Cambridge IGCSE Science - Combined 0653 — 2021 May/June Paper 6 · Variant 2

1 (a) A student investigates the rate of respiration in yeast cells. Yeast is a single-celled organism similar to plant and animal cells. The student uses methylene blue indicator. A solution of methylene blue loses its blue colour as the yeast cells respire. Procedure The student: • puts 5 cm3 yeast suspension into each of three test-tubes labelled A, B and C • adds the volumes of water, 5% glucose solution and methylene blue to test-tubes A, B and C as shown in Table 1.1 Table 1.1 volume of 5% volume of volume of water test-tube glucose solution methylene blue / cm3 / cm3 / cm3 A 5 0 1 B 4 1 1 C 0 5 1 • places the test-tubes in a water-bath at 40 °C • records the time taken for the methylene blue to lose its blue colour in each test-tube • if the time taken is longer than 5 minutes then the student records this value as >300 • repeats the procedure. (i) The student’s results for test-tubes B and C for trial 2 are shown in Fig. 1.1. Record these values in Table 1.2. test-tube B test-tube C START/STOP RESET START/STOP RESET HOUR MIN SEC HOUR MIN SEC Fig. 1.1 Table 1.2 time taken to lose blue colour volume of 5% glucose / seconds test-tube solution / cm3 trial 1 trial 2 A 0 >300 >300 B 1 187 C 5 53 [2] (ii) State a conclusion for the results in Table 1.2. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Suggest why the student repeats the investigation. ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Identify one variable that is kept constant during this investigation. ........................................................................................................................................... ..................................................................................................................................... [1] (v) The student wants to find out if all of the glucose in test-tube C has been used up. Name the testing solution the student uses and give the colour observed for a negative result. testing solution .................................................................................................................. colour for negative result .................................................................................................. [1] (b) The student thinks that carbon dioxide gas is given off when yeast cells respire. State the test for carbon dioxide and include the observation for a positive result. test ............................................................................................................................................ observation ............................................................................................................................... [1] (c) Fig. 1.2 shows a yeast cell. The yeast cell has a cell wall and a large vacuole similar to a plant cell. The yeast cell is magnified 5000 times. A B magnification ×5000 Fig. 1.2 (i) Measure the length of the cell, line AB, on Fig. 1.2 in millimetres to the nearest millimetre. length of line AB on Fig. 1.2 = .................................................. mm [1] (ii) Calculate the actual length of the cell using the equation shown. length of line AB on Fig. 1.2 actual length of cell = magnification actual length of cell = .................................................. mm [1] (iii) In the box provided, make a large, clear line drawing of the yeast cell in Fig. 1.2 showing the cell wall and vacuole. [4] [Total: 13]

Mark scheme: 1(a)(i) (B) 205 ; (C) 44 ; 2 1(a)(ii) more glucose, greater rate of respiration / less time taken ; 1 1(a)(iii) to spot anomalous results / to spot outliers ; 1 1(a)(iv) type of yeast / volume of yeast / temperature / (total) volume of solution / volume of methylene blue ; 1 1(a)(v) Benedict’s AND blue ; 1 1(b) limewater AND milky ; 1 1(c)(i) 48 (mm) ; 1 1(c)(ii) 0.0096 (mm) ; 1 1(c)(iii) M1 clear and continuous outline of yeast cell ; M2 drawing size is half or more of the box ; M3 cell wall shown with two concentric ‘circles’ ; M4 large vacuole shown off centre ; 4

