Cambridge IGCSE Science - Combined 0653 — 2021 May/June Paper 5 · Variant 1

1 You are going to investigate an enzyme-catalysed reaction. Hydrogen peroxide is broken down by catalase, an enzyme found in living cells such as the cells of potato tissue. Oxygen gas is released during the reaction. (a) You are provided with a supply of hydrogen peroxide solution of concentration 6% and a cylinder of potato tissue. Procedure Step 1 Label three test-tubes A, B and C. Step 2 Use a syringe to add the volumes of 6% hydrogen peroxide solution and water to the test-tubes as shown in Table 1.1. Step 3 Shake the test-tubes gently to mix. Table 1.1 volume of 6% hydrogen volume of water percentage concentration of test-tube peroxide solution / cm3 hydrogen peroxide solution / cm3 A 2 4 2 B 4 2 C 6 0 6 (i) Calculate the percentage concentration of hydrogen peroxide solution for test-tube B. Record your answer in Table 1.1 and Table 1.2. [1] (ii) Step 4 Cut three 1 mm slices of potato from the potato cylinder as shown in Fig. 1.1. 1 mm slice of potato potato cylinder Fig. 1.1 Step 5 Drop one potato slice into each labelled test-tube and start the stop-clock. Step 6 Record in Table 1.2 the time taken to the nearest second, for each potato slice to rise to the surface of the liquid. If the time taken is greater than 5 minutes then record this time as >300. Table 1.2 percentage concentration time taken for potato slice test-tube of hydrogen peroxide to rise to the surface solution / seconds A 2 B C 6 (iii) State a conclusion for your results. ........................................................................................................................................... ..................................................................................................................................... [1] (iv) State and explain one safety precaution taken when doing Step 4. ........................................................................................................................................... ..................................................................................................................................... [1] (v) Identify two variables that are controlled in this investigation. 1 ........................................................................................................................................ 2 ........................................................................................................................................ [2] (vi) Suggest why the experiment is not done using 20% concentration of hydrogen peroxide solution. ........................................................................................................................................... ..................................................................................................................................... [1] (b) Add a few drops of biuret solution to the remaining potato cylinder. Record your observations and state a conclusion. observation with biuret solution ................................................................................................ conclusion ................................................................................................................................. [2] (c) A student tests some potato with Benedict’s solution by following the procedure shown in Fig. 1.2. Do not do this procedure. Step 1 put a piece of potato in a test-tube Step 2 add 2 cm3 Benedict’s solution Step 3 shake the test-tube to mix Step 4 ..................................................... Step 5 observe the colour after 3 minutes Fig. 1.2 (i) State the instruction for Step 4. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) State an observation for a positive result. ..................................................................................................................................... [1] [Total: 13]

Mark scheme: 1(a)(i) 4 (%) ; 1 1(a)(ii) all three values recorded ; in whole seconds ; time reducing down the table ; 3 1(a)(iii) the higher the concentration of hydrogen peroxide / H2O2, the shorter the time taken / the faster the potato rises / ORA ; 1 1(a)(iv) (when using scalpel) cut on solid surface / away from fingers / hold with forceps ; 1 1(a)(v) any two from: size / volume / surface area / thickness, of potato ; volume / height, of solution in test-tube ; type of potato / same potato ; 2 1(a)(vi) time too short (to measure) / too fast to measure time ; 1 1(b) blue ; no protein ; 2 1(c)(i) heat (the mixture) ; 1 1(c)(ii) red / orange / yellow / green ; 1

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2 You are provided with five aqueous solutions, F, G, H, J and K. (a) Put approximately 1 cm depth of each solution into separate test-tubes. Add three drops of Universal Indicator solution to each test-tube. Record the colour for each solution in Table 2.1. [1] Table 2.1 solution colour with Universal Indicator pH of solution F G H J K (b) Use the Universal Indicator colour chart to determine the pH of each solution. Record your answers in Table 2.1. [2] (c) (i) Put the solutions in order of decreasing acidity and increasing alkalinity. most acidic .............. .............. .............. .............. most alkaline .............. [1] (ii) Explain why it is difficult to decide where to put F and K in the list in (c)(i). ........................................................................................................................................... ..................................................................................................................................... [1] (d) Add approximately 2 cm depth of solution F into a test-tube. Add a small piece of magnesium to the test-tube. Describe two observations of the reaction. 1 ................................................................................................................................................ 2 ................................................................................................................................................ [2] [Total: 7]

