Cambridge IGCSE Chemistry (9-1) 0971 — 2021 Oct/Nov Paper 6 · Variant 2

1 Carbon dioxide is a colourless gas that is denser than air. Carbon dioxide can be made by reacting marble chips with dilute hydrochloric acid. A student tried to make and collect carbon dioxide gas using the apparatus shown. A gas jar C B (a) (i) Name the substances labelled A and B. A .......................................................................................................................................... B .......................................................................................................................................... [1] (ii) Name the item of apparatus labelled C. ....................................................................................................................................... [1] (b) Explain why very little carbon dioxide gas would be collected using the apparatus shown. .................................................................................................................................................... .............................................................................................................................................. [2] (c) Complete the diagram to show how carbon dioxide gas could be collected and the volume measured. [2] (d) At the end of the experiment there were unreacted marble chips and aqueous calcium chloride in the item of apparatus labelled C. Describe how you would find the mass of unreacted marble chips in apparatus C. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [3] [Total: 9]

Mark scheme: 1(a)(i) A (dilute) hydrochloric acid and B marble chips 1 1(a)(ii) (conical) flask 1 1(b) M1 (carbon dioxide) is denser than air 1 M2 drops out of gas jar 1 1(c) M1 shows collection over water / in a gas syringe 1 M2 in a measuring cylinder / graduations shown on collection vessel 1 1(d) M1 filter 1 M2 wash 1 M3 dry and weigh 1

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2 A student investigated the reaction between two different solutions of aqueous sodium carbonate, solution K and solution L, and two different solutions of dilute hydrochloric acid, acid M and acid N. Three experiments were done. (a) Experiment 1 ● A burette was filled with solution K. Some of solution K was run out of the burette so that the level of solution K was on the burette scale. ● Using a measuring cylinder 25 cm3 of acid M was poured into a conical flask. ● Five drops of methyl orange indicator were added to the conical flask. ● The conical flask was placed on a white tile. ● Solution K was added slowly from the burette to the conical flask, while the flask was swirled, until the solution just changed colour. Use the burette diagrams to complete the table for Experiment 1. 7 31 8 32 9 33 initial reading final reading Experiment 1 final burette reading / cm3 initial burette reading / cm3 volume of solution K added / cm3 Experiment 2 ● The conical flask was emptied and rinsed with distilled water. ● The burette was refilled with solution K. Some of solution K was run out of the burette so that the level of solution K was on the burette scale. ● Using a measuring cylinder 25 cm3 of acid N was poured into the conical flask. ● Five drops of methyl orange indicator were added to the conical flask. ● The conical flask was placed on a white tile. ● Solution K was added slowly from the burette to the conical flask, while the flask was swirled, until the solution just changed colour. Use the burette diagrams to complete the table for Experiment 2. 3 15 4 16 5 17 initial reading final reading Experiment 2 final burette reading / cm3 initial burette reading / cm3 volume of solution K added / cm3 Experiment 3 ● The burette was emptied and rinsed with distilled water. ● The conical flask was emptied and rinsed with distilled water. ● The burette was filled with solution L. Some of solution L was run out of the burette so that the level of solution L was on the burette scale. ● Using a measuring cylinder 25 cm3 of acid N was poured into the conical flask. ● Five drops of methyl orange indicator were added to the conical flask. ● The conical flask was placed on a white tile. ● Solution L was added slowly from the burette to the conical flask, while the flask was swirled, until the solution just changed colour. Use the burette diagrams to complete the table for Experiment 3. 2 26 3 27 4 28 initial reading final reading Experiment 3 final burette reading / cm3 initial burette reading / cm3 volume of solution L added / cm3 [5] (b) State the colour change observed at the end-point in the conical flask in Experiment 1. from ................................................................ to ���������������������������������������������������������������� [1] (c) Describe one other observation made when solution K was added to acid M in Experiment 1. .............................................................................................................................................. [1] (d) (i) Compare the volumes of solution K used in Experiment 1 and Experiment 2. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Suggest why different volumes of solution K were needed in Experiment 1 and Experiment 2. ....................................................................................................................................... [1] (e) Deduce the volume of solution L required to reach the end-point if Experiment 3 is repeated using acid M in place of acid N. volume of solution L = .............................. cm3 [1] (f) Explain why the conical flask was rinsed with water at the start of Experiment 2 and Experiment 3. .............................................................................................................................................. [1] (g) At the start of Experiment 3 the burette was rinsed with water. Describe an additional step that should have been done after rinsing the burette with water but before filling the burette with solution L. Explain your answer. .................................................................................................................................................... .............................................................................................................................................. [2] (h) Explain why the conical flask is placed on a white tile. .............................................................................................................................................. [1] (i) Describe how the reliability of the results can be confirmed. .................................................................................................................................................... .............................................................................................................................................. [1] (j) State one source of error in Experiment 1. Suggest an improvement to reduce this error. source of error ............................................................................................................................ improvement .............................................................................................................................. [2] [Total: 18]

