Separation Of Mixture

           

Introduction

            The method of separation of impurities in liquid to obtain a pure substance is called separation of mixture. The components of mixture may be two or more than two and their separation method depends on the nature of the substances like melting points, boiling points, particle size, etc. the impurities present in water may be insoluble or soluble. They can be separated by following methods.

Method of separation of insoluble substances from a liquid:

1.      Decantation

2.      Filtration

3.      Centrifuging

Method of separation of soluble substances from a liquid:

1.      Evaporation

2.      Distillation

3.      Crystallization

4.      Chromatography

5.      Sublimation

Decantation

            The process of separation of clear liquid from the sediment (solid substance) is known as decantation. This method is applied when one component is water or liquid and the other solid (heavier than liquid).  Example, mixture of sand and water.

Filtration

            It is a process by which insoluble solid particles from a solution is separated by filter paper. For a complete and quick separation of impurities filtration process is widely used in laboratory. It is better process than sedimentation or decantation.

            The solution is carefully poured in a funnel fitted with filter paper. The pores in filter paper allow only liquid to pass through. The diameter of the pore is about 10-5 cm and the solid which cannot pass through filter paper remain in the filter paper. This is called residue. The clear liquid which we get is called filtrate.

Experiment: A mixture of sand and water is taken. With the help of a rod the solution is allowed to pass through the filter paper. The filtrate is collected in a beaker and residue remains in the filter paper.

Fig:Filtration

While pouring solution in the funnel, following precautions are to be taken.

1.The filter paper should not be torned.

2.The filter paper should be attached with the inner surface of funnel.

3.The filter paper should occupy ¾ of the funnel.

4.A glass rod should be used while transferring the liquid in the funnel.

Centrifuging

            The method of separation of solid from a solution with the help of a centrifuge, a device used for this process, is called centrifuging. This method is effective when solid substance to be separated is very less. This method is used to separate precipitate from a solution. Cream is separated from butter by this method.

Fig:Centrifuging

Experiment: in this method the solution is passed in a test tube and fitted as shown in a centrifuge. The apparatus is rotated at high speed so that the test tube also rotates. As a result the solid gets deposited at the bottom of the test tube. Then, by decantation it may be poured in the other test tube.

Evaporation 

            By this method a solvent is changed into vapour and solid is collected. The solution is heated to high temperature. When the components are miscible, this method is applied. Likewise, salt water can be separated by this method. The solute is deposited due to dehydration.

Experiment: The given mixture is taken in a evaporating dish, China dish (or in a watch glass). The dish is heated slowly so that it will not crack. Sometimes heating may be done on a steam bath. The solvent evaporates gradually and the solid remains deposited. Thus, if a solute under heating does not melt & solvent vaporizes, this method may be applied.

Fig: Evaporation