Cambridge IGCSE Sciences - Co-ordinated (Double) 0654 — 2019 May/June Paper 6 · Variant 2

1 A student tests the nutrient content of three colourless solutions A, B and C. She tests each solution with each of the following: • iodine solution • biuret solution • ethanol and water. Solution A tests positive with the ethanol and water. Solution B tests positive with the biuret solution. Solution C tests positive with the iodine solution. All other results are negative. (a) (i) Use the information to complete Table 1.1 with the colours the student observes in each test. You should include the observations for negative results. Table 1.1 test solutions solution A solution B solution C biuret solution ethanol and water iodine solution [4] (ii) Use the results to state the nutrient content of each solution. Solution A contains ........................................................................................................... Solution B contains ........................................................................................................... Solution C contains ........................................................................................................... [3] [Total: 7]

Mark scheme: 1(a)(i) purple/lilac for solution B with biuret solution ; white (emulsion) for solution a with ethanol and water ; blue-black for solution C with iodine solution; all correct ‘negative’ observations ; test solutions solution A solution B solution C biuret solution blue purple / lilac blue ethanol and water white cloudy / milky emulsion clear / colourless clear / colourless iodine solution orange / brown / yellow orange / brown / yellow blue-black 4 1(a)(ii) A – fat / lipid ; B – protein ; C – starch ; 3

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2 A student carries out an experiment to find the concentration of a sample of hydrochloric acid by reacting it with sodium hydroxide solution of concentration 0.2 mol/dm3. • He adds 20 cm3 of hydrochloric acid into a beaker. • He places 5 drops of Universal Indicator solution in the beaker. • He measures 25 cm3 of sodium hydroxide solution in a measuring cylinder. • He pours sodium hydroxide solution from the measuring cylinder into the beaker containing hydrochloric acid and Universal Indicator slowly whilst stirring the mixture. • He stops adding sodium hydroxide solution as soon as the Universal Indicator shows a neutral colour. • He records in Table 2.1, to an appropriate accuracy, the volume VR of sodium hydroxide solution remaining in the measuring cylinder for experiment 1. • He repeats this procedure three more times. These are experiments 2, 3 and 4. Table 2.1 volume VR of volume V of sodium hydroxide sodium hydroxide experiment solution remaining solution added to in the measuring the beaker / cm3 cylinder / cm3 1 14.0 2 3 16.5 4 14.5 (a) (i) Fig. 2.1 shows the measuring cylinder containing the remaining sodium hydroxide solution for experiment 2. cm3 15 10 Fig. 2.1 Read the scale of the measuring cylinder in Fig. 2.1 and record in Table 2.1, to an appropriate accuracy, the volume VR of sodium hydroxide solution remaining. [1] (ii) When the student pours the sodium hydroxide solution from the measuring cylinder into the hydrochloric acid, he finds it very difficult to add the exact amount needed to achieve a neutral solution. State the name of a piece of apparatus he should use instead of the measuring cylinder to add an exact amount of sodium hydroxide solution. ........................................................................................................................................... ..................................................................................................................................... [1] (b) (i) For each experiment calculate the volume V of sodium hydroxide solution added to the beaker using the equation shown. V = 25.0 – VR Record the values of V in Table 2.1. [1] (ii) Select and record the values of V that should be used in calculating the average volume of sodium hydroxide solution used. You should justify your selection. values of V selected .......................................................................................................... ........................................................................................................................................... explanation ........................................................................................................................ ........................................................................................................................................... [2] (iii) Use the volumes you have selected in (b)(ii) to calculate the average volume VA of sodium hydroxide solution added to the beaker to neutralise the hydrochloric acid. VA = ...................................................cm3 [1] (iv) Calculate the concentration C of the hydrochloric acid using the equation shown. (concentration of sodium hydroxide solution × VA) C = volume of hydrochloric acid used 0.2 × VA = 20 Give your answer to an appropriate number of significant figures. C = ............................................mol/dm3 [2] (c) Suggest why the student uses Universal Indicator solution rather than litmus paper. ................................................................................................................................................... ............................................................................................................................................. [1] (d) Another student carries out the same experiment but uses sodium hydroxide solution of concentration 0.1 mol/dm3 instead of 0.2 mol/dm3. Using (b)(iii) and (b)(iv), predict the average volume VA of sodium hydroxide solution that would be needed to neutralise the hydrochloric acid. VA using 0.1 mol/dm3 sodium hydroxide = ...................................................cm3 [1] [Total: 10]

