Cambridge IGCSE Sciences - Co-ordinated (Double) 0654 — 2024 Oct/Nov Paper 5 · Variant 3

1 You are going to investigate an enzyme-controlled reaction. Catalase is an enzyme found inside living cells such as yeast cells. It catalyses the breakdown of hydrogen peroxide, releasing oxygen gas. When a suspension of yeast cells is mixed with hydrogen peroxide solution the oxygen released produces a foam. You are provided with a suspension of yeast cells and different concentrations of hydrogen peroxide solution. (a) (i) Read through the procedure in (a)(ii) and draw a table to record your results. [2] (ii) Procedure • Stir the suspension of yeast cells with a clean glass rod. • Use a clean syringe to put 2 cm3 of yeast cell suspension into a clean boiling tube. • Use a clean syringe to add 2 cm3 of 6% hydrogen peroxide solution to the boiling tube with the yeast suspension. • Start the stop-watch. • At 2 minutes, measure the height h in your boiling tube, as shown in Fig. 1.1. • Record in your results table in (a)(i) this value in millimetres to the nearest millimetre. foam solution h Fig. 1.1 Repeat the procedure with 4% hydrogen peroxide solution, 2% hydrogen peroxide solution and 0% hydrogen peroxide solution. [4] (iii) State the relationship between the concentration of the hydrogen peroxide solution and the height h of the foam and solution. ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Explain why repeating the experiment would allow you to have more confidence in your results. ........................................................................................................................................... ..................................................................................................................................... [1] (v) Suggest why it is important to stir and mix the yeast suspension at the start of the procedure in (a)(ii). ........................................................................................................................................... ..................................................................................................................................... [1] (vi) Suggest why it is important to use a clean syringe to add the hydrogen peroxide solution each time. ........................................................................................................................................... ..................................................................................................................................... [1] (vii) Describe one difficulty in measuring the height h. ........................................................................................................................................... ..................................................................................................................................... [1] (viii) Suggest a piece of apparatus that can be used to measure the amount of gas produced in a reaction more accurately. ........................................................................................................................................... ..................................................................................................................................... [1] (b) The amount of hydrogen peroxide in a solution of hydrogen peroxide is described as a percentage. A student has a solution of 10% hydrogen peroxide. Calculate the volumes of water and 10% hydrogen peroxide solution needed to make 10 cm3 of 6% hydrogen peroxide solution. volume of 10% hydrogen peroxide solution = ........................................................ cm3 volume of water = .................................................. cm3 [1] [Total: 13]

Mark scheme: Question Answer Marks 1(a)(i) columns / rows with headings separated and headings hydrogen peroxide and height / h ; 2 correct units in the headings for hydrogen peroxide and h ; 1(a)(ii) value for 6% ; 4 full set of values ; value for 6% > 4% > 2% > 0% and 0% value > 0 ; all readings in mm ; 1(a)(iii) as concentration increases height increases ; 1 1(a)(iv) identify / exclude anomalies ; 1 1(a)(v) even distribution of cells / yeast / enzymes / prevents cells / yeast / enzyme settling ; 1 1(a)(vi) contamination with other concentrations (of hydrogen peroxide) ; 1 1(a)(vii) top not level / test-tube curved (at the bottom) ; 1 1(a)(viii) gas syringe ; 1 1(b) 6 (cm3) hydrogen peroxide and 4 (cm3) water ; 1

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2 The pH of saliva in the mouth is approximately 7. Eating and drinking lowers the pH of saliva in the mouth. This can cause tooth decay. Mouthwash is sometimes used to raise the pH of saliva in the mouth. Plan an investigation to determine the relationship between the volume of mouthwash used and the pH of saliva. You are provided with: • mouthwash • a solution of saliva at pH 3. You may use any laboratory apparatus. You are not required to do this investigation. In your plan include: • the apparatus needed • a brief description of the method • the measurements you will make • the variables you will control • how you process your results to draw a conclusion. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [7]

Mark scheme: 2 one marking point from each section and any other two marks 7 apparatus universal indicator (+ colour chart) and its use ; measuring cylinder / burette / syringe and its use ; method saliva add mouthwash and measure pH for at least 2 different volumes / amounts of mouthwash ; measurements volume of mouthwash ; measure pH of solution (after mixing) ; indication of pH colour chart used ; 5 different volumes of mouthwash ; repeat each volume to identify / exclude anomalies ; control variables concentration of mouthwash ; volume / amount of saliva ; processing and conclusion plot graph of pH against volume of mouthwash ; description of how to use shape of the graph ; as volume of mouthwash increases does pH of saliva increase / decrease / stay same ;