Mark scheme: 2(a) solution colour with Universal Indicator F red G green H blue-green J purple K red ; 1 2(b) solution pH of solution F 0 to 3 G 5 to 8 H 9 to 13 J 10 to 14 K 0 to 3 ;; F and K pH values correct ; pH of G < H / J ; 2 2(c)(i) F and K (either order) G H and J (either order) ; 1 2(c)(ii) both have the same colour with Universal Indicator / both have the same pH value ; 1 Question Answer Marks 2(d) any two from: fizzes / bubbles / effervescence ; form a (colourless) solution ; temperature increases ; 2

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3 Sodium carbonate is a white solid that reacts with dilute hydrochloric acid as shown in the word equation. sodium hydrochloric sodium carbon + + + water carbonate acid chloride dioxide When sodium carbonate is added to dilute hydrochloric acid the reaction fizzes (bubbles). When the fizzing stops the reaction is complete. The time it takes for the reaction to be completed is called the reaction time. Plan an investigation to find out how the reaction time depends on the temperature of the hydrochloric acid. You are provided with: • sodium carbonate powder • dilute hydrochloric acid You may use any common laboratory apparatus in your plan. You are not required to do this investigation. In your plan, include: • the apparatus needed • a brief description of the method and explain any safety precautions you would take • what you would measure • which variables you would keep constant • how you would process your results to draw a conclusion. You may include a labelled diagram if you wish. You may include a table that can be used to record the results if you wish. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [7]

Mark scheme: 3 One marking point from each section and any two others (e.g. if one section missed max 6 etc.): 1 apparatus thermometer AND stopwatch / stop-clock ; balance (to measure sodium carbonate) / measuring cylinder / syringe (to measure acid) ; water bath ; 2 brief description and safety precautions idea of adding sodium carbonate to acid of different temperatures ; repeat experiment at same temperature ; idea of doing at least five different temperatures ; wearing safety goggles to protect eyes because acid is corrosive / wear goggles to avoid powder or acid getting into the eyes ; 3 measurements time (in seconds) (from when powder added) to when fizzing / bubbles, stop / when volume of gas collected stays constant ; measure temperature of acid (before mixing with sodium carbonate) / suggests suitable values for temperature between 0 and 90 °C ; 4 variables constant same volume / amount / cm3, of acid ; same concentration of acid ; same mass of sodium carbonate / amount of sodium carbonate ; 5 processing calculate average times from repeated experiments ; plot graph of reaction time against temperature / look at pattern in reaction time as temperature increases / shorter time means a faster rate ORA ;

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4 You are going to investigate how forces affect an elastic band. You are provided with an unstretched elastic band. (a) (i) Measure the length l of your elastic band to the nearest millimetre, by pushing your band together and measuring against a ruler as shown in Fig. 4.1. Fig. 4.1 Record your value in Table 4.1. [1] (ii) Suggest one practical difficulty in obtaining an accurate value for length l. ........................................................................................................................................... ..................................................................................................................................... [1] (b) Procedure • Suspend the elastic band from the nail. • Attach the mass hanger to the elastic band as shown in Fig. 4.2. nail boss elastic band 100 g mass hanger stand Fig. 4.2 • Measure and record in Table 4.1 the stretched length l of the elastic band. • Carefully add a 100 g mass to the mass hanger. • Measure and record in Table 4.1 the stretched length l of the elastic band. • Continue adding masses 100 g at a time, recording length l after each addition, until 500 g is suspended from the elastic band. • Record the length l at 500 g in Table 4.1 and Table 4.2. • Now carefully remove one 100 g mass from the hanger. • Measure length l and record it in Table 4.2. • Continue removing 100 g masses from the mass hanger, recording length l each time. • Remove the mass hanger and record in Table 4.2 the final length l of the elastic Table 4.1 mass force length l / g / N / mm 0 0.0 100 (mass hanger) 1.0 200 2.0 300 3.0 400 4.0 500 5.0 Table 4.2 mass force length l / g / N / mm 500 5.0 400 4.0 300 3.0 200 2.0 100 (mass hanger) 1.0 0 0.0 [3]

Mark scheme: 4(a)(i) 1 4(a)(ii) difficult to get the elastic band to lie flat along ruler / difficult to hold elastic band along ruler without stretching it at all / reference to the limit of precision of ruler ; 1 4(b) all values of l recorded AND l increases as mass increases ; l at 500 g is the same in both tables AND l decreases in Table 4.2 ; final value for l (in Table 4.2) is greater than initial l in Table 4.1 ; 3 4(c)(i) suitable linear scale chosen ; points for increasing force plotted correctly ; 2 4(c)(ii) good best-fit curve judgement for increasing force and curve labelled ; 1 4(c)(iii) points for decreasing force plotted correctly on same grid ; 1 4(c)(iv) good best-fit curve judgement for decreasing force and curve labelled ; 1 4(d) evidence of use of approximating squares or alternative method ; 5 < area < 25 (square centimetres) ; 2 4(e) 0.15 (J) ; 1

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