Mark scheme: 2(a)(i) 14.0 ; 1 2(a)(ii) use of a (dropping) pipette / dropper ; 1 2(b)(i) 11(.0), 11(.0), 8.5, 10.5 ; 1 2(b)(ii) 11(.0) and 11(.0) and 10.5 ; suitable justification in terms of closeness of values ; 2 2(b)(iii) correct average of selected values of V ; 1 2(b)(iv) correct answer (0.108) ; 2 significant figures ; 2 2(c) continuous change with UI ; 1 2(d) double the value in 2(b)(iii) ; 1

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3 A student measures the resistance of an unknown resistor R. She sets up the circuit as shown in Fig. 3.1. V resistance wire R C P Q l A power supply Fig. 3.1 (a) • She places the sliding contact C at a distance of l = 10.0 cm from end P of the resistance wire. • She closes the switch. Fig. 3.2 shows the reading on the ammeter and Fig. 3.3 shows the reading on the voltmeter. 0.4 0.6 0.2 0.8 A 0 1.0 Fig. 3.2 0.4 0.6 0.2 0.8 V 0 1 Fig. 3.3 (i) Read the ammeter scale and record the current I in the circuit. I = .......................................................A [1] (ii) Read the voltmeter scale and record, in Table 3.1, the reading V on the voltmeter. [1] Table 3.1 l / cm V / V 10.0 30.0 1.15 50.0 1.40 70.0 1.65 90.0 1.90 (b) (i) • She repeats the procedure using values of l of 30.0 cm, 50.0 cm, 70.0 cm and 90.0 cm. • She opens the switch. Her results are shown in Table 3.1. On the grid provided, plot a graph of V (vertical axis) against l. Start both axes from the origin (0, 0). [3] (ii) Draw the best-fit straight line. [1] (c) Extend your line until it crosses the vertical axis. Measure the intercept Y that your line makes on the vertical axis. Y = .......................................................V [1] (d) The resistance of the unknown resistor R is given by the equation shown. Y resistance = I I is the current that you measured in part (a)(i). Use this equation to calculate a value for the resistance of R. resistance of R = ...................................................... Ω [2] (e) Suggest one practical reason why, despite the student carrying out the experiment with care, her value for the resistance of R is only approximate. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 10]

Mark scheme: 3(a)(i) 0.16 ; 1 3(a)(ii) 0.9(0) ; 1 3(b)(i) axes labelled with units ; suitable choice of scales and linear must start from origin (≥ half the grid used) ; 4 plots correct to half a small square ; 3 3(b)(ii) good best-fit line judgement ; 1 3(c) intercept correct from candidate’s graph ± half a small square ; 1 3(d) calculation of resistance correct ; R = (5 ± 1) Ω ; 2 3(e) difficult to measure to nearest mm / zero error on voltmeter or ammeter / difficult to read (an analogue) scale with precision / cell might run down / circuit heats and resistance changes ; 1

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4 Fig. 4.1 shows a life size photograph of a walnut. X Y Fig. 4.1 (a) In the box below, make an enlarged detailed pencil drawing of the walnut in Fig. 4.1. [2] (b) (i) Draw the line XY on your drawing. XY is an estimate of the diameter of the walnut in your drawing. Measure and record this diameter d1 of the drawing of the walnut in millimetres to the nearest millimetre. d1 = ...................................................mm [1] (ii) Measure and record the length of the line XY on Fig. 4.1 in millimetres to the nearest millimetre. This is an estimate of the diameter, d2 of the actual walnut. d2 = ...................................................mm [1] (iii) Use your values for d1 (drawing) and d2 (actual walnut) to calculate the magnification of your drawing. magnification = ......................................................... [1] (c) Explain why d2 is not an accurate measurement of the diameter of the actual walnut. ............................................................................................................................................. [1] [Total: 6]