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3 You are going to investigate the reactivity of metals by heating some metal carbonates. Some metal carbonates break down and release carbon dioxide when they are heated. The carbonate of a more reactive metal takes a longer time to break down than the carbonate of a less reactive metal. (a) (i) Procedure • Half fill a test-tube with limewater and put it in the test-tube rack. • Put three spatula loads of copper carbonate into a clean hard-glass test-tube. • Record in Table 3.1 the colour of the copper carbonate before heating. • Attach the delivery tube and stopper to the hard-glass test-tube as shown in Fig. 3.1. • Place the delivery tube into the limewater as shown in Fig. 3.1. • Hold the test-tube of copper carbonate with test-tube holders near the stoppered end of the test-tube. copper carbonate delivery tube hard-glass test-tube Bunsen burner test-tube heatproof mat limewater Fig. 3.1 • Carefully heat the copper carbonate and start the stop-clock. • When the limewater starts to go milky take the delivery tube out of the limewater. You must take the delivery tube out of the limewater while you are still heating. • Stop the stop-clock and stop heating. • The hard-glass test-tube is very hot – do not touch it. • Put the hard-glass test-tube on the heatproof mat with the test-tube holders still attached. • Record in Table 3.1 the time in seconds, to the nearest second. • Put the hard-glass test-tube in the test-tube rack to allow it to fully cool. • Record the colour of the hot solid after heating in Table 3.1. • Carefully remove the delivery tube from the hard-glass test-tube. • Rinse the test-tube containing the limewater for reuse with the next metal carbonate. Repeat the procedure with iron(II) carbonate, magnesium carbonate and zinc carbonate instead of the copper carbonate. If the limewater doesn’t go milky after 300 seconds record the time as ‘> 300’. Table 3.1 colour of metal time for the colour of hot metal rate of reaction carbonate limewater to solid after carbonate per 100 s before heating go milky / s heating copper carbonate iron(II) carbonate magnesium carbonate zinc carbonate [7] (ii) The colour of the solid remaining after zinc carbonate is heated changes as it cools down. State the colour of the cold solid. ..................................................................................................................................... [1] (b) Calculate the rate of reaction for each metal carbonate. Use the equation shown. 100 rate of reaction = time Record in Table 3.1 your values to three significant figures. [2] (c) Using the results in Table 3.1 place the metals copper, iron, magnesium and zinc in order of reactivity, starting with the most reactive. most reactive ................................................. ................................................. ................................................. least reactive ................................................. [1] (d) In the procedure it is important to take the delivery tube out of the limewater before stopping heating. Explain why this is important for safety reasons. ................................................................................................................................................... ............................................................................................................................................. [1] (e) Suggest one improvement to the procedure that would give more confidence in the order of reactivity of the metals given in (c). Do not include repeating the experiment. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 13]

Mark scheme: 3(a)(i) 7 metal colour before colour after carbonate heating heating copper green black iron green / brown black / dark red / dark brown magnesium white white zinc white yellow colours for copper carbonate ; colours before heating for other three ; colours after for other three ; time for copper carbonate ; times for all four ; times in seconds to nearest second ; in order (smallest to largest) copper, iron, zinc, magnesium ; 3(a)(ii) white ; 1 3(b) 4 correct calculations ; 2 3sf ; 3(c) magnesium 1 zinc iron copper ; 3(d) the limewater would “suck – back” into the heated tube and it would break ; 1 3(e) any one from: 1 weigh the metal carbonates collect / measure the gas given off ; have the heating the same for each carbonate e.g. same height of flame / same distance from flame / same heat of the flame ;

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4 You are going to investigate the reactivity of metals by measuring voltages in electrochemical cells. When two different metals are dipped into an aqueous salt solution, they produce a voltage. The larger the difference in reactivity between the two metals, the greater the voltage produced. (a) Procedure • Half fill a small beaker with aqueous salt solution. • Assemble the apparatus as shown in Fig. 4.1, keeping the copper strip towards the edge of the beaker. + – V copper strip attached to wire by a crocodile clip aqueous salt solution Fig. 4.1 • Attach a strip of iron to the positive terminal of the voltmeter and place it carefully at the opposite side of the beaker to the strip of copper as shown in Fig. 4.2. Do not let the metals touch. + – V iron strip attached copper to wire by a crocodile clip aqueous salt solution Fig. 4.2 • Record in Table 4.1 the voltage reading on the voltmeter as soon as the iron strip is placed into the aqueous salt solution. • Remove the iron strip. Repeat the procedure using strips of magnesium, zinc and copper instead of iron. Table 4.1 metal attached to voltage / V the positive terminal iron magnesium zinc copper [5] (b) Using the results in Table 4.1 place the metals copper, iron, magnesium and zinc in order of reactivity, starting with the most reactive. most reactive ................................................. ................................................. ................................................. least reactive ................................................. [1] (c) Question 3 and question 4 use two different procedures to determine the order of reactivity of the metals. Suggest which procedure allows the order of reactivity to be determined with more accuracy. Tick (✓) the box. procedure from question 3 procedure from question 4 Explain your answer. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 7]

Mark scheme: 4(a) voltage for iron ; 5 voltage for all metals ; voltages to the same number of decimal places ; Cu is 0 ; order Mg > Zn > Fe > Cu ; 4(b) Mg ; 1 Zn ; Fe ; Cu ; 4(c) (procedure from question) 4 and / difficult to ascertain when cloudy or same level of cloudiness / voltage more precise ; 1

