To make use of the materials around us, we need methods for physically separating the many and varied mixtures that we come across. One of the distinctive characteristics of a mixture of substances is that it is usually possible to separate the constituents by physical means. There are many different physical methods used to separate a wide variety of mixtures. The particular method employed to separate any given mixture depends upon the nature of its constituents. 

The Different Methods of Mixture Separation.

1. Mechanical Separation.



2. Magnetic separation

If the solid mixture contains iron, the iron can be removed using a magnet. This method is used to separate scrap iron from other metals. Magnetic iron ore can be separated from other material in the crushed ore by using an electromagnet. In the process of recycling metals, iron objects can be picked out from other scrap metals using electromagnets.


3. Evaporation

This method is used to separate substances that form a solution. In such a mixture, the solute is completely dissolved in a solvent to make a uniform solution. To separate these substances, the solution is heated so that the solvent evaporates, leaving the solid residue behind. A mixture of salt or sugar in water can be separated by applying this method.



4. Simple distillation

Separating a liquid from a solution can be carried out by distillation. The boiling point of a liquid is usually very much lower than that of the dissolved solid. The liquid can easily be evaporated off in a distillation flask. It is condensed by passing it down a water-cooled condenser and then collected as the distillate. This method can be used to obtain pure water from impure water or from water with dissolved impurities. The process may be used to separate a liquid from a solution or to separate two liquids whose boiling points differ by an appreciable temperature interval. This is a way of getting a pure solvent out of a solution.



5. Fractional distillation

Separating the liquids from a mixture of two (or more) miscible liquids is again based on the fact that liquids will have different boiling points. However, the boiling points are closer together than for solid-in-liquid solutions. It is difficult to separate mixtures of liquids whose boiling points differ by only a few degrees. In this case, fractional distillation is used.
For example, ethanol boils at 78°C whereas water boils at 100°C. 



When a solution of ethanol and water is heated, ethanol and water vapours enters the fractionating column. Evaporation and condensation take place as the vapours rise up the column. Ethanol passes through the condenser first as the temperature of the column is raised above the boiling point. Water condenses in the column and flows back into the flask because the temperature of the column is below its boiling point of 100°C.

The temperature on the thermometer stays at 78°C until the ethanol has distilled over.
Eventually, the thermometer reading rises above 78C°. This is a sign that all the ethanol has been separated, so heating can be stopped. By watching the temperature carefully, the two liquids (fractions) can be collected separately.

Various forms of fractionating column can be used. Their general purpose is to provide surfaces, e.g. flat discs, on which ascending vapour can condense. Glass beads in the column provide a large surface area for condensation.

6. Sublimation

This is a technique used to separate a mixture of solids where one of the solids sublimes.
Examples of solids which sublime are ammonium chloride, iodine, solid carbon dioxide and
naphthalene. A mixture of any of these solids with another solid can be separated by sublimation.

Let us consider a mixture of iodine and sodium chloride. The mixture is placed in a beaker and covered with a filter funnel as shown in the diagram below. Then, as the mixture is heated, the ammonium chloride sublimes. The ammonium chloride vapour rises and condenses on the cooler walls of the filter funnel. The sodium chloride is left in the beaker.
Sublimation of ammonium chloride

7. Chromatography

This method is commonly used to separate a mixture of coloured substances (solids or dyes). An example of this is the separation of dyes that make up black ink. Chromatography works better when a solvent is used. The commonest solvent is water, though other solvents such as ethanol or ether may be used for those substances that do not dissolve in water.
 

There are two types of chromatography, namely column chromatography and paper chromatography. The two types of chromatography follow the same principle, but paper chromatography is the simplest form to set up, and hence is more commonly used. 

On which principle does chromatography work? 

Uses of Chromatography

Chromatography is used in many different ways. The following are some of the application of chromatography:
1. It can be used to find out the components of a liquid or solid, or even to identify different
substances.
2. It can be used by security agents and medical personnel to analyse blood and urine
samples.
3. Causes of pollution in water and in animals that live in water can also be detected using
chromatography.
4. In chemistry, chromatography is used to test the purity of substances and in separation of
mixtures.

8. Layer separation

Mixtures of two immiscible liquids can be separated with a separating funnel. The mixture is placed in a separating funnel and allowed to stand. The liquids separate into two different layers. The lower dense layer is then "tapped" off at the bottom.


For example, when a mixture of kerosene and water is poured into the funnel, the kerosene floats to the top as shown in figure above. When the tap is opened, the water runs out. The tap is closed again when all water has gone, leaving the kerosene in the funnel.

9. Solvent extraction

Solvent extraction, also known as liquid-liquid extraction, refers to the separation of materials of different chemical types and solubilities by selective solvent extraction. That is, some materials are more soluble in one solvent than in another. The method is used to refine petroleum products, chemicals, vegetable oils, and vitamins.

This method is used is to separate a solid from a solution in which there is more than one solid dissolved. 


The solvent extraction works on two principles:
e.g. Separating iodine from sodium chloride by solvent extraction
1. One solid in the solution must be more soluble in the extracting solvent than the other.
2. The extracting solvent must not be miscible with the solvent in which the mixture of
solids is dissolved. Neither should it react with it.

10. Centrifugation

A centrifuge is used to separate small amounts of suspension. Centrifugation is used with
insoluble solids where the particles are very small and spread throughout the liquid. In
centrifugation, test tubes containing suspensions are spun round very fast. 

                           

The solid gets thrown to the bottom. Here, it is no longer the force of gravity on the solid that causes settling. Instead, there is a huge centrifugal force acting on the particles due to the high speed spinning of the samples. This causes the solid to be deposited at the bottom of the centrifuge tube.

Separation by centrifugation

After centrifugation, the liquid can be decanted (poured out) from the test tube, or removed with a small pipette. This makes the solid to be left behind.



11. Decantation

This is another method that can be used to separate mixtures called suspensions. However, in this case, separation will be successful if the suspended particles are large enough. Otherwise, the decantation exercise should be accompanied by filtration if you want to get a clear liquid. 



Once the solid has settled to the bottom of the container (sedimented), the liquid can be carefully poured off. This is called decantation. Decantation can be applied to separate such components as mixtures of mud, sand or gravel in water and so on.

12. Crystallization

This process involves evaporation but the speed of evaporation is much slower. In principle the salt solution can be left in the evaporating basin for a long period until all the water has
evaporated but in practice this takes longer time. The process begins by evaporating away the liquid. However, because the crystals are needed, evaporation is stopped after the solution has been concentrated enough. 



The concentrated solution is allowed to cool slowly and crystallize. The crystals so formed can be filtered off and dried. A similar process is used to extract salt from the sea. Salty sea water is placed in wide basins and put in the sun. Water evaporates off, leaving the salt crystals in basins.

13. Winnowing or threshing

This is a method used to separate grains from husks or bran. The process makes use of the
differences in density of the constituents in the mixture. When the winnower is shaken around, grains, being denser than husks or bran, sink to the bottom of the winnower
The less dense husks or bran moves to the top. They are then blown off the winnower by wind or breath, or sometimes picked by hand and separated from the grains.



14. Filtration

This method is best applicable in separation of components of mixtures called suspensions.
A mixture of chalk dust or flour with water can be separated by filtering the suspension. The
suspended particles get trapped in the filter paper. The trapped particles are called the residue. The water is called the filtrate