Cambridge IGCSE Chemistry (9-1) 0971 — 2024 May/June Paper 6 · Variant 1

1 A student carries out a titration to find the concentration of a sample of dilute hydrochloric acid. The student: ● adds 25.0 cm3 of aqueous potassium hydroxide to the apparatus labelled A in Fig. 1.1 B 25.0 cm3 aqueous potassium hydroxide A Fig. 1.1 ● adds a few drops of a suitable indicator to the apparatus labelled A ● uses a burette to add dilute hydrochloric acid to the aqueous potassium hydroxide and indicator mixture in the apparatus labelled A. (a) Name the items of apparatus labelled A and B in Fig. 1.1. A ................................................................................................................................................. B ................................................................................................................................................. [2] (b) The student adds the indicator after the volume of the 25.0 cm3 of aqueous potassium hydroxide has been measured. (i) Explain why the student adds an indicator to the aqueous potassium hydroxide. ....................................................................................................................................... [1] (ii) Name a suitable indicator. ....................................................................................................................................... [1] (c) Describe how the student can determine the volume of dilute hydrochloric acid used in this titration. .................................................................................................................................................... .............................................................................................................................................. [2] (d) The student observes the colour changes that occur as they add dilute hydrochloric acid from the burette. State one other thing the student should do as they add the dilute hydrochloric acid to the aqueous potassium hydroxide. .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 7]

Mark scheme: 1(a) M1 A (conical) flask 1 M2 B volumetric pipette 1 1(b)(i) so that it can change colour at the end point 1 1(b)(ii) methyl orange / thymolphthalein 1 1(c) M1: take initial and final (burette) readings 1 M2 final reading minus initial reading 1 1(d) swirl (A) / swirl (the flask) / mix (the contents of A) 1

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2 A student investigates the temperature change when magnesium reacts with dilute sulfuric acid. The student does five experiments. Experiment 1 ● Use a 25 cm3 measuring cylinder to pour 20 cm3 of dilute sulfuric acid into a boiling tube. ● Use a thermometer to measure the initial temperature of the acid in the boiling tube. Record the initial temperature. ● Add a coiled 5 cm length of magnesium ribbon to the acid in the boiling tube. At the same time start a timer. ● Continually stir the contents of the boiling tube using the thermometer. ● After 45 seconds, measure the temperature of the mixture in the boiling tube. Record this temperature. ● Rinse the boiling tube with distilled water. Experiment 2 ● Use the 25 cm3 measuring cylinder to pour 20 cm3 of dilute sulfuric acid into the boiling tube. ● Use a 10 cm3 measuring cylinder to add 2.0 cm3 of distilled water to the acid in the boiling tube. ● Place a bung in the boiling tube and invert the tube to mix the acid and water. ● Use the thermometer to measure the initial temperature of the contents of the boiling tube. Record the initial temperature. ● Add a coiled 5 cm length of magnesium ribbon to the contents of the boiling tube. At the same time start a timer. ● Continually stir the contents of the boiling tube using the thermometer. ● After 45 seconds, measure the temperature of the mixture. Record this temperature. ● Rinse the boiling tube with distilled water. Experiment 3 ● Repeat Experiment 2, adding 4.0 cm3 of distilled water instead of 2.0 cm3. Experiment 4 ● Repeat Experiment 2, adding 6.0 cm3 of distilled water instead of 2.0 cm3. Experiment 5 ● Repeat Experiment 2, adding 10.0 cm3 of distilled water instead of 2.0 cm3. (a) Use the information in the description of the experiments and the thermometer diagrams to complete Table 2.1. Table 2.1 volume of volume of thermometer initial temperature temperature dilute distilled diagramexperiment temperature after 45 s increase sulfuric acid water after 45 s / °C / °C / °C / cm3 / cm3 / °C 40 1 25.0 35 30 40 2 25.5 35 30 40 3 25.5 35 30 35 4 26.0 30 25 35 5 26.0 30 25 [5] (b) (i) State which Experiment, 1, 2, 3, 4 or 5, had the smallest temperature change. ....................................................................................................................................... [1] (ii) Explain why the temperature change was smallest in the experiment you have given in (b)(i). ............................................................................................................................................. ....................................................................................................................................... [1] (c) Complete a suitable scale on the y-axis and plot your results from Experiments 1 to 5 on Fig. 2.1. Draw a line of best fit. temperature increase / C 0.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 volume of distilled water / cm3 Fig. 2.1 [4] (d) Use your graph in Fig. 2.1 to deduce the temperature increase when Experiment 2 is repeated with 7.5 cm3 of distilled water instead of 2.0 cm3. Show clearly on Fig. 2.1 how you worked out your answer. .............................. °C [2] (e) The average rate of temperature increase in each experiment is calculated using the equation shown. temperature increase average rate of temperature increase = 45 seconds Calculate the average rate of temperature increase in Experiment 1. Give units for the rate you have calculated. average rate of temperature increase = .............................. units = .............................. [2] (f) (i) Explain why the results of the experiment are more accurate if the boiling tube is wrapped in cotton wool. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Explain why a 25.0 cm3 volumetric pipette cannot be used to accurately measure the volume of the distilled water added. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) State one other way in which the apparatus can be changed to give more accurate results. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (g) Sketch on Fig. 2.1 the graph you would expect if all of the experiments were repeated using a 2 cm length of magnesium ribbon instead of the 5 cm length. Label your line g. [1] [Total: 20]

