Separation of Substances

1. INTRODUCTION
In our day to day life we might have separate the useful substance from the mixture.

For example rice and pulses are cleaned before they cooked. We find tiny stones, small insects and husk among the grains. We need to separate them before they are cooked.

Substances are mostly available in mixtures. A few substances are available in pure form. Hence, we need to separate the useful substance from the mixture by using some methods.

Before studying the various separation methods we should know the need to separate the components of a mixture.

Need for separation of substances
Substances need to be separated from each other to remove impurities or to get two or more useful products.

Separation of substances can be carried out under two conditions

• Dry conditions

• Wet conditions

2. SEPARATION OF SUBSTANCES UNDER DRY CONDITIONS
There are various methods for separating substances, such as handpicking, threshing, winnowing and sieving.

I. Handpicking
It involves simply picking out substances by hand and separating them from others. The substances being separated may be impurities that have to be thrown away or it may be that both the substances being separated are useful – such as if you separate green grapes from black ones from a mixture of the two.

II. Threshing
This is done after harvesting of crops. Once wheat is harvested, the stalks are dried. Then, each grain of wheat is separated from the stalks to which it is attached, so that it can be ground into flour. This method of separating grains from stalks is called threshing. It is basically the beating of the dry stalks to shake off the dried grains

It can be done by hand, by cattle, or by using machines. Traditionally, threshing was done by hand, but cattle help to do this job quickly. Nowadays, threshing machines are also used to separate large quantities of grain at a time. One of these machines is a combine harvester

III. Winnowing
Once the wheat has been threshed, the grains should be cleared of the husk and chaff before grinding into wheat flour.

The process of separation of lighter and heavier particles from the mixture by wind or blowing air is called winnowing.

The chaff is separated from the grains by winnowing. In this method the farmer gently drops the mixture to the ground from a height. The heavier grains are collected at a place, while the lighter chaff blows away.

IV. Sieving
Sieving is used to separate substances that are of different sizes. In this method a sieve is used. The smaller components of the mixture can pass through the pores of the sieve whereas bigger components of the mixture are retained by the sieve.

For example, Wheat flour has some fine powdered wheat as well as some bigger impurities. When this flour is put through a sieve, the fine powder falls through the small holes in the sieve, while the thicker impurities remain as they are too big for these holes. And hence, the substances are separated.

V. Magnetic separation
This method is used when one of the components is magnetic. One of the components of the mixture is a magnetic substance (iron, cobalt; nickel and steel or their oxides are magnetic in nature).

Example: The mixture of iron filings and sulphur powder can be separated by using magnets.

3. METHODS OF SEPARATION OF SUBSTANCES UNDER WET CONDITIONS
Some solid particles are insoluble in water. For example chalk powder, dust particles, sand, and tiny pieces of straw are insoluble in water. To separate this type of particles generally we use the following methods

I. Sedimentation and Decantation
The process of separating insoluble substances which are heavier than liquid by allowing them to settle down on their own due to gravity is called sedimentation. The particles which settle down during sedimentation are called sediments.

Decantation follows sedimentation. In this process the clear upper liquid is poured out from the container without disturbing the sediment. The upper clear liquid is called supernatant. For example muddy water contains heavier particles like sand and soil. Leave this muddy water undisturbed for some time. The heavier soil and sand particles settle down. And the clear upper water is poured out from the container.

II. Filtration
Filtration is a process by which insoluble solids can be removed from a liquid by using a filter paper.
A filter paper is a special type of paper which has pores that are tiny enough to let only liquids pass through it. If you pass a solution through filter paper, any undissolved solid particles will get left behind on the paper whereas the liquid will filter through.
The liquid that passes through is called the filtrate and the undissolved solid particles are called residue.
Example: A mixture of chalk powder and water can be separated by this method. To separate the soluble solids form liquids we use evaporation and condensation methods.

III. Evaporation
The process of conversion of water into vapour by heating it to its boiling point is called evaporation. The salt can be easily obtained from the salty water by the process of evaporation. If we boil this water, the water evaporated completely, leaving behind only the salt.

IV. Condensation
The process of conversion of water vapour into its liquid form is called condensation.

Example: The water vapour condenses and we receive the same water in the form of rain.
If you pour cold water in a glass, you will observe the formation of water droplets on the outer surface of the glass This is due to the condensation of water vapour present in atmosphere.

4. OTHER METHODS OF SEPARATION

I. Distillation
Distillation is used to separate and collect a liquid from a solution of a soluble solid. The solution is heated in a flask until the liquid boils, the vapour produced in the flask is passed into a condenser where it condenses into a liquid. The pure liquid (distillate) is collected in a beaker. Eg: Pure water can be separated from the salt water by distillation process.

II. Sublimation
This method is used to separate volatile solids from a nonvolatile solid. The mixture is taken in a china dish, upon heating it, the volatile substance vaporizes and gets deposited on the walls of the funnel and the non-volatile substances are left in the dish.
Eg: Iodine & sand where Iodine sublimes.

III. Crystallization
Crystallisation is used to separate liquid-solid compounds. In this process, liquid is evaporated by heating, leaving the solid remains in the crystalline form.
Eg: Salt, Ammonium chloride from their aqueous solutions.

IV. SEPARATION OF SUBSTANCES USING MORE THAN ONE METHOD
A mixture of sand and salt can be separated by combination of methods. The first method is sedimentation and decantation. This mixture is put in water and left for the sand to settle for some time. Then, we will decant the salty water, which will separate the sand from the mixture. Now the salt can be separated from the water by evaporation. The water will boil away, leaving the salt behind. So, the mixture of the sand, salt and water has been separated successfully using a combination of sedimentation, decantation, evaporation and condensation.

5. SOLUTION AND SOLUBILITY

I. Solution
A solution is prepared by dissolving a substance in a liquid.

Solute + Solvent = Solution

Salt + water = Salt solution

II. Saturated solution
A solution is said to be saturated if it cannot dissolve more of the substance in it at a particular temperature.

III. Unsaturated solution
A solution is said to be unsaturated if it can dissolve more of the substance in it at a particular temperature.

IV. Solubility
Solubility is the property of a chemical substance which is either solid, liquid or a gas which is called as a solute dissolves in another chemical substance called solvent which can be solid, liquid or a gas to form a homogeneous solution of the solute in the solvent. The solubility of a substance depends upon the solvent as well as temperature and pressure.
The maximum amount of a substance that can dissolve in 100 g of a solvent at a given temperature is known as the solubility of the substance.

Solubility of the Compounds
All common salts of lithium, sodium, potassium, rubidium, cesium and ammonium ion are soluble in water.
All binary compounds of chlorine, bromine, iodine are soluble except with silver, mercury and lead are soluble in water.

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