Cambridge IGCSE Science - Combined 0653 — 2019 Oct/Nov Paper 6 · Variant 2

1 A student investigates the concentration of vitamin C in two samples of fruit juice, A and B. Iodine solution and starch can be used to estimate the concentration of vitamin C in a sample of fruit juice. Iodine solution is added to a starch fruit juice mixture until the solution remains blue- black. The more iodine solution that needs to be added to produce a permanent blue-black colour, the higher the concentration of vitamin C in the sample. (a) • The student puts 20 cm3 of fruit juice A into a beaker. • He adds 1 cm3 starch solution to the fruit juice using a measuring cylinder and stirs. • He puts 10 cm3 iodine solution into a second beaker. • He uses a pipette to add a few drops of the iodine solution to the fruit juice and starch mixture and stirs. • He continues adding iodine solution until the colour changes to blue-black. • He pours the remaining unused iodine solution into a measuring cylinder. • He records in Table 1.1 the volume of unused iodine solution remaining for experiment 1. Table 1.1 volume of unused average volume, VA, volume of iodine fruit juice A iodine solution of iodine solution solution added experiment remaining added / cm3 / cm3 / cm3 1 8.0 2.0 2 7.5 2.5 3 He repeats the experiment twice more. He records in Table 1.1 the volume of unused iodine solution remaining for experiment 2 and experiment 3. The reading on the measuring cylinder for the volume of remaining iodine solution for experiment 3 is shown in Fig. 1.1. cm3 10 8 Fig. 1.1 (i) Read and record in Table 1.1 the volume shown in Fig. 1.1 to the nearest 0.5 cm3. [1] (ii) Calculate the volume of iodine solution that has been added to the fruit juice in experiment 3. Record this value in Table 1.1. [1] (iii) Calculate the average volume, VA, of iodine solution added to fruit juice A. Record this value in Table 1.1. [1] (b) The student repeats (a) with fruit juice B. He records the volumes in Table 1.2. Table 1.2 volume of unused average volume, VB, volume of iodine fruit juice B iodine solution of iodine solution solution added experiment remaining added / cm3 / cm3 / cm3 1 9.5 0.5 2 5.5 4.5 3 9.5 0.5 Calculate the average volume, VB, of iodine solution added to fruit juice B. Record this value in Table 1.2. [2] (c) State what can be concluded about the concentration of vitamin C in fruit juice A compared to fruit juice B. Use the results in Table 1.1 and Table 1.2. ................................................................................................................................................... ............................................................................................................................................. [1] (d) (i) Calculate the concentration of vitamin C in fruit juice A using the equation shown. VA × 0.025 concentration = 20 Give your answer to an appropriate number of significant figures. concentration = ...............................................g / cm3 [2] (ii) Suggest one piece of apparatus that can be used to measure the 1 cm3 of starch solution more accurately. ..................................................................................................................................... [1] (e) Fig. 1.2 shows half of a fruit that contains vitamin C. Fig. 1.2 In the box below, make an enlarged detailed drawing of the cut surface of the fruit. [3] (f) State the name of a reagent that could be used to test the fruit for the presence of protein. ............................................................................................................................................. [1] [Total: 13]

Mark scheme: 1(a)(i) 8.5 ; 1 1(a)(ii) 1.5 ; 1 1(a)(iii) 2.0 ; 1 1(b) average calculated ; outlier excluded (ave = 0.5) ; 2 1(c) A has more vitamin C / A is more concentrated (than B) ora ; 1 1(d)(i) correct calculation (0.0025 or 0.003) ; answer to appropriate significant figures (1 or 2) ; 2 1(d)(ii) burette / syringe / graduated pipette ; 1 1(e) enlarged circle (greater than 5 cm diameter) ; continuous line around outside and 9–11 segments ; pith and centre shown ; 3 1(f) biuret ; 1

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2 A student has a sample of a green powder, E. (a) • She places a small amount of powder E into a test-tube and heats it gently. • She bubbles the gas formed into limewater in a test-tube. The limewater turns milky. • The green powder changes into a black powder F. (i) Draw a labelled diagram of the assembled apparatus and chemicals she uses in (a). [2] (ii) State the identity of the gas formed. ..................................................................................................................................... [1] (iii) State the identity of the anion in powder E. ..................................................................................................................................... [1] (b) • The student places the black powder F into a beaker of dilute sulfuric acid and heats it. • She filters the mixture formed into two test-tubes. • The liquid in the two test-tubes is solution G and is blue in colour. • She adds aqueous sodium hydroxide to one test-tube of solution G. • A pale blue precipitate forms. • She performs a flame test on the sample of solution G in the other test-tube. • The flame colour she observes is blue-green. State the identity of the cation in solution G, powder E and powder F. ............................................................................................................................................. [1] (c) State the identities of black powder F and blue solution G. Use the results in (a) and (b). identity of F ............................................................................................................................... identity of G .............................................................................................................................. [2] [Total: 7]

Mark scheme: 2(a)(i) test tube and bung and delivery tube above solid and 1 label ; delivery tube into container without bung, under the level of the limewater (with label on limewater) ; 2 2(a)(ii) carbon dioxide / CO2 ; 1 2(a)(iii) carbonate ; 1 2(b) copper (ion) / Cu2+ ; 1 2(c) F (copper) oxide ; G (copper) sulfate ; 2

