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
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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. . 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. . Paraffin and water). • Simple distillation: For miscible liquids with boiling points widely far apart. • Fractional distillation: For separating miscible liquids with closely related boiling points (e. g. Petrol and paraffin, ethanol and water). METHODS OF SEPARATING SOLID-SOLID MIXTURES • Sublimation: Used when one of the solids sublime on heating (iodine and sand). • Magnetisation: Used when the mixture consist of a metal and non-metal (iron filing and sulphur). • Hand picking and winnowing: Used when the solid particles of one of the solids are big (Stones and rice). METHODS OF DRYING SOLIDS Pressing between filter papers. • Sun drying. • Oven drying. • Desiccators with drying agents. APPLICATION OF METHODS OF SEPARATING MIXTURES • Filtration: Sewage and water treatment (stone and sand filter beds). [pic] • Simple distillation: Production of local alcohol. [pic] • Fractional distillation: Petroleum refinery, manufacture of spirits and separation of liquid air into oxygen and nitrogen. • Fractional crystallisation: used in the extraction of sodium chloride (common salt) from lake Magadi in Kenya and lake Katwe in Uganda. • Chromatography: Used in analysing chemicals e. g. lant extracts in chemotherapeutics research laboratories. • Centrifugal: Used in clinical laboratories separate blood into cells (corpuscles) and plasma. TEACHER’S DEMOSTRATION EXPERIMENTS FOR SEPARATING MIXTURES FILTRATION • Illustrate with a mixture of chalk powder and water. • Apparatus: Filter funnel, Filter paper and a conical flask. DECANTING • Illustrate with a mixture of sand and water. • Apparatus: Two beakers. CENTIFUGAL • Illustrate with a mixture of chalk powder and water. • Apparatus: A centrifuge and beaker. SIMPLE DISTILLATION • Illustrate with a mixture of sodium chloride and water( common salt). Requirements: simple distillation kit, running water and heat source. FRACTIONAL DISTILLATION • Illustrate with a mixture of ethanol and water. • Requirements: Fractionating column , simple distillation kit, running water and heat source. SEPARATION OF IMMISCIBLE LIQUIDS • Illustrate with a mixture of paraffin and water. • Apparatus: Separating funnel and a conical flask. CHROMATOGRAPHY • Illustrate with black ink. • Requirements: Filter paper, beaker and water. EVAPOURATION • Illustrate with sodium chloride solution. • Requirements: Evaporating dish and heat source. CRYSTALLISATION Illustrate with sodium chloride solution. • Requirements: Evaporating dish, filter papers, filter funnel, conical flask ,a beaker and heat source. SUBLIMATION • Illustrate with a mixture of iodine and sand. • Requirements: Evaporating dish, filter funnel and heat source. MAGNETISATION • Illustrate with a mixture of copper powder and sulphur powder. • Requirements: Magnets. STUDENTS’ EXPERIMENTS Ask students to conduct simple experiments to bring out the concepts of uniform and non uniform mixtures, miscible and immiscible liquids, soluble and insoluble solids, solute and solvent.
For instance, by asking students to mix the following: i. Sand and water. ii. Sodium chloride and water. iii. Paraffin and water. iv. Alcohol and water. v. Iron and sulphur. Additional students’ experiments may include: i. Filtration: Filter muddy water using filter paper and various materials as filter paper( e. g. cotton wool, cotton cloth, grass etc ). ii. Distillation: Distillation of local alcohol. iii. Separating funnel: Separation of a mixture of cooking oil and water. iv. Sublimation: Separation of a mixture ammonium chloride and sand. v.
Evaporation: copper (II) sulphate solution. vi. Crystallisation: Copper (II) sulphate crystals from copper (II) sulphate solution. vii. Chromatography: Chlorophyll using ethanol as solvent. viii. Magnetisation: Separation of a mixture of iron filing and sulphur. EXTENSION WORK AND STUDENT’S PROJECTS Ask students to perform individual projects on decanting, filtration, distillation, crystallisation and chromatography. Decanting ? Extraction oil from seeds: Roast, pound, grind, boil with water until oil floats then decant off the oil. Filtration Prepare sand filter beds. ? Make filter funnels from plastic bottles and large leaves. Distillation ? Model distillation kit using clay. ? Distil local alcohol. Crystallisation ? Growing of copper (II) sulphate crystals. ? Chemical forest made growing crystals of different chemicals in glass water. Chromatography ? Analysis of dyes in coloured sweets, sodas and a mixture of phenolphthalein and methyl orange indicators. STUDY TOUR Arrange to carry out class study tours to the following facilities: • Filtration: National water and sewage treatment plants ( e. g.
