IGCSE CHEMISTRY STUDY NOTES UNIT 2 EXPERIMENTAL TECHNIQUES

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Table of Contents

2.2.2 Methods of purification

CORE ............................................................................................. 2

• Describe and explain methods of purification by the use of a suitable

solvent, filtration, crystallisation and distillation (including use of a

fractionating column). ............................................................................. 2

Suggest suitable purification techniques, given information about

the substances involved ............................................................... 11

ASSESSMENT ............................................................................... 11

QUESTION-BANK .......................................................................... 11

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2.2.2 Methods of purification CORE Describe and explain methods of purification by the use of a suitable solvent, filtration, crystallisation and distillation (including use of a fractionating column).

Use of a suitable solvent:

• generally applied to a mixture of two solids wherein one of the solids is soluble in the chosen solvent and the other is insoluble.

• This is done by adding the chosen solvent to the mixture to dissolve the soluble solid followed by filtration.

Filtration:

• experimental technique used to separate insoluble solid/s from a liquid mixture

Filtration

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CORE Crystallisation:

• used for obtaining crystals of a (soluble) solid from its (aqueous) solution

NOTE: For IGCSE 0620 exams, crystallisation from aqueous solutions only will be assessed. Crystallisation steps:

• The aqueous solution is added to an evaporating dish and heated till some solid appears.

• The mixture (mother liquor) is then cooled for crystals to form.

• Residual liquid (if any) is removed by filtration.

• The crystals are washed with small quantity of distilled water.

• The crystals are dried by pressing between dry filter papers or by placing in a warm oven.

Crystallization assembly

The evaporating dish can be placed directly on the wire gauze. Indirect heating is applied for controlled heating / to prevent over heating / when solutions contain flammable solvents such as Ethanol.

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CORE Distillation:

• Simple distillation – used for separating a soluble solid from its solution (salt from sea water) or for separating simple liquid-liquid mixtures where the liquids have widely different boiling points

Simple distillation assembly in a school laboratory

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CORE Distillation:

• Fractional distillation – used for separating complex mixtures (many components) of liquids with different boiling points using a fractionating column examples: fractional distillation of crude oil (petroleum) fractional distillation of liquid air fractional distillation of aqueous ethanol (from fermentation mixture)

FRACTIONATING COLUMN (packed with glass beads to increase the surface area for condensation of vapours)

Fractional distillation assembly in a school laboratory

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EXAM TIP:

For IGCSE 0620 exams, when asked to suggest a technique for separating liquid-liquid mixtures, always state FRACTIONAL DISTILLATION.

QUESTION 1

The diagrams show the apparatus used to obtain crystals of calcium chloride from a mixture of solid calcium chloride and solid calcium carbonate.

Calcium chloride is soluble in water and calcium carbonate is insoluble in water.

(a) Complete the boxes to name the apparatus.

SOLUTION:

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QUESTION 1 (continued)

(b) (i) Write down the order in which the apparatus should be used in this experiment.

SOLUTION:

B → C → A

Explanation:

B: The mixture of solid Calcium chloride and solid Calcium carbonate is added to distilled

water and thoroughly stirred. Calcium chloride is water-soluble, hence dissolves while Calcium carbonate remains undissolved.

C: The undissolved Calcium carbonate is removed from the mixture by filtration. It is left

behind on the filter paper as the residue while Calcium chloride passes through the filter paper as part of the filtrate.

A: The filtrate is collected in an evaporating dish which is then heated for evaporation.

This is done to speed up the crystallisation process.

(ii) Name the separation process in C.

SOLUTION:

Filtration

(c) (i) What has been added to the mixture in B?

SOLUTION:

Distilled water

Explanation: The question states: Calcium chloride is soluble in water and calcium carbonate is insoluble in water.

Water is therefore a suitable solvent to separate the solids. Distilled water is 100% pure water and is therefore used to prevent the addition of impurities.

(ii) What is the general name given to the liquid in the dish in C?

SOLUTION:

Filtrate (the clear liquid that passes through the filter paper)

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QUESTION 1 (continued)

(d) How would you know when to stop heating the dish in A?

SOLUTION:

Heating is stopped when some solid appears on the glass rod / inner sides of dish.

Explanation: Whilst heating the filtrate, a glass rod is used to stir it for uniform heating. After some time, crystals start forming on the glass rod as well as on the inner sides of the dish. This is an indication that the solution is saturated and ready for crystallisation. The heating is stopped and the dish is allowed to cool for crystals to form.

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QUESTION 2

The diagram shows the apparatus used to separate a mixture of water, boiling point 100°C, and ethanol, boiling point 78°C.

(a) Complete the boxes to name the apparatus.

(b) Label the arrows on the condenser.

(c) Identify one mistake in the apparatus.

SOLUTION:

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QUESTION 2 (continued)

(d) Which liquid would collect first? Explain your answer.

SOLUTION:

Ethanol would collect first. Upon heating, the liquids begin to boil. The liquid with the lower boiling point – Ethanol in this case – boils over first. The vapours of Ethanol pass through the condenser and form liquid Ethanol that is collected in the receiving beaker.

BOILING POINT OF:

• Water = 100°C

• Ethanol = 78°C

(e) Why would it be better to use an electrical heater instead of a Bunsen burner to heat the water and ethanol mixture?

SOLUTION:

Ethanol is a flammable liquid and will catch fire if the vapours come in contact with the flame (in the event of a leakage). This is a potential safety hazard and can be avoided by using an electrical heater instead of a burner flame.

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Suggest suitable purification techniques, given information about the substances involved

A sample contains a mixture of powdered limestone (calcium carbonate), sugar and wax.

Suggest a way to obtain a pure sample of sugar?

SOLUTION:

Dissolve the mixture in water, filter and evaporate the filtrate to obtain sugar crystals.

Explanation:

Sugar is soluble in water. Limestone and wax are insoluble in water. Sugar will therefore dissolve in water and pass out in the filtrate. Upon heating the filtrate, water will evaporate leaving sugar behind.

ASSESSMENT

ASSESSMENT 2.2.2

QUESTION-BANK

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