WJEC 2019 Online Exam Review

GCSE Chemistry Unit 2 Higher Tier 3430UB0

All Candidates' performance across questions

Question Title N Mean S D Max Mark F F Attempt %

1a 7608 2.5 0.8 3 83.2 99.2

1b 7644 2.3 1.2 5 45.8 99.6

2a 7526 1.3 1.3 4 33.3 98.1

2b 7505 1.1 0.8 2 53.7 97.8

2c 7293 0.5 0.5 1 54.7 95

3a 7208 0.6 0.9 3 19.9 93.9

3b 6430 0.2 0.6 2 8.9 83.8

3c 6935 1.3 1.1 3 44.5 90.4

4a 6852 0.1 0.4 2 6.4 89.3

4b 7624 2.7 0.9 4 66.7 99.4

4c 6026 0.2 0.5 3 6 78.5

5a 7110 1.3 1 3 44.5 92.7

5b 7096 0.4 0.8 2 20.5 92.5

6 6995 1 1.1 6 16.6 91.2

7a 7273 0.3 0.8 4 8.5 94.8

7b 6676 0.6 1 3 18.5 87

7c 7386 1.2 0.8 2 61 96.3

8a 7608 0.8 0.4 1 80.8 99.2

8b 7610 0.1 0.3 1 13 99.2

8c 6968 0.1 0.3 1 11.8 90.8

8d 7264 0.5 0.8 5 9.2 94.7

83.245.8

33.353.754.7

19.98.9

44.56.4

66.76

44.520.5

16.68.5

18.561

80.813

11.89.2

0 10 20 30 40 50 60 70 80 90 100

1a1b2a2b2c3a3b3c4a4b4c5a5b

67a7b7c8a8b8c8d

Facility Factor %

Qu

est

ion

GCSE Science (Double) Chemistry - Unit 2: Higher Tier

13 © WJEC CBAC Ltd.

Question Marking details Marks available AO1 AO2 AO3 Total Maths Prac

4 (a) award (1) for any of following • getting crystals to form • temperature below room temperature • cooling to 4°C / 10°C place the boiling tube in ice (1)

2

2

2

(b) (i) all points plotted correctly (2) 4/5 points plotted correctly (1) tolerance ±½ small square suitable straight line / curve drawn (1)

3

3

3

(ii) no - maximum of around 4.9 g will dissolve at this temperature accept any sensible explanation using graph ecf possible from incorrect graph plotting / poor line

1 1 1

(c) put 5.0 g sample in 50 g of water and mix/stir well (1) accept any volume which will not dissolve all of the solid filter off undissolved solid, dry and weigh (1) work out how much dissolved and hence value for solubility (1)

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1

1

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Question 4 total 1 4 4 9 3 6

Sticky Note

This was poorly answered by the majority of candidates. Those who had carried out a similar experiment would have had a much better chance of seeing the point of the question. This candidate has completely misunderstood. No marks awarded.

Sticky Note

Responses to this question suggest that very few candidates had tried the method described briefly on page 38 of the Guidance for Teaching document. This candidate has made a reasonable effort at describing a method but it is not what is required. They did score 1 mark for stating that a volume of water of less than 100 cm3 should be used. No other credit was awarded.

Sticky Note

This is one of the better responses seen. The candidate was awarded 1 mark for stating that the first two results are below room temperature. They should have gone on to say that an ice bath (or similar) would be required to cool the tube to such a low temperature.

Sticky Note

The candidate has attempted to describe a method but it is not what is required. Only candidates who were familiar with the method described on page 38 of the Guidance for Teaching document gained significant credit here. The method described tests how quickly a compound dissolves, which is a different thing to its solubility. However, 1 mark was awarded for suggesting a volume of water less than 100 cm3.

Sticky Note

This candidate has no idea what the question is about. No marks awarded.

Sticky Note

This is one of the better responses seen and the candidate clearly has some knowledge of the method required. A mark was awarded for filtering the undissolved compound and a further mark for partially meeting the other two marking points. A very good effort - 2 marks awarded.

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(3430UB0-1)06

Examineronly

4. A student investigates the solubility of ammonium chloride by adding different masses to 10 g of water.

He uses the apparatus shown.

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thermometer

solution

crystals of ammonium chloride

10 g of water is placed in a boiling tube and 3.0 g of ammonium chloride is added.

The tube is heated until all the solid dissolves.

The tube is allowed to cool.

The temperature at which solid ammonium chloride first appears is recorded.

The experiment is repeated using different masses of ammonium chloride.

The results are shown in the table.

Mass of ammonium chloride in 10 g of water (g) 3.0 3.3 4.1 5.2 5.9 6.6

Temperature at which solid ammonium chloride first appears (°C) 4 10 30 52 68 80

(a) What practical problem is the student likely to come across in finding the first two results? Suggest how this problem might be overcome. [2]

(3430UB0-1) Turn over.

