Cambridge IGCSE Science - Combined 0653 — 2023 Feb/March Paper 6 · Variant 2

1 When acid is added to milk, the milk clots, forming lumps. A student investigates the effect of varying the concentration of the acid. Procedure The student: Step 1 puts 5 cm3 of milk into each of five labelled test-tubes, A, B, C, D, and E Step 2 sets up a water-bath at 30 °C Step 3 puts the five test-tubes of milk into the water-bath Step 4 adds 1 cm3 of 0.4 M hydrochloric acid to test-tube A and swirls to mix (Note: M is a unit of concentration where 2 M is twice as concentrated as 1 M.) Step 5 repeats Step 4 with the four other test-tubes, using the different concentrations of hydrochloric acid as shown in Table 1.1 Table 1.1 test-tube concentration of hydrochloric acid / M A 0.4 B 0.3 C 0.2 D 0.1 E 0.0 Step 6 removes test-tube A from the water-bath and observes the contents Step 7 decides on the clotting score using the scale in Table 1.2 Table 1.2 clotting score description 1 no clotting 2 small lumps 3 large lumps 4 almost all solid 5 all solid Step 8 repeats Step 6 and Step 7 with the four other test-tubes. The student’s results are shown in Fig. 1.1. Key A B C D E = solid = milk Fig. 1.1 (a) (i) Use Fig. 1.1 and Table 1.2 to determine the clotting score for each test-tube. Record in Table 1.3 the clotting score for each test-tube. Table 1.3 test-tube concentration of clotting score hydrochloric acid / M A 0.4 B 0.3 C 0.2 D 0.1 E 0.0 [3] (ii) State the relationship between the concentration of hydrochloric acid and the clotting score of the milk. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Hydrochloric acid changes the shape of protein molecules making them stick together. Use this information and your results from Table 1.3 to state a conclusion for this investigation in terms of the milk and protein. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) The student measures the final temperature of the water-bath. Fig. 1.2 shows the reading on the thermometer. °C 30 20 Fig. 1.2 Record this temperature to the nearest 0.5 °C. temperature = .................................................... °C [1] (v) The starting temperature of the water-bath was 30 °C. Suggest if temperature is a source of error in this investigation. Tick (3) the appropriate box. • temperature is not a source of error • temperature is a source of error Give a reason for your answer. ........................................................................................................................................... ..................................................................................................................................... [1] (vi) Identify one other possible source of error in this investigation. ........................................................................................................................................... ..................................................................................................................................... [1] (b) The student filters the contents of each test-tube and measures the mass of any solid in the filter paper. The results are shown in Table 1.4. Table 1.4 test-tube concentration of mass of solid hydrochloric acid / g / M A 0.4 4.9 B 0.3 3.4 C 0.2 2.8 D 0.1 1.2 E 0.0 0.0 (i) Calculate the percentage of milk clotted in test-tube B. Use the equation shown where 5.1 is the mass (g) of the 5 cm3 of milk. mass of solid in test-tube B percentage of milk clotted = × 100 5.1 percentage clotted = .......................................................... [1] (ii) Suggest if the results in Table 1.4 support the relationship in (a)(ii). Explain your answer. ........................................................................................................................................... ..................................................................................................................................... [1] (c) A student notices that milk removed from a refrigerator clots after several days. Fig. 1.3 shows bacteria growing in the milk when viewed using a microscope. Line F on Fig. 1.3 is the length of one bacterium. F magnification = ×15 000 Fig. 1.3 (i) Measure the length of line F on Fig. 1.3. length = .................................................. mm [1] (ii) Calculate the actual length of the bacterium. Use the equation shown. length on Fig. 1.3 actual length = magnification Give your answer to two significant figures. actual length = ................................................... mm [2] [Total: 13]