Mark scheme: 2(a) M1 all five volumes of dilute sulfuric acid are 20 1 M2 all five volumes of water are correct (0, 2, 4, 6, 10) 1 M3 all temperatures and temperature changes are recorded to nearest 0.5 C 1 M4 all five temperatures after 45 seconds are recorded correctly (37.0, 34.0, 31.5, 30.0, 27.5) 1 M5 all temperature increases correct (12.0, 8.5, 6.0, 4.0, 1.5) 1 2(b)(i) 5 1 2(b)(ii) larger / largest volume (being heated) 1 2(c) M1 suitable sensible scale on y-axis 1 M2 and M3 all five points plotted correctly 2 M4 best fit line 1 Question Answer Marks 2(d) M1 correct working shown on graph 1 M2 correct value from their working to a point on the line 1 2(e) M1 (12  45 =) 0.27 or 0.267 or 0.3 1 M2 C / s 1 2(f)(i) any two from:  insulator  so reduces / prevents heat loss / traps heat  temperature (more) accurate / does not cool down / prevents temperature decreasing 2 2(f)(ii) volume used is not fixed / volume needed is not the same in each experiment / volume used is not 25 cm3 1 2(f)(iii) use a burette instead of a measuring cylinder 1 2(g) sketch line is below results line for all volumes of water 1

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3 A student tests two substances: solution E and solid F. Tests on solution E Solution E is aqueous chromium(III) bromide. Solution E is divided into two portions. Record the expected observations. (a) To the first portion of solution E, the student adds aqueous sodium hydroxide dropwise and then in excess. observations adding dropwise .................................................................................................... .................................................................................................................................................... observation in excess ................................................................................................................. .................................................................................................................................................... [2] (b) To the second portion of solution E, the student adds about 1 cm3 of dilute nitric acid and a few drops of aqueous silver nitrate. observations ............................................................................................................................... .............................................................................................................................................. [1] Tests on solid F Table 3.1 shows the tests and the student’s observations for solid F. Table 3.1 tests observations test 1 Heat about half of solid F in a boiling tube until the white solid forms a colourless liquid, there is no further change. steam comes out from the boiling tube and condensation is seen at the top of the boiling tube, after a while the colourless liquid becomes a white solid test 2 The remaining solid F is dissolved in water to form solution F. Solution F is divided into three portions. To the first portion of solution F in a boiling no change tube, add aqueous sodium hydroxide dropwise and then in excess. Warm the product and hold damp red litmus the damp red litmus paper remains red paper at the mouth of the boiling tube. test 3 To the second portion of solution F, add 1 cm3 white precipitate of dilute nitric acid followed by a few drops of aqueous barium nitrate. test 4 To the third portion of solution F, add a the solution becomes light purple few drops of acidified aqueous potassium manganate(VII). (c) The observations in test 1 show that solid F is hydrated. Describe a chemical test to show that the condensation at the top of the boiling tube contains water. test ............................................................................................................................................. result .......................................................................................................................................... [2] (d) From the tests and observations in Table 3.1 it is not possible to identify the cation in solid F. Give another test that can be carried out to help identify the cation in solid F. .................................................................................................................................................... .............................................................................................................................................. [1] (e) Identify the anion in solid F. .............................................................................................................................................. [1] [Total: 7]

Mark scheme: 3(a) M1 green precipitate 1 M2 precipitate dissolves (giving a green solution) / soluble (in excess) / precipitate disappears 1 3(b) cream precipitate 1 3(c) M1 anhydrous copper(II) sulfate M2 (changes from white to) blue 1 OR M1 anhydrous cobalt(II) chloride M2 (changes from blue to) pink 1 3(d) flame test 1 Question Answer Marks 3(e) sulfate / SO42– 1

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4 A mixture contains three compounds: ● liquid ethanol ● solid sodium chloride ● solid zinc carbonate. Table 4.1 gives some information about these three compounds. Table 4.1 name of compound solubility in water solubility in ethanol ethanol soluble sodium chloride soluble insoluble zinc carbonate insoluble insoluble Describe how to obtain a pure sample of each of the three compounds, ethanol, sodium chloride and zinc carbonate, from the mixture. You are provided with common laboratory apparatus. ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ........................................................................................................................................................... ..................................................................................................................................................... [6]

Mark scheme: 4 max 6 to obtain ethanol MP1 filter the initial mixture (ethanol obtained as filtrate) and then any five from: to obtain zinc carbonate MP2 add water to the residue / mixture MP3 stir / warm after adding water MP4 filter after adding water MP5 wash residue from filtration after the addition of water MP6 dry the solid zinc carbonate to obtain sodium chloride MP7 heat filtrate to remove water and obtain solid sodium chloride If candidates do not have a valid method of obtaining ethanol then MP1 cannot be awarded and the maximum score is 5. If ethanol is obtained AFTER adding more ethanol to the mixture, then do not award MP1, maximum 5 5

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