Class 9 Chemistry Is Matter Around Us Pure Separation of Mixture of a Solid and a Liquid

Separation of Mixture of a Solid and a Liquid

  • All the mixtures containing a solid and a liquid are separated by the one of the following processes:-
  1. By filtration
  2. By centrifugation
  3. By evaporation
  4. By crystallization
  5. By chromatography
  6. By distillation
  7. By Filtration
    1. Filter paper is a round piece of special paper which has millions of pores in it.
    2. The liquids can pass through the tiny pores of a filter paper but solid particles are left behind.
    3. The solid which remains behind on the filter paper is called residue.
    4. The liquid which passes through the filter paper is called filtrate.
    5. A mixture of sand and water can be separated by filtration.
    6. Filtration cannot remove any solid substances which are dissolved in a liquid.

Class_9_Is_Matter_Around_Us_Pure_Matter_Filtration

  1. By Centrifugation
  • Centrifugation is a method for separating the suspended particles of a substance from a liquid in which the mixture is rotated at a high speed in a centrifuge.
  • The mixture of fine suspended particles in a liquid is taken in a test-tube. This tube is placed in centrifuge machine and rotated rapidly for some time.
  • As the mixture rotates round rapidly, a force acts on the heavier suspended particles in it and brings them down to the bottom of the test-tube.
  • The clear liquid, being lighter remains on the top.
  • Separating cream from milk:-
    • The process of centrifugation is used in diaries to separate cream from milk.
    • The milk is put in a closed container in big centrifuge machine.
    • When the machine is switched “on”, the milk is rotated at a very speed in its container. As a result milk separates into “cream” and skimmed milk.
    • The cream being lighter floats over the skimmed milk.

Class_9_Is_Matter_Around_Us_Pure_Matter_Centrifugation

  1. By Evaporation
  • The changing of liquid into vapours (or gas) is called evaporation.
  • This process is used to separate a solid substance that has dissolved in water (or any other liquid).
  • The use of this process for separating a mixture is based on the fact that the liquids vaporise easily whereas solids do not vaporise easily.
  • For Example: - The common salt dissolved in water can be separated by the process of evaporation.
  • This process is used to on a large scale to obtain common salt from sea-water.
  • Evaporation is also used in recovering dissolved solid substances from liquid mixtures (or solutions) but the liquid itself cannot be recovered by this method. The liquid vaporises and gets lost to the air.

Class_9_Is_Matter_Around_Us_Pure_Matter_Evaporation

Getting salt from sea water by the process of evaporation.

  1. By Crystallisation
  • The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallization.
  • Crystallisation technique is better than simple evaporation technique as –
    • Some solids decompose or some, like sugar, may get charred on heating to dryness.
    • Some impurities may remain dissolved in the solution even after filtration.
    • On evaporation these contaminate the solid.
  • Following are the steps involved in obtaining pure solid substance from impure sample:-
  1. The impure solid substance is dissolved in the minimum amount of water to form a solution.
  2. The solution is filtered to remove insoluble impurities.
  3. The clear solution is heated gently on a water bath till a concentrated solution or saturated solution is obtained.
  4. Allow the hot, saturated solution to cool slowly.
  5. Crystals of pure solid are formed. Impurities remain dissolved in solution.
  6. Separate the crystals of pure solid by filtration and dry.
  7. For Example: -
    1. Impure copper sulphate can be purified by the method of crystallisation.

 

Class_9_Is_Matter_Around_Us_Pure_Matter_Crystallization

Class_9_Is_Matter_Around_Us_Pure_Matter_Crystallization_1

 

  1. Separation by Chromatography
  • Chromatography is a technique of separating two or more dissolved solids which are present in a solution in very small quantities.
  • This separation is based on the fact that though two or more substances are soluble in the same solvent but their solubilities may be different.
  • A special type of paper called “chromatography paper” is used for carrying out separations by chromatography.
  • Black ink is a mixture of several coloured substances which can be prepared by paper chromatography.
  • Applications of chromatography:-
  1. It is used to separate solutions of coloured substances.
  2. Used in forensic science to detect and identify trace amounts of substances (like poisons) in the contents of bladder or stomach.
  3. Also used to separate small amounts of products of chemical reactions.

Class_9_Is_Matter_Around_Us_Pure_Matter_Chromatography

Separation of dyes in black ink using chromatography

  1. Separation by Distillation
  • The process of distillation is used to get both salt as well as water from salt-water mixture.
  • It is process of heating a liquid to form vapour, and then cooling the vapour to get back liquid.
  • Liquid <-> Vapour (or Gas)

         (Heating/Cooling)

  • The liquid obtained by condensing the vapour is called “distillate”.
  • When a homogeneous mixture of solid and a liquid is heated in a closed distillation flask, the liquid, being volatile, forms vapour.
  • The vapours of liquid are passed through a “condenser” or “Liebig condenser” where they get cooled and condense to form pure liquid.
  • This pure liquid is collected in a separate vessel.
  • The solid, being non-volatile, remains behind in the distillation flask.

Class_9_Is_Matter_Around_Us_Pure_Matter_Distillation

Separation of two miscible liquids by distillation

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