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5 You are going to investigate the changes in potential difference V across a length of resistance wire in an electrical circuit. Fig. 5.1 shows a circuit with a resistance wire. The circuit is assembled for you. A l R X Y S V metre rule resistance wire Fig. 5.1 (a) Procedure • Close the switch. • Record the current I in the circuit. • Open the switch. I = ...................................................... A [1] (b) • Place the sliding contact S at a distance of l = 10.0 cm from end X of the resistance wire and then close the switch. • Record in Table 5.1 the reading V on the voltmeter. • Open the switch. Table 5.1 l / cm V / V 10.0 20.0 40.0 60.0 80.0 [1] (c) Repeat the procedure in (b) for values of l of 20.0 cm, 40.0 cm, 60.0 cm and 80.0 cm. [2] (d) Suggest one practical reason why your values for the length l of resistance wire are only approximate. ................................................................................................................................................... ............................................................................................................................................. [1] (f) (i) Extend the best-fit line until it crosses the vertical axis. Record the intercept c that the line makes on the vertical axis. c = ......................................................... [1] (ii) Calculate the numerical ratio r. Use the equation shown and the current I you measured in (a). c r = I r = ......................................................... [1] (g) The teacher says that the ratio r in (f)(ii) is expected to be 3.3. Two values are considered to be equal within the limits of experimental accuracy if they are within 10% of each other. Compare your ratio r from part (f)(ii) with the expected ratio 3.3. State if your value of r is close enough to 3.3 so that the ratios can be considered equal, within the limits of experimental accuracy. Justify your statement with a calculation. statement .................................................................................................................................. ................................................................................................................................................... justification ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [2] [Total: 12]

Mark scheme: 5(a) I value < 0.5 A and to at least 2 decimal places ; 1 5(b) V value < 2 V and to at least 1 decimal place ; 1 5(c) all V values recorded ; 2 V values increasing ; 5(d) difficult to position crocodile clip on the wire to the nearest mm / width of contact on the crocodile clip ; 1 5(e)(i) suitable linear scales, starting from the origin and plotted points cover ⩾ ½ the grid used ; 2 points plotted correctly to ± ½ small square ; 5(e)(ii) good judgement best-fit straight line ; 1 5(f)(i) intercept correct from candidate’s graph ± 1 small square ; 1 5(f)(ii) r calculation correct ; 1 5(g) Comparing the candidates value to the true value of 3.3 with calculations ;; 2 e.g. using the values of 3.4 (the students value) and 3.3 the value in the question) 10% of 3.3 is 0.33 ; 3.3 + 0.33 = 3.63 and 3.4 is smaller and this is within 10% ; OR (3.4 ÷ 3.3) = 0.97 ; 0.97  100 = 97% and so this is within 10% ; OR (3.4 – 3.3) ÷ 3.3 = 0.033 ; 0.033  100 = 3.3 % and is within 10% ;

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6 You are going to use two different methods to measure the density of water. Method 1 (a) Record the mass m1 of the empty 100 cm3 measuring cylinder to the nearest gram. m1 = ...................................................... g [1] (b) Remove the measuring cylinder from the balance and approximately half-fill it with water. Record the volume V1 of the water in the measuring cylinder. V1 = ........................................................ cm3 Record the mass m2 of the measuring cylinder containing the water to the nearest gram. Keep the measuring cylinder containing the water for use in method 2. m2 = ............................................................ g [1] (c) Calculate the density ρ1 of the water. Use the equation shown. (m2 – m1) ρ1 = V1 ρ1 = .............................................. g / cm3 [1] (d) State how you ensure that your reading of the volume of water in the measuring cylinder is as accurate as possible. ................................................................................................................................................... ............................................................................................................................................. [1] Method 2 (e) (i) Use the balance to record the mass m3 of the test-tube to the nearest gram. m3 = ...................................................... g [1] (ii) Procedure • Use the measuring cylinder containing the water used in method 1. • Slowly lower the test-tube into the measuring cylinder until it floats, approximately vertically, as shown in Fig. 6.1. The test-tube should not touch the bottom of the measuring cylinder. cm3 test-tube measuring cylinder water Fig. 6.1 Record the new water level V2 in the measuring cylinder. V2 = ........................................................ cm3 Calculate the volume V3 of water displaced by the test-tube. Use the equation shown. V3 = V2 – V1 V3 = .................................................. cm3 [1] (f) Calculate the density ρ2 of the water using your values from (e)(i) and (e)(ii). Use the equation shown. m3 ρ2 = V3 ρ2 = .............................................. g / cm3 [1] (g) Suggest why method 1 is done before method 2. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 8]

Mark scheme: 6(a) m1 recorded to the nearest gram ; 1 6(b) V1 present and m2 > m1 ; 1 6(c) 1 correct ; 1 6(d) perpendicular viewing of scale / read scale at eye level ; 1 6(e)(i) m3 present ; 1 6(e)(ii) V2 present and > V1 ; 1 6(f) 2 within 10% of 1 ; 1 6(g) measuring cylinder will be wet / contains some water when its dry mass is measured ; 1

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