some time.Take a metal plate with some ice on it. Hold the plate just above the spout of the kettle.

When the steam comes in contact with the metal plate cooled with ice, it condenses andforms liquid water. The water drops that you observed falling from the plate, weredue to condensation of steam. The process of conversion of water vapour into its liquid form is called condensation. After, all water has evaporated, salt is left behind in kettle.

We have thus, separated salt, sand and water using processes of dissolution, evaporation and condensation.

Distillation

Principle- This method is applied for the separation of a mixture containing two miscible liquids having large difference in their boiling points or a mixture containing solid dissolved in liquid.

Method- When the mixture is heated, the vapours of the low boiling liquid, i.e., acetone are first formed. These travel upwards. On passing through the condenser, they get condensed to form liquid acetone which is collected in the beaker. The boiling point of acetone is 329 K (56°C), therefore, the thermometer reading becomes constant at 329 K when acetone is distilling. Water is left behind in the flask. 

Simple distillation 

Separation of the components of a mixture containing two miscible liquids which boil without decomposition and have difference (30–50 K) in their boiling points can be carried out by simple distillation. This is because at the boiling point of each liquid, the vapours almost entirely consist of that liquid. Simple distillation is applicable in following cases

1. Two miscible liquids that boil without decomposition and have large difference (greater than 25 K) in their boiling points.

2. A solution that contains a volatile solvent and a non-volatile solute.

Crystallisation 

While evaporating a solution, a stage comes when the amount of solvent becomes very little. This solution is called a concentrated solution. When it is allowed to cool slowly, crystals of the substance dissolved start separating. 

The process is called crystallisation. It is used to obtain a substance in the pure state

Process of crystallisation of copper sulphate

4.0Methods to Separate A liquid - Liquid Mixture

Separating funnel

The separation of two immiscible liquids is based on the difference in their densities. The apparatus used for separation is the separating funnel.

How will you separate a mixture of kerosene and water?

Water Explanation

Kerosene and water are two immiscible liquids. The mixture can be separated by using a separating funnel.

The mixture of kerosene with water form two distinct layers of liquids. Kerosene being lighter will form a top layer.

On opening the stop cock water will be collected in beaker leaving the kerosene in the separating funnel.

Water (An important solvent) 

It is because of its property of dissolving a large number of substances that water is so essential for life. 

(i) Our body cannot absorb food substances unless they are soluble in water. In the process of digestion, food is reduced to simple substances that are soluble in water. They can then be dissolved in water and absorbed by the body. 

(ii) Several waste materials produced in the body are dissolved in water and excreted.
A number of chemical reactions occur inside our body. They all occur in the presence of water.

(iii) Plants can absorb nutrients from the soil only if they are soluble in water. Minerals from the roots and food from the leaves are transported to different parts of the plant in the form of solution in water.

(iv) Gases such as oxygen and carbon dioxide which are soluble in water are important for survival of aquatic life. Fish breathe through their gills. Aquatic plants use the carbon dioxide dissolved in water for photosynthesis. Carbon dioxide is dissolved in aerated drinks to give them their tangy taste.

The solubility of gases in water decreases with increase in temperature. Thus, dissolved gases escape from water when it is heated. This can easily be seen by gently heating water in a pot. You will see small bubbles coming out much before water starts boiling. These are bubbles of air dissolved in water.

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