Mark scheme: 4(a) clear and continuous outline ; surface detail and larger than original ; 2 4(b)(i) correct measurement ; 1 4(b)(ii) 38 ; 1 4(b)(iii) correct calculation ; 1 4(c) not smooth / is not a sphere/ is irregular ; 1

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5 A student investigates how the concentration of hydrochloric acid affects its reaction with marble chips (calcium carbonate). She uses hydrochloric acid which has a concentration of 2.0 mol/dm3. • She half fills a small test-tube with water and adds one drop of detergent. • She places this test-tube in a beaker. • She places 2 marble chips into a large test-tube. • She adds 10.0 cm3 hydrochloric acid to the marble chips. • She quickly connects the bung of a delivery tube to the large test-tube containing the hydrochloric acid and marble chips, so that the gas produced bubbles through the water and detergent. • She immediately starts the stopclock. • She stops the stopclock when the bubbles in the water and detergent reach the top of the small test-tube. • She records in Table 5.1 this time in seconds to the nearest second. (a) Draw a labelled diagram to show the apparatus connected together as in the procedure. [2] (b) The student makes 10.0 cm3 of hydrochloric acid of concentration 1.5 mol/dm3 by adding 2.5 cm3 of water to 7.5 cm3 of the original hydrochloric acid and mixing well. These volumes are shown in Table 5.1. Table 5.1 volume of volume of concentration of time / s hydrochloric water / cm3 hydrochloric acid / acid / cm3 mol per dm3 10.0 0 2.0 48 7.5 2.5 1.5 5.0 5.0 1.0 109 She repeats the procedure using this acid of lower concentration (1.5 mol/dm3) instead of the original acid (2.0 mol/dm3). The reading on the stopclock is shown in Fig. 5.1. min sec Fig. 5.1 (i) Read the time on the stopclock in Fig. 5.1 and record this time in Table 5.1. [1] (ii) The student repeats the procedure with hydrochloric acid of concentration 1.0 mol/dm3, and records the information in Table 5.1. Use the results in Table 5.1 to deduce the relationship between rate of reaction and concentration of hydrochloric acid for this reaction. ........................................................................................................................................... ..................................................................................................................................... [1] (c) (i) Another student suggests that three concentrations for the hydrochloric acid is not enough to deduce a valid relationship between rate of reaction and concentration of hydrochloric acid. Suggest why this student may be correct. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) The student decides to repeat the procedure using hydrochloric acid of 0.5 mol/dm3. State the volumes of the original hydrochloric acid (2.0 mol/dm3) and water required to make hydrochloric acid of concentration 0.5 mol/dm3. volume of original hydrochloric acid (2.0 mol/dm3) = ........................................................ cm3 volume of water = ........................................................ cm3 [1] (iii) Suggest two further improvements for this procedure, other than testing more different concentrations of hydrochloric acid. 1 ........................................................................................................................................ ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... [2] (d) Suggest an alternative method for measuring the rate of the reaction between hydrochloric acid and marble chips (calcium carbonate). You must include what is timed in your answer. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] [Total: 10]

Mark scheme: 5(a) apparatus connected together ; delivery tube and at least one test-tube labelled ; 2 5(b)(i) 63 ; 1 5(b)(ii) rate decreases as concentration decreases ; 1 5(c)(i) need more points if plot a graph / if one is misread two is not enough for a trend / too narrow a range ; 1 5(c)(ii) (acid) = 2.5 and (water =) 7.5 ; 1 5(c)(iii) repeat each concentration of acid (and take average) ; use pipette/burette (for measuring volumes) ; weigh marble chips / control temperature ; same volume of water ; measure volume water (with detergent/small test-tube) ; same amount of detergent ; max 2 5(d) collect gas above water / collect gas in a gas syringe / reaction vessel on a mass balance / bubble into limewater ; (time for) volume of gas / (time for) drop in mass / (time to) go cloudy ; 2