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3 Fizzy drinks are fizzy because they have carbon dioxide dissolved in them. The carbon dioxide is dissolved under pressure. As soon as the top is taken off a bottle of fizzy drink, the carbon dioxide gas starts to bubble out of the drink. If the top is left off the bottle, the carbon dioxide will start to leave the drink. The drink will eventually ‘go flat’ (not fizzy) when all of the carbon dioxide has left the drink. This will also happen if the drink is poured into a glass. The drink will lose all of its carbon dioxide more quickly if it is heated. Plan an experiment to compare the amount of dissolved carbon dioxide in the three fizzy drinks lemon soda, orange soda and sparkling water. You may use any common laboratory apparatus and samples of lemon soda, orange soda and sparkling water. Include in your answer: • the apparatus needed, including a labelled diagram if you wish • a brief description of the method, including any safety precautions and why these are needed • the measurements you will make • what you will control • how you will process your results • how you will use your results to draw a conclusion. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [7]

Mark scheme: 3 one mark from each section and then any other 3 marks from: 1. apparatus flask / test-tube / boiling-tube and heating apparatus, e.g. Bunsen burner ; (gas) syringe / measuring cylinder to collect gas / burette / weighing scale ; measuring cylinder to measure fizzy drink ; 2. method and safety drink in a container with delivery tube into gas syringe or upturned container in water ; heat / leave (until no more gas is given off) / shake (until no more gas is given off) ; wear goggles when heating / wear goggles to prevent (hot) fizzy drink from going into eyes / take precaution against hot apparatus, e.g. tongs, wait for apparatus to cool before touching, use heat gloves to protect against burning ; repeat for same type of fizzy drink ; 3. measurements and control same amount of drink used / quotes volume of drink used / measures volume or mass of drink used ; measure volume of CO2 evolved / measure mass lost ; to constant volume of gas / to constant mass ; 4. processing and use of results calculate average amount of CO2 evolved for each type of drink (if measurements repeated) ; compare volume/mass of CO2 / largest volume collected has most CO2 / smallest volume collected has least CO2 / largest loss in mass has most CO2 / smallest loss in mass has least CO2 ; 7

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4 A student investigates how the period of a simple pendulum changes as its length changes. Fig. 4.1 shows the apparatus used by the student. The length of a pendulum is the distance from the point of support to the centre of the bob. point of support l bob Fig. 4.1 (a) (i) Measure the length l on Fig. 4.1 to the nearest 0.1 cm. length l of pendulum in Fig. 4.1 = ................................................... cm [1] (ii) Fig. 4.1 is drawn to a one-tenth scale. Calculate the actual length L of the pendulum. Record this length L in Table 4.1 on page 10. [1] (b) (i) The student measures the time for 10 complete oscillations of the pendulum. She repeats this measurement. Fig. 4.2 shows the two stop-clock readings for her measurements. 1st 2nd Fig. 4.2 Read and record in Table 4.1 the times for 10 oscillations of the pendulum to the nearest 0.01 s. [2] Table 4.1 time for 10 oscillations / s average time for 2 length L / cm period T / s T / s2 10 oscillations / s 1st 2nd 40.0 12.63 12.69 12.66 1.266 1.60 35.0 11.98 11.95 11.97 1.197 1.43 25.0 10.10 10.19 10.15 1.015 1.03 20.0 9.22 9.32 9.27 0.927 0.86 (ii) Calculate the average time for 10 oscillations of the pendulum of length L. Record this time in Table 4.1. [1] (iii) Calculate the period T and T 2 for the pendulum of length L. Record these values in Table 4.1. [1] (c) The student repeats the experiment for pendulums with length L = 40.0, 35.0, 25.0 and 20.0 cm. Her results are recorded in Table 4.1. (i) Describe one precaution that the student should take to ensure her timings of 10 complete oscillations are as accurate as possible. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Explain why it is better to time 10 oscillations rather than one oscillation to determine the period T. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (d) (i) On the grid plot a graph of T 2 against L. T2 / s2 0 0 L / cm [2] (ii) Draw the best-fit straight line. [1] (e) Describe the relationship between the length L of the pendulum and T 2. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 13]

Mark scheme: 4(a)(i) 5(.0) (cm) ; 1 4(a)(ii) 50.0 (cm) ; 1 4(b)(i) 14.18 ; 14.16 ; 2 4(b)(ii) 14.17 ; 1 4(b)(iii) correct calculation of T and T2 (1.417 and 2.01) ; 1 4(c)(i) (time for complete swing is when bob crosses «) retort stand upright / line drawn on card / any sensible description that shows attempt to time from a point in swing to same point in swing ; 1 4(c)(ii) reaction time of humans / human error in timing / one oscillation is too short to time accurately / difficult to start/stop at exactly the right time ; reduced percentage uncertainty / uncertainty spread over more oscillations / uncertainty less significant over more oscillations ; 2 4(d)(i) axes right way round with linear scale so that plots occupy at least half of grid, starting from 0 ; correct plots to ± ½ small square ; 2 4(d)(ii) appropriate best-fit straight line based on plots ; 1 4(e) T2 is (directly) proportional to L ; 1

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