Mark scheme: 2(a) M1 Experiment 1 burette readings completed correctly (31.6 and 8.0) 1 M2 Experiment 2 burette readings completed correctly (15.9 and 4.1) 1 M3 Experiment 3 burette readings completed correctly (26.4 and 2.7) 1 M4 All subtractions to give volume added correct (23.6, 11.8, 23.7) 1 M5 All readings / volumes are given to 1 dp or better 1 2(b) (from) red (to) orange 1 2(c) effervescence / fizzing / bubbles 1 Question Answer Marks 2(d)(i) M1 greater volume used in experiment 1 / smaller volume used in experiment 2 1 M2 twice as much in experiment 1 / half as much in experiment 2 1 2(d)(ii) solution M is more concentrated than solution N 1 2(e) 47.4 1 2(f) to clean / to remove residue from previous experiment 1 2(g) M1 rinse with solution L 1 M2 to remove water / avoid diluting solution L / avoid changing concentration (of L) 1 2(h) to see colour change clearly / easily / accurately / better 1 2(i) repeat the experiments and compare the results 1 2(j) M1 source of error: measuring cylinder / error in volume of solution M 1 M2 improvement: use a pipette 1

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3 Solid O and liquid P were analysed. Solid O was ammonium bromide. Tests were done on each substance. tests on solid O Complete the expected observations. Solid O was dissolved in water to form solution O. Solution O was divided into four approximately equal portions in four test‑tubes. (a) To the first portion of solution O, approximately 2 cm3 of aqueous ammonia was added. observations ......................................................................................................................... [1] (b) To the second portion of solution O, approximately 2 cm3 of aqueous sodium hydroxide was added. The mixture formed was warmed. A gas was given off. (i) The gas given off was tested with damp red litmus paper. observations .................................................................................................................. [1] (ii) Identify the gas given off. ....................................................................................................................................... [1] (c) To the third portion of solution O, approximately 1 cm3 of dilute nitric acid followed by a few drops of aqueous silver nitrate were added. observations ......................................................................................................................... [1] (d) To the fourth portion of solution O, approximately 1 cm3 of aqueous chlorine was added. observations ......................................................................................................................... [1] tests on liquid P tests observations test 1 A few drops of liquid P were placed in a burned with an orange flame and lots of smoke; crucible. A lighted splint was applied to the soot was left around the top of the crucible surface of liquid P in the crucible. test 2 A few drops of liquid P were added to a colour changed from orange to colourless test‑tube containing 1 cm3 of aqueous bromine. (e) State what conclusions can be made about liquid P. .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 7]

Mark scheme: 3 Tests on solid O 3(a) no change / colourless / no reaction / no observation 1 3(b)(i) (red litmus) becomes blue 1 3(b)(ii) ammonia / NH3 1 3(c) cream precipitate 1 3(d) (solution becomes) orange / yellow 1 3 Tests on a liquid P 3(e) any two from: flammable / fuel unsaturated / alkene / any named alkene organic / contains carbon / hydrocarbon 2

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4 Cobalt is a metal. Cobalt is between copper and iron in the reactivity series. The mineral spherocobaltite contains the compound cobalt(II) carbonate and no other metal ions. Cobalt(II) carbonate is insoluble in water and reacts with dilute acids to form an aqueous solution of a salt. Describe how you would obtain a sample of cobalt metal starting with a large lump of spherocobaltite. You have access to all normal laboratory apparatus and chemicals. ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ..................................................................................................................................................... [6]

Mark scheme: 4 reduction method Any 6 from: • crush rock / break into smaller pieces / powder • using a suitable method, e.g. pestle and/or mortar, hammer • add more reactive metal / suitable gas • carbon / coke / zinc / aluminium / magnesium / CO / hydrogen specified • heat • In a suitable container (e.g. crucible, evaporating basin) • cobalt displaced / cobalt formed electrolysis method Any 6 from: • crush rock / break into smaller pieces / powder • using a suitable method, e.g. pestle and / or mortar, hammer • add a (dilute) acid • suitable strong acid named (e.g. HCl, H2SO4, HNO3) • electrolysis (of solution) • specified inert material for electrodes (e.g. carbon, platinum) • cobalt obtained at the negative electrode / cathode displacement method Any 6 from: • crush rock / break into smaller pieces / powder • using a suitable method, e.g. pestle and / or mortar, hammer • add a (dilute) acid • suitable strong acid named (e.g. HCl, H2SO4, HNO3) • add metal more reactive than cobalt • name of metal added specified (e.g. iron, zinc, magnesium) • Cobalt displaced / cobalt formed 6

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