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6 A student investigates the cooling rates of different volumes of hot water in a beaker. He pours 200 cm3 of hot water into a beaker. He places a thermometer into the water and when the reading stops rising, measures the temperature θ0 of the hot water and starts a stopwatch. The initial temperature of the hot water is shown in Fig. 6.1. 90 80 hot water Fig. 6.1 (a) Read and record, in the second column of Table 6.1, the temperature θ0 at time t = 0. [1] Table 6.1 time t / ...... temperature of 200 cm3 of temperature of 100 cm3 of water water θ / ....... θ / ...... 0 87.0 83.0 84.0 81.0 81.0 79.0 78.5 77.5 76.0 76.0 74.0 180 74.5 72.5 (b) The student measures the temperature of the hot water every 30 s for 180 s. He records his results in Table 6.1. (i) Complete the headings in Table 6.1 by inserting the correct units. [1] (ii) Complete the time column in Table 6.1. [1] (c) State one precaution that the student should take to ensure that the temperature readings are as accurate as possible. ................................................................................................................................................... ............................................................................................................................................. [1] (d) (i) Calculate the fall in temperature θX of the 200 cm3 of hot water during the 180 s for which it cooled. θX = .....................................................°C [1] (ii) Calculate the average rate of fall in temperature R1 of the 200 cm3 of hot water over the 180 s. Use your answer to (d)(i) and the equation shown. θX R1 = 180 R1 = .................................................. °C/s [1] (e) The student empties the hot water from the beaker and repeats the experiment using 100 cm3 of hot water instead of 200 cm3. He records his results in the third column of Table 6.1. Calculate the average rate of fall in temperature R2 of the 100 cm3 of hot water over the 180 s. R2 = .................................................. °C/s [1] (f) Write a conclusion stating how the volume of hot water affects its rate of cooling. Justify your conclusion by referring to the results. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (g) Refer to Table 6.1. Suggest one improvement that could be made to the investigation to make it a fairer comparison of the cooling rates of the two different volumes of water. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 10]

Mark scheme: 6(a) 85(.0) ; 1 6(b)(i) S and °C and °C ; 1 6(b)(ii) 30, 60, 90, 120, 150 ; 1 6(c) read at 90° / perpendicular / at eye level to the scale / stir before reading / keep thermometer at same level / wait until reading stops rising initially / thermometer not touching sides or bottom (of beaker) ; 1 6(d)(i) θX = 10.5 (°C) ; 1 6(d)(ii) R1 = 0.0583 (°C/s) ; 1 6(e) R2 = 0.0805 (°C/s) ; 1 6(f) smaller volume greater temperature change/drop / smaller amount of water cools more quickly ; quoting data from table i.e. temperatures quoted for the same time (or the time quoted as well) ; 2 6(g) start with the same initial temperatures ; 1

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7 A student has three solutions containing different concentrations of reducing sugar. Plan an investigation to test the three solutions for the presence of reducing sugar and place them in order of concentration. In your answer, include: • a brief description of the method • which variables you will keep the same and any safety precautions • the observations for positive and negative results • how you would use the results to place them in order of concentration of reducing sugar. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [7] [Total: 7]

Mark scheme: 7 Minimum of 1 mark from each section and any two other marking points method heat ; Benedict’s solution ; safety waterbath (not naked flame) / goggles ; variables same volume of food solution ; same volume / concentration of testing solution ; run test for the same time ; observations start/negative is blue ; positive is green / yellow / orange / red ; conclusion colours in order : green/yellow – orange – red ; max 7

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