Cambridge IGCSE Science - Combined 0653 — 2020 May/June Paper 5 · Variant 2

1 You are going to investigate the effect of substrate concentration on the rate of a reaction. Catalase is an enzyme that breaks down the substrate, hydrogen peroxide, to release oxygen gas. catalase hydrogen peroxide water + oxygen The oxygen gas causes paper disks to rise in a solution of hydrogen peroxide. The greater the rate of reaction, the faster the disks will rise to the surface. (a) You are provided with catalase solution and five different concentrations of hydrogen peroxide solution labelled, 5%, 4%, 3%, 2% and 1%. Step 1 Use the forceps to dip one of the small paper disks in the catalase solution, as shown in Fig. 1.1. catalase beaker solution Fig. 1.1 Step 2 Place the small disk onto the paper towel. Step 3 Repeat step 1 and step 2 with nine more disks. Step 4 Use the forceps to pick up one of the soaked disks and drop it into the solution labelled 5%. Step 5 Start the stop-clock. The disk will sink to the bottom of the beaker and then slowly rise to the surface. If the disk does not sink immediately, push it down gently with the forceps. Step 6 Record, to the nearest second, the time it takes for the disk to rise to the surface. If it takes more than 2 minutes then record the time as greater than 120 s. Step 7 Repeat step 4 to step 6 with a second disk. Step 8 Repeat step 4 to step 7 using the solutions labelled 4%, 3%, 2% and 1%. (i) Record your results in Table 1.1. Table 1.1 time for the disk to rise to the surface / spercentage concentration of hydrogen peroxide 1st disk 2nd disk average 5 4 3 2 1 [3] (ii) Calculate the average time for each concentration and record these values in Table 1.1. Ignore any values that are greater than 120 s. [1] (iii) Describe the relationship between the percentage concentration of hydrogen peroxide and the average time for the disks to rise to the surface. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Suggest why two disks are used in each beaker. ........................................................................................................................................... ..................................................................................................................................... [1] (b) (i) On the grid, plot a graph of the average time for the disks to rise (vertical axis) and the percentage concentration of hydrogen peroxide. [3] (ii) Draw the curve of best fit. [1] (iii) Determine the average time for a disk to rise to the surface in a 3.7% hydrogen peroxide solution. Show on your graph how you arrived at your answer. average time = ...................................................... s [1] (c) Describe one possible source of error in this investigation. ................................................................................................................................................... ............................................................................................................................................. [1] (d) State one safety hazard associated with this investigation and explain how the risk from this hazard is reduced. safety hazard ............................................................................................................................ explanation ............................................................................................................................... ................................................................................................................................................... [1] [Total: 13]

Mark scheme: 1(a)(i) results for 5% recorded ; all results recorded ; 5% faster than 1% ; 3 1(a)(ii) averages correctly calculated for results ; 1 1(a)(iii) higher percentage concentration of hydrogen peroxide, shorter time to rise to the surface ; 1 1(a)(iv) to spot anomalous results ; 1 1(b)(i) suitable scale chosen ; labels correct for both axes, including units ; all points plotted correctly ; 3 1(b)(ii) suitable curve of best fit drawn ; 1 1(b)(iii) correct value from candidate’s graph and indication shown on graph ; 1 1(c) difficulty knowing when disk reaches top / unequal blotting / unknown catalase concentration ; 1 1(d) use of hydrogen peroxide AND wear, safety glasses / gloves ; 1