Bugolobi sewage treatment plant and Gabba water treatment plant in Uganda). • Distilleries: Uganda distilleries Luzira- Kampala and Kenya petroleum refinery Mombasa. • Chromatography: Chemotherapeutic research laboratories( e. g. Chemotherapeutics research laboratory Wandedeya- Kampala Uganda • Centrifugal: Clinical laboratories( e. g. In hospitals). ICT APPLICATION Use animation capabilities to show separation of a mixtures of red, blue, green, and yellow dyes by chromatography. REFERENCES: • Form One Chemistry by Patel. • Complete Junior Chemistry by Atkinson New Certificate Chemistry by Holderness and Lambert. • ’O’ Level Chemistry for East Africa by M. C. Sinclain. • Key Science by Eileen. • GCSE Chemistry First Examination By Blackie. WEB LINKS • GCSE Chemistry Notes on Elements, compounds, separation. http:/www. wpbschoolhouse. btinternet. co. uk/page01/EleCpdMix/EleCmdMix. htm • Separation process. http://en. wikipedia. org/wiki/separation_ process • Separation of mixtures and chromatography. http://www. 800mainstreet. com/e3/e3. html # top • Separation of mixtures and compounds. http://www. iit. edu~smile/ ch9130. tml • GCSE Chemistry mixtures- Definition-Separation-Physical http. //www. gcsescience. com/e4. html Separation of mixtures. http://dbhs. wvsd. k12. ca. us/webdocs/matter/2. 6 SeparationofMixtures. html STUDENTS’ EXERCISES 1. Name one method that can be used to separate the mixture of the following substances; i) Sodium chloride and chalk powder ii) Coloured dyes in sun –sip straw berry drink. iii) Sodium chloride and aluminum chloride iv) Petrol and water v) Paraffin and diesel vi) Iron filings and ash. vii) Copper(II) oxide and copper(II) sulphate iii) Copper(II) sulphate and water. 2. Name two drying agents that are used to dry substances. 3. Explain how you can separate a mixture containing sodium chloride, ammonium chloride and chalk powder. 4. State the difference between the following: i) Solute and solvent ii) Miscible and immiscible liquids iii) Filtrate and distillate iv) Melting and boiling point 5. The picture above shows a local distillation set up. i) What is the role of the copper tube? ii) State why the tube is made of copper and not plastic? iii) Why is the tube coiled in the middle? [pic] 6.
Name separation apparatus you can see in the pictures below. [pic] . What is the effect of impurity on melting and boiling points of substances? ANSWERS TO STUDENTS’ EXERCISES 1. (i) Filtration then evaporation of the filtrate. (ii) Chromatography (iii) Sublimation (iv) Use of a separating funnel (v) Fractional distillation (vi) Magnetism (vii) Dissolving in water, filtration and crystallisation (viii) Crystallisation 2. Concentrated sulphuric acid and anhydrous calcium chloride. 3. The mixture is first heated where the ammonium chloride comes off as a vapour (sublimes).
The remaining mixture of sodium chloride and chalk is now then shaken with water to dissolve all sodium chloride. The mixture is then filtered. The filtrate which is sodium chloride solution is then evaporated carefully on an evaporating dish. The residue is chalk. 4. i. The solute is the substance that dissolves in a solvent. ii. Miscible liquids have a homogenous layer while immiscible liquids are separated by a clearly defined boundary. iii. Filtrate is a clear solution or substance obtained by the filtration process while a distillate is a condensed liquid obtained by distillation process. v. Melting point occurs at lower temperature than boiling point. Melting point is defined as the temperature at a solid substance changes to liquid state at atmospheric pressure. Boiling point is the temperature of a vapour of a substance boiling at atmospheric pressure. 5. (i) To convey alcohol vapour from the drum to the collection point. (ii) It is resistant to decomposition by heat. (iii) To increase the surface area for condensation of the alcohol vapour. 6. Distillation and centrifuge. 7. Impurities decrease or increase melting and boiling points.