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Methods of Separation of Mixtures

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Methods of Separation of Mixtures - Lesson Summary

Separation process:  The process of separating the constituent substances of a mixture by physical methods, taking advantage of the differences in their physical properties is called separation process.
Commonly used separation methods are

Separation using magnets:  This method is used when one of the components is magnetic.
Example: The mixture of iron filings and sulphur powder can be separated by using magnets.

Evaporation:  Evaporation is the process of vaporizing the solvent to obtain the solute. Evaporation is used to separate a mixture containing a non-volatile, soluble solid from its volatile, liquid solvent.
We can separate salt from a solution by evaporating the water from the solution.

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.
Centrifugation:  If the solid particles are very small and pass through a filter paper, then centrifugation  process is used for the separation of insoluble solid particles from a solid-liquid mixture.

Principle involved in centrifugation:  The principle is that when the liquid is spun rapidly, the denser particles are forced to the bottom and the lighter particles stay at the top.
Example: Centrifugation is used for blood and urine testing in diagnostic laboratories, in dairies to separate butter from cream, and in washing machines to squeeze out water from clothes.

Separating funnel:  When two liquids do not mix, they form two separate layers and are known as immiscible liquids. These two liquids can be separated by using a separating funnel.

A separating funnel is a special type of glass funnel, which has a stop-cock in its stem to regulate the flow of liquid. It will separate the immiscible liquids into two distinct layers depending on their densities. The heavier liquid forms the lower layer while the lighter one forms the upper layer. Remove the stopper and open the tap to run the lower layer into a beaker. You will be left behind with just the upper layer in the funnel. Collect this liquid into another beaker.

Examples: Kerosene and water mixture is separated by using separating funnel method.
This method is also used to extract iron from its ore. 

Sublimation:  Sublimation is the process in which solid directly changes to gaseous state.
Example: Salt and a sublimable solid such as ammonium chloride, can be separated by the process of sublimation.

Chromatography:  Chromatography is a method used to separate mixture that comprises solutes that dissolve in the same solvent.This method gets its name from the Greek word for colour —Kroma, as it was first used for separating colours.

Principle: Chromatography is based on differential affinities of compounds towards two phases, i.e stationary and mobile phase.

The fraction with greater affinity towards stationary phase travels shorter distance while the fraction with less affinity towards stationary phase travels longer distance.
Chromatography is used for separating colors in a dye, pigments from natural colors and drugs from blood.
Based on nature of stationary and mobile phases chromatography is classified into following types

      •  Paper chromatography
      •  Column chromatography
      •  Thin layer chromatography
      •  Gas chromatography

Paper chromatography: In paper chromatography the stationary phase is paper and the mobile phase is any suitable liquid.

Seperation of components of ink:
      •  First take a thin, long strip of filter paper. Use a pencil to draw a line on it, about 3 cm above the lower edge. Then, put a small drop of black ink.
      •  On the filter paper in the centre of the line and allow it to dry.
      •  Finally, lower the filter paper into a jar containing water so that the drop of ink on the paper is just above the water level. Don’t disturb the jar.
      •  After some time you will observe different coloured spots on the paper.

The ink has water as the solvent and the dye is soluble in it. As the water rises, it takes the particles of dye along with it. Since a dye is made of two or more colours, the colour which is the most soluble rises faster and higher. This is why there are differently coloured spots on the paper.

Distillation:  This method is used for the separation of a mixture containing two miscible liquids that boil without decomposing and have a large difference between their boiling points.
Process of conversion of a liquid into vapour by boiling, and then recondensing the vapour into liquid is called distillation. 

Apparatus:  Distillation process requires a distillation flask, thermometer, heating assembly, a receiver flask and condenser as the apparatus. A distillation flask is a round-bottomed flask with a tube at its neck.  This tube is attached to a Leibig condenser. The Leibig condenser is a long glass tube within a glass jacket, with an inlet and outlet for water. The open end of the flask is fitted with a one-holed rubber cork through which a thermometer is introduced.

Principle:  Distillation process involves heating a liquid to its boiling point such that the liquid passes into its vapour state. The vapour's are condensed in a condenser and transformed into liquid form. The pure liquid is collected from the condenser in a receiver.
Example: A mixture of acetone and water can be separated by the process of distillation.

Separation of mixture of water and acetone:  Put the mixture into a distillation flask. Heat the mixture. You will see that the acetone, which has a lower boiling point, vaporizes first and then condenses in the condenser. It can be collected from the condenser outlet. Water gets left behind in the flask. 

Fractional distillation method:  In case the difference in the boiling points of the liquids is less than 25K temperature, we use the fractional distillation method.
The apparatus is almost the same as used in distillation. The only difference is that a fractioning column is fitted in between the distillation flask and the condenser. A simple fractioning column is made up of a tube packed with glass beads. The beads provide the surface for the vapours to cool and condense again and again. The fractioning columns obstruct the smooth upward flow of vapours.

Example: A mixture of n-hexane and n-heptane can be separated through the process of fractional distillation.
Put the mixture into a distillation flask. Heat the mixture. The vapours of, n-hexane has a lower boiling point pass through and get condensed in the condenser. n-heptane, which has a higher boiling point, condenses and flows back into the distillation flask.

The gases in the air are separated from one another by the fractional distillation of liquid air.
Air is made up of different gases like nitrogen, oxygen and carbon dioxide. These gases are separated from one another by the fractional distillation of liquid air.

Steps involved are,
Air is compressed in the compressor and cooled in the refrigeration unit. Thus, the air gets liquefied.
The liquid air is passed through a filter to remove impurities and then fed into a tall fractional distillation column.
On warming, liquid nitrogen distils first because it has the lowest boiling point of -196 0C. Liquid argon has a slightly higher boiling point of -186 0C, so it distils next. Liquid oxygen has the highest boiling point of -1830C, it left behind. 

Crystallisation: Crystallisation is a separation and purification method which involves the precipitating of solid crystals from its saturated solution on cooling.
In this process the impure sample is dissolved in minimum amount of suitable solvent. The formed solution is heated to get a saturated solution. On cooling, this saturated solution produce pure crystals of the sample.

Crystallisation is used for:  Purification of salt that we get from sea water and separation of crystals of alum from impure samples.


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