SS1 Chemistry Lesson Note on Separating a Mixture of an Insoluble Solid and a Liquid

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SEPARATING A MIXTURE OF AN INSOLUBLE SOLID AND A LIQUID.

1. DECANTATION: The mixture is allowed to stand for some time until it separates into two distinct layers and an upper clear liquid layer. The clear liquid can be carefully poured or decanted into a second container.
2. CENTRIFUGATION: This involves the use of a centrifuge to separate solid particles from a liquid as shown in the diagram below. As the centrifuge spins the mixture, the solids separates and settles at the bottom of the test tube while the liquids on the top layer can easily be decanted.
3. FILTRATION: As shown below, the mixture is poured into s porous material (filter paper) folded inside a funnel. The solid particle that remains inside the liquid that drips through the filter paper is known as the filtrate.

EVALUATION:

1. Explain briefly, how a mixture of sand and water can be separated.
2. Explain how plasm can be separated from blood.

PERIOD 4: SEPARATING A SOLUBLE SOLID FROM A LIQUID

1. Evaporation: Evaporation method is used to recover a solid solute from a solution in which it is soluble to give a solution. The method is suitable if the solid has a high melting point i.e. cannot be decomposed by heating.

Evaporation is based on the large different between the boiling points of the solid and solvent. For example, common salt can be recovered from its aqueous solution by complete evaporation of water. The solvent (liquid) is usually sacrificed.

Evaporation process

Note: Evaporation method is not suitable for salts that can easily be destroyed by heating.

2. Crystallization: Crystallization is a method used to separate salts which decompose easily on heating from their solutions. The salt solution (the mixture) is heated to drive away some of the liquid (i.e. to evaporate some of the liquid) (i.e. to evaporate some of the liquid) until solution becomes concentrated or saturated. The concentrated solution remaining is allowed to cool slowly resulting in the formation of crystals. Crystal formation can be induced by (i) adding crystals of the same salt to serve as seed. (ii) Scratching the inside of the vessel containing the solution.

Note: If all the liquid is evaporated a powder will be obtained and not crystals. This powder might also contain impurities which otherwise would have remained in the solution and not contaminate the crystals. Many crystals formed on cooling saturated solution contain water which is chemically combined and loosely bonded to the crystals. This water is called water of crystallization. Salts which contain water of crystallization are said to be hydrated. Those which do not are anhydrous. Those are often powders.

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