Mark scheme: Question Answer Marks 1(a)(i) all results recorded ; 3 number score used instead of, words / descriptions ; correct values assigned ; 1(a)(ii) higher concentration of acid gives higher clotting score / ORA ; 1 1(a)(iii) the higher the concentration of acid the more the protein has changed shape ; 1 1(a)(iv) temperature recorded as 24.5 (°C) ; 1 1(a)(v) not a source of error because 1 all test-tubes exposed to same temperature ; 1(a)(vi) idea that time in water bath not controlled / clotting score is subjective / insufficient mixing ; 1 1(b)(i) 67 / 66.7% ; 1 1(b)(ii) (yes because ….) 1 both show same trend / results (in Table 4.1) show more, solid / clotting when more acid is added ; 1(c)(i) 47 (mm) ; 1 1(c)(ii) 47 ÷ 15 000 ; 2 0.0031 / 3.1  10–3 (mm) ;

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2 A student investigates the properties of aqueous sodium hydroxide. (a) Procedure The student: Step 1 pours 15.0 cm3 of aqueous sodium hydroxide into a glass beaker Step 2 records in Table 2.1 the temperature of the aqueous sodium hydroxide to the nearest 0.5 °C Step 3 fills a burette to the 0.0 cm3 mark with dilute hydrochloric acid as shown in Fig. 2.1 0 10 burette containing dilute 20 hydrochloric acid 30 40 50 15.0 cm3 of aqueous sodium hydroxide Fig. 2.1 Step 4 adds 5.0 cm3 of dilute hydrochloric acid from the burette to the beaker containing aqueous sodium hydroxide Step 5 stirs the mixture Step 6 records in Table 2.1 the temperature of the contents of the beaker to the nearest 0.5 °C Step 7 repeats Step 4 to Step 6 until a total of 25.0 cm3 of dilute hydrochloric acid is added. (i) Fig. 2.2 shows the thermometer reading in Step 2. °C 30 20 10 Fig. 2.2 Record in Table 2.1 this temperature to the nearest 0.5 °C. Table 2.1 total volume of hydrochloric acid added temperature of reaction mixture / cm3 / °C 0.0 5.0 28.0 10.0 32.0 15.0 34.5 20.0 33.0 25.0 31.5 [1] (ii) On the grid, plot a graph of temperature of reaction mixture (vertical axis) against the total volume of hydrochloric acid added. Do not start the temperature scale at 0 °C. (v) Explain why in Step 5 the mixture in the beaker is stirred before the temperature is measured. ..................................................................................................................................... [1] (vi) Another student repeats the investigation. This student uses a measuring cylinder to measure each of the 5.0 cm3 volumes of dilute hydrochloric acid instead of a burette. Explain one advantage of using a burette other than the precision of measurement. ........................................................................................................................................... ..................................................................................................................................... [1] (vii) Thermal energy (heat) is lost from the reaction mixture during the experiment. Describe one change in the apparatus to reduce this loss of thermal energy. ..................................................................................................................................... [1] (b) Procedure The student: • puts some aqueous sodium hydroxide into a clean test-tube • adds about 20 drops of an unknown solution G to the aqueous sodium hydroxide. (i) The student observes a green precipitate. Circle the metal ion present in solution G. aluminium calcium iron(II) iron(III) zinc [1] (ii) The student does not measure the volumes of the two solutions. Explain why measuring the volumes is not important in this procedure. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) The student repeats the procedure but this time uses 20 drops of aqueous copper(II) sulfate instead of solution G. Describe what the student observes in the test-tube. ..................................................................................................................................... [1] [Total: 13]

Mark scheme: 2(a)(i) 21.0 ; 1 2(a)(ii) y-axis labelled temperature in °C AND x-axis labelled (total) volume (of acid added) in cm3 ; 3 suitable linear scales so that points occupy at least half of the grid ; plots correct  ½ small square ; 2(a)(iii) best fit curve drawn ; 1 2(a)(iv) as the volume of acid increases the temperature increases ; 2 and (after increase) then decreases ; 2(a)(v) to ensure an even temperature ; 1 2(a)(vi) less thermal energy lost between experiments / less cooling / do not need to keep refilling burette ; 1 2(a)(vii) use a plastic beaker / use a polystyrene cup / wrap beaker with insulation ; 1 2(b)(i) iron(II) ; 1 2(b)(ii) it is a qualitative test not a quantitative test / only depends on the observation not any numerical values ; 1 2(b)(iii) blue precipitate / ppt ; 1