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7Examiner

only (b) (i) On the grid below, plot the temperature at which solid ammonium chloride first

appears against the mass of ammonium chloride in 10 g of water. Draw a suitable line. [3]

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0

10

20

30

40

50

60

70

80

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Mass of ammonium chloride in 10 g of water (g)

Tem

pera

ture

at w

hich

sol

id

am

mon

ium

chl

orid

e fir

st a

ppea

rs (°

C)

(ii) The student is given a boiling tube containing 5.0 g of ammonium chloride in 10 g of water. He stirs the ammonium chloride in the water and heats it to a temperature of 45 °C.

State whether all the ammonium chloride dissolves. Give a reason for your answer. [1]

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(3430UB0-1)08

Examineronly

(c) The student is asked to use a different method to find the exact solubility of another compound in water at room temperature. He knows that it has a value of approximately 7 g per 100 g of water at this temperature.

He is given a 5.0 g sample of the compound and common laboratory equipment but no heating apparatus.

Describe how he would carry out his method and how he would find the solubility. [3]

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14 © WJEC CBAC Ltd.

Question Marking details Marks available AO1 AO2 AO3 Total Maths Prac

5 (a) mixture is heated until one liquid boils / evaporates then condenses and is collected in a different container (1) liquids must have different boiling points (1) liquid E has the lower boiling point so is the one removed / liquid F has the higher boiling point so is the one left in the flask (1)

3

3

3

(b) 74 / 74.1 (2) if answer incorrect award (1) for 32

43.2 × 100

2

2

2

Question 5 total 3 2 0 5 2 3

10

(3430UB0-1)10

Examineronly

5. (a) A bottle contains a mixture of liquids E and F. Liquid E has a boiling point of 57 °C and liquid F has a boiling point of 95 °C.

Describe the process of distillation and explain why it can be used to separate these liquids. [3]

(b) One molecule of liquid E contains two oxygen atoms. The percentage by mass of oxygen in liquid E is 43.2 %.

Use the following equation to calculate the relative molecular mass (Mr) of liquid E. [2]

Ar(O) = 16

Mr = . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5

mass of oxygenMr

× 100 = 43.2

15 © WJEC CBAC Ltd.

Question Marking details Marks available AO1 AO2 AO3 Total Maths Prac

6 Indicative content • removal of temporary hardness by boiling • hydrogencarbonate ions are not thermally stable and decompose easily on

heating • this forms a layer of calcium carbonate (inside kettles) • calcium hydrogencarbonate → calcium carbonate + water + carbon dioxide • Ca(HCO3)2 → CaCO3 + H2O + CO2 • boiling does not remove permanent hardness • removal of permanent hardness by adding washing soda/sodium carbonate • this forms a (white) precipitate • sodium carbonate + calcium sulfate → calcium carbonate + sodium sulfate • Na2CO3 + CaSO4 → CaCO3 + Na2SO4

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5-6 marks Detailed description of how hard water is softened using both methods; one correct equation There is a sustained line of reasoning which is coherent, relevant, substantiated and logically structured. The candidate uses appropriate scientific terminology and accurate spelling, punctuation and grammar. 3-4 marks Good description of how water is softened using both methods There is a line of reasoning which is partially coherent, largely relevant, supported by some evidence and with some structure. The candidate uses mainly appropriate scientific terminology and some accurate spelling, punctuation and grammar. 1-2 marks Brief description of how water is softened using one of the methods There is a basic line of reasoning which is not coherent, largely irrelevant, supported by limited evidence and with very little structure. The candidate uses limited scientific terminology and inaccuracies in spelling, punctuation and grammar. 0 marks No attempt made or no response worthy of credit.

Question 6 total 6 0 0 6 0 3

Sticky Note

This is obviously a top band answer and 6 marks were awarded. The explanation of removal of temporary hardness by boiling and both temporary and permanent hardness using washing soda is detailed and correct. The equations are good and the slip in calling sodium ions Na– has been ignored. An excellent response!

Sticky Note

There is enough understanding evident here to award a top band mark but 5 rather than 6 this time. Explanations are detailed but, based on the first three lines, it is not clear that the candidate knows exactly how calcium and magnesium ions come to be dissolved in hard water. Still a very good effort!

Sticky Note

Another very good effort which was again awarded 5 marks. Not quite worthy of the top mark as it is not clear that the candidate appreciates that hardness exists as a result of dissolved ions.

(3430UB0-1) Turn over.11

11Examiner

only6. Explain how the processes of boiling and adding washing soda remove hardness from water.

Include equations in your answer. [6 QER]

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