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2 You are going to investigate the reaction of some metals with dilute hydrochloric acid and with aqueous copper sulfate. The order of reactivity of some metals is shown. potassium most reactive sodium magnesium aluminium zinc iron lead copper silver least reactive (a) Procedure – reaction with dilute hydrochloric acid • Measure 10 cm3 of dilute hydrochloric acid with a measuring cylinder and pour into a large test-tube. • Measure to the nearest 0.5 °C the initial temperature of the dilute hydrochloric acid. • Record in Table 2.1 this initial temperature of hydrochloric acid for the metal, iron. • Add 1 spatula of iron powder to this test-tube. • Stir the mixture and start the stop-clock. • Measure to the nearest 0.5 °C the final temperature of the mixture after 2 minutes. • Record in Table 2.1 this final temperature. (i) Repeat the procedure in (a) using separate samples of magnesium powder, zinc powder and copper powder instead of the iron powder. Table 2.1 initial temperature of final temperature of temperature change metal hydrochloric acid the mixture / °C / °C / °C iron magnesium zinc copper [3] (ii) Calculate the temperature change for each metal. Record this temperature change in Table 2.1. [1] (iii) State which metal produces the largest temperature change. Suggest why this metal produces the largest temperature change. metal ................................................................................................................................. suggestion ......................................................................................................................... [1] (iv) Explain the temperature change produced with copper. ..................................................................................................................................... [1] (v) Suggest one change to the apparatus which would improve the accuracy of the results. ........................................................................................................................................... ..................................................................................................................................... [1] (vi) The gas given off in this experiment is hydrogen. State the test for hydrogen gas and give the result of a positive test. test .................................................................................................................................... positive result .................................................................................................................... [1] (b) Procedure – reaction with aqueous copper sulfate • Measure 10 cm3 of aqueous copper sulfate with a measuring cylinder and put into a large test-tube. • Measure to the nearest 0.5 °C the initial temperature of the aqueous copper sulfate. • Record in Table 2.2 this initial temperature of aqueous copper sulfate for the metal, magnesium. • Add 1 spatula of magnesium powder to this test-tube. • Stir the mixture and start the stop-clock. • Measure to the nearest 0.5 °C the final temperature of the mixture after 2 minutes. • Record in Table 2.2 this final temperature. (i) Repeat the procedure in (b) using iron powder instead of the magnesium powder. Table 2.2 initial temperature of final temperature of temperature change metal aqueous copper sulfate the mixture / °C / °C / °C magnesium iron [2] (ii) Allow the mixture to settle. Look carefully at the test-tube containing iron. Describe the contents of the test-tube. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Explain the change observed in (ii). ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Calculate the temperature change for magnesium and iron. Record your results in Table 2.2. Predict the change in temperature you would expect if zinc is added to aqueous copper sulfate. temperature change for zinc = .......................................................... °C [1] [Total: 13]

Mark scheme: 2(a)(i) two temperatures for iron ; all temperatures recorded and to 0.5 °C ; final temperature of magnesium greatest and copper smallest ; 3 2(a)(ii) all changes in temperature calculated correctly ; 1 2(a)(iii) magnesium AND most reactive (metal of these four) ; 1 Question Answer Marks 2(a)(iv) copper doesn't react with acids ; 1 2(a)(v) pipette (instead of measuring cylinder) ; ALLOW weigh metal ; 1 2(a)(vi) lighted splint AND pops ; 1 2(b)(i) all temperatures recorded ; final temperature Mg > Fe ; 2 2(b)(ii) pink solid / colourless / very pale blue / green, solution ; 1 2(b)(iii) displacement / iron takes the place of copper / iron more reactive than copper ; 1 2(b)(iv) temperature change calculated correctly for Mg and Fe AND (predicted change for zinc) between Mg and Fe ; 1

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3 You are going to investigate the resistance of a piece of wire. (a) The circuit shown in Fig. 3.1 has been set up for you. power supply A V 0 cm 100 cm X metre rule resistance wire crocodile clip Fig. 3.1 (i) • Connect the crocodile clip at X so that the length of resistance wire in the circuit is 100.0 cm. • Close the switch. • Measure and record the potential difference (p.d.) across the wire and the current through the wire. • Open the switch. p.d. = ........................................................... V current = ............................................................ A [1] (ii) Calculate the resistance R1 of the 100.0 cm length of wire. Use your results in (a)(i) and the equation shown. p.d. resistance = current R1 = ..................................................... Ω [1] (b) (i) • Remove the crocodile clip and connect it so that the length of resistance wire in the circuit is 50.0 cm. • Close the switch. • Measure and record the p.d. across the wire and the current through the wire. • Open the switch. p.d. = ........................................................... V current = ............................................................ A [1] (ii) Calculate the resistance R2 of the 50.0 cm length of wire. Give your answer to two significant figures. R2 = ..................................................... Ω [1] (c) A student suggests that the resistance of a 100.0 cm length of the wire should be double the resistance of a 50.0 cm length of the wire. State whether your values of R1 and R2 support the student’s suggestion. Justify your answer with reference to your results. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] (d) Another student suggests that the resistance of a wire is proportional to its length. (i) Describe how the student can extend this experiment to test their suggestion. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Explain how the student can process the results to reach a conclusion. ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 7]

