Separation

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TEACHER’S GUIDE

CHEMISTRY

TOPIC: OBSERVATIONS AND EXPERIMENTS

USEFUL TIPS:

Student’s experiments can be done in groups.
Groups can be given different mixtures. Each group then carry out their own experiment, makes a write up of their procedure and presents to the rest of the class together with the samples obtained.

UNIT 1: MIXTURES AND PURE SUBSTANCES

This unit is suitable for senior one (S1)

BRIEF DESCRIPTION OF THE UNIT:

This unit deals with:
• Methods of separating mixtures.
• Methods of drying solids.
• Methods of determining purity of solids and liquids.

LEARNING OBJECTIVES
Students should be able to:
➢ Define the key terms such as solution, solvent, solute, filtrate, residue, immiscible and miscible liquids, distillate, sublimation, melting and boiling points and chromatography. ➢ Use melting and boiling points to differentiate between pure and impure substances. ➢ Select appropriate methods for separating given mixtures. ➢ Describe appropriate methods of separating given mixtures. ➢ Perform simple experiments involving separation of mixtures.

TIME REQUIRED: Minimum: 16 periods of 40 minutes each

MAIN CONTENT AND CONCEPTS TO EMPHASISE:

• Criteria of purity: melting and boiling points for determining whether a substance is a mixture or pure. • Types of mixtures: uniform and non-uniform mixtures. • Provide definitions for the following terms:

i. Solutions i.e. alloys as solid solutions and air as gaseous solution. ii. Solvent i.e. water as a universal solvent, ethanol, etc. iii. Solute.
iv. Filtrate.
v. Residue.
vi. Immiscible and miscible liquids.
vii. Distillate.
viii. Sublimation i.e. examples of substances that sublime – iodine, ammonium salts, iron (III) chloride, aluminium chloride, etc. ix. Melting and boiling points.
x. Chromatography.

TEACHING/LEARNING MATERIALS:

• Criteria for purity: Teacher demonstration experiments. i) Melting point: Illustrate with pure naphthalene and with a mixture of naphthalene and a little camphor and water. Apparatus required include, a 250cm3 glass beaker, boiling tube, thermometer (0o -100oC), Tripod stand and a wire gauze. (ii) Boiling points: Illustrate with pure ethanol and a mixture of ethanol and water. • Drying of solids: Sun drying, filter paper and drying agents (e.g. anhydrous calcium chloride, concentrated sulphuric acid, silica gel, calcium oxide etc). • Apparatus required: A U-tube, wash bottle, Desiccators and oven.

TEACHER’S NOTES

METHODS OF SEPARATING INSOLUBLE SOLIDS FROM LIQUIDS
• Decanting: For separating insoluble solids with heavy particles from liquids. I.e. mixture of sand and water. • Filtration: For separating insoluble solids with fine particle from liquids e.g mixture of chalk powder and water. • Centrifugal: For separating fluids of different densities (e.g. Cream from milk) or liquids from solids in suspensions.

METHODS OF SEPARATING SOLUBLE SOLIDS FROM LIQUIDS
• Evaporation (e.g. sodium chloride from water).
• Simple distillation: Used when both solid and liquid is to recovered. • Crystallisation: For separating soluble solids from liquids by allowing them to form crystals. • Fractional crystallisation: For separating two or more solutes with different solubility at the same temperature form a liquid.

METHODS OF SEARATING COLOURED SUBSTANCES IN SOLUTION

• Chromatography: Uses the difference in rates of movement of solutes over a porous medium like filter paper by a moving solvent (e.g. ink, chlorophyll, dyes, etc).

METHODS OF SEPARATING LIQUID-LIQUID MIXTURES

• Separating funnel: For immiscible liquids (e.g. Paraffin and water). • Simple distillation: For miscible liquids with boiling points widely far apart. • Fractional distillation: For separating miscible liquids with closely...
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