IGCSE Chemistry (0620) Past Papers' Solutions - CHAPTER ......COMPLETE CHEMISTRY FOR CAMBRIDGE IGCSE: ANSWERS EDUCATALYST 1 CHAPTER 2 SEPARATING SUBSTANCES PAGE 13 1 a. A substance

COMPLETE CHEMISTRY FOR CAMBRIDGE IGCSE: ANSWERS

EDUCATALYST 1

CHAPTER 2 SEPARATING SUBSTANCES

PAGE 13

1 a. A substance which dissolves in a solvent is said to be soluble in that solvent b. A substance which does not dissolve in a solvent is said to be insoluble in that solvent c. A solution formed when water is used as the solvent. (aqua = water) 2 a. silver nitrate b. It is about 20 times more soluble than Potassium sulfate. c. A colourless solution will be obtained with undissolved white solid settled at the bottom d. Some of the undissolved solid will gradually start dissolving upon heating the solution. 3 a. sodium and potassium b. ammonium salts


EDUCATALYST 2

PAGE 13

4 a. sugar, salt, instant coffee, soap powder b. flour, pepper, plastic, wood, stainless steel 5 propanone in nail polish remover; white spirit or turpentine as a thinner for oil paints; ethanol in perfumes and after-shave 6 Oxygen – dissolved Oxygen is used by fishes Carbon dioxide – dissolved in water to make carbonated beverages


EDUCATALYST 3

PAGE 15

1. A pure substance contains particles of one substance only. 2. No. An impurity is generally an undesirable substance. 3. The presence of an impurity alters the melting and boiling points of a pure substance. The melting point decreases while the boiling point increases in the presence of an impurity. 4. Yes. A gas may contain particulate matter. For example: air may contain dust particles, harmful gases from industrial emissions or car exhausts


EDUCATALYST 4

PAGE 17

1 a. The clear liquid obtained from filtration (that passes through the filter paper). For example: Upon filtering a mixture of sand and seawater, the seawater passes through the filter paper, hence constitutes the filtrate. b. The solid that remains behind on the filter paper is called the residue. For example: Upon filtering a mixture of sand and seawater, the sand remains behind on the filter paper, hence constitutes the residue. 2 a. Sugar is water-soluble. The sugar particles dissolve in water and can therefore pass through the filter paper. Filtering works only for insoluble particles. b. The method of crystallisation can be used. The solution can be heated to remove the water by evaporation. Sugar crystals will be left behind. 3 The solution of Potassium nitrate can be heated to evaporate the water, until some solid appears (indication that it is saturated). It is then left to cool for crystals to form. The crystals formed can be obtained by filtration to remove the residual liquid, if any. 4 The salt and sugar mixture can be dissolved in Ethanol. The sugar dissolves while the salt remains undissolved. The mixture can be filtered to separate the salt. The filtrate must be heated to evaporate the ethanol and obtain the sugar by crystallisation. Precaution: Ethanol is inflammable (catches fire), hence evaporation must be carried out using an electric heater.


EDUCATALYST 5

PAGE 17

5 A bar magnet can be moved over the mixture. The little bits of Iron wire will cling to the magnet and can thus be separated from the sand.


EDUCATALYST 6

PAGE 19

1 Pure water can be obtained from seawater by simple distillation. The seawater is heated in a distillation flask. The water evaporates and passes through the condensor where it condenses back to form pure water. Dissolved salts are left behind in the flask.

2 Condensers bring about CONDENSATION of the vapours that pass through them. The cold water continuously circulates through the condenser. It cools the vapours passing through it so that condensation can occur. 3 Ethanol and water are miscible liquids, not a mixture of solid dissolved in a liquid. They can be separated by fractional distillation. 4 Using the fractional distillation of a water and ethanol mixture as example: The glass beads in the fractionating column provide a large surface area on which condensation can take place. Once the beads warm up to 78°C, the ethanol passes over into the condenser. But the water vapour condenses on the beads and drips back into the flask. 5 The chromatogram of X has no green dot as seen in the chromatogram of C.


EDUCATALYST 7

PAGE 19

6 Each coloured circle can be cut out and soaked in a little water separately. The dye will get extracted in the water. The paper can be separated by decantation and the rest of the solution can be evaporated to obtain the dye.


EDUCATALYST 8

PAGE 21

1. Paper chromatography works by partitioning of the solutes between the solvent and the chromatography paper. It depends on the relative affinity of the solutes for both the paper and solvent. A solute that is more soluble in the solvent rises higher up the paper while a solute that is less soluble in the solvent is left behind on the paper. 2 a. A locating agent shows the location of a colourless substance on the chromatogram by producing a colour. b. Amino acids are colourless, hence need to be located on the chromatogram by spraying it with a suitable locating agent. 3 A solute has a fixed Rf value for a given solvent under a given set of experimental conditions. A solute can therefore be identified by comparison of its Rf value with a set of reference Rf values obtained for known solutes under identical experimental conditions. 4 a. Only C. A has a spot at the same height as the C spot. b and c. Rf values and names of amino acids: A 0.14, lysine; 0.26, glycine; 0.73, leucine B 0.38, alanine C 0.73, leucine D 0.43, proline E 0.6, valine.


EDUCATALYST 9

PAGE 24: Core

1 a. A iii B v C vi D i E iv F ii 2 a. Ice-cold water cools the tube and condenses the water-vapour to form water. b. This is needed to prevent the steam from escaping the tube without condensation. c i. In the test-tube (containing the salt-water) being heated c ii. In the lower part of the delivery tube d. simple distillation e. salt


EDUCATALYST 10

PAGE 25: Core

3 a. i. 100°C (the boiling point of water) ii. It is lower than the boiling point of seawater. (Presence of impurities raises the boiling point.) b. distillation flask c. i. C ii. at the lower inlet / end of the condenser d. Only insoluble impurities can be removed by filtration. Soluble impurities like dissolved salts can be removed by distillation. 4 a. Bunsen burner, filter funnel, conical flask, tripod, gauze, filter paper, stirring rod, beaker b. Add distilled water to the sample of Gypsum taken in a beaker. Stir well using a stirring rod. Place the beaker on a wire gauze on tripod. Heat the mixture with stirring till it starts boiling. Position a filter paper cone in the filter funnel supported on a conical flask. Filter the hot mixture carefully. Gypsum remains behind as the residue. Wash it with some distilled water using a wash bottle. Remove the solid residue and dry by placing in a warm oven


EDUCATALYST 11

PAGE 25: Core

5 a. mixture b. Separation of liquids requires fractional distillation. Simple distillation would not be a suitable technique. c. Nitrogen has the lowest boiling point while oxygen has the highest boiling point. 6 a. The mixture can be separated by filtration.

b. Simple distillation can be used.

salt & sugar in

Ethanol

sugar dissolved in Ethanol

salt


EDUCATALYST 12

PAGE 25: Core

6 c.

7 a. B b. A c. Red

sugar dissolved in

Ethanol

Pure Ethanol


EDUCATALYST 13

PAGE 25: Extended

8 a. Valine and Proline b. Proline c. The Rf values will not change as the distance travelled by the spots will decrease proportionately. 9 Candidates are advised to refer to the instructions on pages 20 and 21.

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IGCSE Chemistry (0620) Past Papers' Solutions - CHAPTER ......COMPLETE CHEMISTRY FOR CAMBRIDGE IGCSE: ANSWERS EDUCATALYST 1 CHAPTER 2 SEPARATING SUBSTANCES PAGE 13 1 a. A substance