Mark scheme: 3(a)(i) values of p.d. AND current recorded ; 1 3(a)(ii) correct value of R1 calculated (Ω) ; 1 3(b)(i) values of p.d. and current recorded AND current value higher than in (a)(i) ; 1 3(b)(ii) R2 correct to 2 s.f. AND R1> R2 ; 1 3(c) (expect yes) R1 is approximately double R2, it is within the limits of experimental accuracy / the value is close to double ; 1 3(d)(i) take several more readings of p.d. and current at different lengths of wire / at least two more different lengths used ; 1 3(d)(ii) plot graph of resistance against length, straight line through origin if resistance is proportional to length / compare ratio of length to resistance for different values in a results table ; 1

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4 Fig. 4.1 shows an empty test-tube, floating in a measuring cylinder containing pure water. If the same test-tube is put into a measuring cylinder containing a solution of salt and water it will float with a smaller length of the test-tube below the surface. This is because the salt solution has a greater density than pure water. empty test-tube measuring cylinder pure water Fig. 4.1 The higher the concentration of salt in the solution the greater the density of the solution. Five salt solutions of different, known concentrations are placed in beakers and labelled A, B, C, D and E. Another salt solution of unknown concentration is placed in another beaker and labelled X. Plan an investigation to determine the concentration of salt solution X using the following apparatus: • a measuring cylinder • a test-tube • a 30 cm ruler. You may use any other common laboratory apparatus in your plan. You are not required to do this investigation. In your plan, include: • a brief description of the method • how you will ensure that your results are as accurate as possible • how you will process your results • how you will determine the concentration of X. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [7] NOTES FOR USE IN QUALITATIVE ANALYSIS Tests for anions anion test test result carbonate (CO32–) add dilute acid effervescence, carbon dioxide produced chloride (Cl –) acidify with dilute nitric acid, then white ppt. [in solution] add aqueous silver nitrate nitrate (NO3–) add aqueous sodium hydroxide then ammonia produced [in solution] aluminium foil; warm carefully sulfate (SO42–) acidify, then add aqueous barium white ppt. [in solution] nitrate Tests for aqueous cations cation effect of aqueous sodium hydroxide effect of aqueous ammonia ammonium (NH4+) ammonia produced on warming – calcium (Ca2+) white ppt., insoluble in excess no ppt. or very slight white ppt. copper (Cu2+) light blue ppt., insoluble in excess light blue ppt., soluble in excess, giving a dark blue solution iron(II) (Fe2+) green ppt., insoluble in excess green ppt., insoluble in excess iron(III) (Fe3+) red-brown ppt., insoluble in excess red-brown ppt., insoluble in excess zinc (Zn2+) white ppt., soluble in excess, giving a white ppt., soluble in excess, giving colourless solution a colourless solution Tests for gases Flame tests for metal ions gas test and test result metal ion flame colour ammonia (NH3) turns damp, red litmus paper blue lithium (Li+) red carbon dioxide (CO2) turns limewater milky sodium (Na+) yellow chlorine (Cl 2) bleaches damp litmus paper potassium (K+) lilac hydrogen (H2) ‘pops’ with a lighted splint copper(II) (Cu2+) blue-green oxygen (O2) relights a glowing splint

Mark scheme: 4 one marking point from each section and any three others: method pour sample of one salt solution into measuring cylinder ; (carefully) put test-tube into measuring cylinder ; use ruler to measure depth of test-tube in water / diagram showing use of ruler to measure depth of test-tube in water ; repeat with all known salt concentrations and unknown solution X ; mention of controlling variables ; accuracy repeat readings and take an average / or repeat to identify anomalies ; description of how to avoid parallax error when taking readings ; use a thin ruler that can fit inside the measuring cylinder next to test-tube / put ruler next to test-tube ; ensure measuring cylinder is rinsed with next solution ; processing results recording depths for each concentration (in, e.g. table of results) ; plot graph of depth against concentration ; conclusion description of using, graph / results table to find concentration of X ;

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