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3 A student investigates the extension of a spring as different loads are suspended from the spring. The student uses the apparatus shown in Fig. 3.1. 0 cm mark clamp boss top of spring bottom of spring clamp stand metre rule bench 100 cm mark Fig. 3.1 The student measures the length of the spring, l0 , with no load suspended from it. l0 = 23 mm (a) State one practical precaution that the student takes to ensure that the value of l0 is as accurate as possible. ................................................................................................................................................... ............................................................................................................................................. [1] (b) The student suspends loads of 0.20 N, 0.40 N, 0.60 N and 0.80 N from the spring. For each load, L, the length, l , is measured and recorded in Table 3.1. (i) Fig. 3.2 shows the spring with a load of 0.60 N suspended from it. l 0.60 N (drawn actual size) Fig. 3.2 Measure the length, l , of the spring. Record l in Table 3.1. Table 3.1 L l e k / N / mm / mm / N per mm 0.00 23 0 – 0.20 27 4 0.05 0.40 35 12 0.03 0.60 0.80 49 26 0.03 [1] (ii) Calculate the extension e of the spring for the load of 0.60 N. Use the equation shown. e = l – l 0. Record this value of e in Table 3.1. [1] (iii) The spring constant, k, of the spring is a measure of its elastic stiffness. Calculate the spring constant, k, for the load of 0.60 N. Use the equation shown. L k = e Record this value of k in Table 3.1. [1] (c) One of the values of k is anomalous. State which value is anomalous. Give a reason for your answer. anomalous value ..................................... N per mm reason ....................................................................................................................................... ................................................................................................................................................... [2] (d) Suggest what the student can do to have more confidence in their values of l. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 7]

Mark scheme: 3(a) keep your eyes level with the, point being measured / top of the spring / avoid parallax / look perpendicular to the, point 1 behind measured / top of the spring / fiducial aid / keep ruler as close as possible to the spring ; 3(b)(i) 43  1 ; 1 3(b)(ii) 20 ; 1 3(b)(iii) 0.03 ; 1 3(c) anomalous value = 0.05 ; 2 reason: other values are all 0.03 / is different from the other k values ; 3(d) measure as load is both added and removed, to check that l is the same / check spring has not stretched / repeat the 1 measurement and see if it is the, same / similar / close to original ;

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4 Plan an investigation to find out how much (electrical) energy is required to increase the temperature of different liquids. You are provided with: • four different liquids: water, salt solution, vegetable oil and vinegar • an electric heater and power supply, assembled as shown in Fig. 4.1. power supply A V heater glass beaker liquid Fig. 4.1 The energy, E, transferred by the heater is calculated using the equation shown E = V × I × t where V is the potential difference across the heater I is the current through the heater t is the time in seconds that the heater is switched on You may use any common laboratory apparatus in your plan. In your plan, include: • any other apparatus needed • a brief description of the method, including what you will measure and how you will make sure your measurements are accurate • the variables you will control • a results table to record your measurements (you do not need to enter any readings in the table) • how you will process your results to draw a conclusion. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [7]

Mark scheme: 4 One mark from each section and any two other marking points: 7 additional apparatus: (Max two from this section) measuring cylinder / balance ; stop-watch ; thermometer ; method and measurements: measure volume / mass of liquid before starting / states a volume or mass for the liquid ; measure temperature of liquid before starting and after heating ; heat the liquid for a fixed time and measure temperature (increase) / heat the liquid for a fixed temperature increase and measure the time taken ; control variables: use constant mass or volume of each liquid ; voltage and / or current of heater ; use the same initial temperature of liquid ; table of results: table shown with four liquids and columns for voltage, current, temperature and / or time (depending on their method) ; with units for each column shown ; conclusion: describes how to calculate energy transferred for each liquid ; compare the results in the table to see which is greatest ; repeat to, identify anomalies / increase confidence (take average) ;

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