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2 A student investigates solid F. (a) Procedure The student: • adds some water to a test-tube • measures the temperature of the water to the nearest 0.5 °C • adds two spatula loads of solid F to the water • stirs the mixture until all of solid F dissolves • measures the temperature of the solution to the nearest 0.5 °C • pours the solution into two test-tubes for use in (b) and (c). (i) Fig. 2.1 shows the two thermometer readings. °C °C 30 30 20 20 10 10 temperature of water temperature of solution Fig. 2.1 Record in Table 2.1 these two temperatures to the nearest 0.5 °C. Table 2.1 temperature of water / °C temperature of solution / °C [2] (ii) In an endothermic process heat energy is taken in from the surroundings. State if solid F dissolving in water is an endothermic process. Explain your answer using the results in Table 2.1. ........................................................................................................................................... ..................................................................................................................................... [1] (b) The student adds three drops of Universal Indicator solution to one of the test-tubes containing the solution of F. The Universal Indicator turns red-orange. Fig. 2.2 shows a pH colour chart. red orange yellow green blue-green blue purple 1 4 6 7 8 10 14 Fig. 2.2 (i) Use the pH colour chart in Fig. 2.2 to estimate the pH of the solution of F. ..................................................................................................................................... [1] (ii) Explain why using a pH colour chart does not give the student an accurate value for the pH of the solution of F. ........................................................................................................................................... ..................................................................................................................................... [1] (c) The student adds an equal volume of aqueous sodium hydroxide to the other test-tube containing a solution of F. The student warms the mixture carefully and tests the gas formed with damp red litmus paper. The student concludes that ammonia gas is formed. (i) Describe what happens to the piece of damp red litmus paper during the test for ammonia gas. ..................................................................................................................................... [1] (ii) Identify the positive ion present in the solution of F. ..................................................................................................................................... [1] [Total: 7]

Mark scheme: 2(a)(i) (temperature of water) 20.5 (°C) ; (temperature of solution) 16.0 (°C) ; 2 2(a)(ii) it is endothermic because the temperature decreases ; 1 2(b)(i) any value greater than 1 and less than 4 ; 1 2(b)(ii) some colours have a range of pH values / difficult to distinguish between colours / only gives pH to a whole number ; 1 2(c)(i) (damp red litmus paper) goes blue ; 1 2(c)(ii) ammonium ; 1

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3 Sodium hydrogencarbonate is a white solid that reacts with dilute nitric acid as shown in the word equation. sodium nitric sodium carbon + + + water hydrogencarbonate acid nitrate dioxide When sodium hydrogencarbonate is added to dilute nitric acid the reaction mixture 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 concentration of the dilute nitric acid. You are provided with: • sodium hydrogencarbonate powder • dilute nitric acid • distilled water You may use any common laboratory apparatus in your plan. 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 balance (to measure mass of sodium hydrogencarbonate) ; measuring cylinder (to measure volume of nitric acid) ; stopwatch / timer ; 2 brief description and safety precautions idea of adding sodium hydrogencarbonate to acid of different concentrations ; add different amounts of acid and water (to vary concentration) ; repeat experiment at each concentration ; do at least five different concentrations ; wearing safety goggles to protect eyes because acid is corrosive / wear goggles to avoid powder getting into the eyes ; 3 measurements time (in seconds) (from when powder added) to when fizzing / bubbles stop ; volume / cm3, of acid / water ; 4 variables constant same amount / mass / g, of sodium hydrogencarbonate ; total volume of acid (and water) ; same temperature ; 5 processing calculate average times from repeated experiments ; plot graph of reaction time against concentration / look at pattern in reaction time as concentration increases / shorter time means a faster rate ORA ;

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4 A student determines the cross-sectional area of a piece of resistance wire using measurements of current and voltage. Fig. 4.1 shows the cross-sectional area of a wire. wire cross-sectional area Fig. 4.1 (a) Procedure The student: • sets up the circuit shown in Fig. 4.2 A V C X 0 resistance wire metre rule Fig. 4.2 • completes the circuit by connecting C to the resistance wire at length l = 400 mm from point X, the zero end of the metre rule. • closes the switch • records in Table 4.1 the current I flowing in the circuit and the potential difference V between X and C • opens the switch. (i) Fig. 4.3 shows the voltmeter and ammeter readings for l = 400 mm. Record these values in Table 4.1. 2 3 0.4 0.6 1 4 0.2 0.8 0 5 0 1 V A Fig. 4.3 Table 4.1 length, l potential difference, V current, I resistance, R / mm / V / A / Ω 400 500 1.60 0.73 2.2 700 1.85 0.59 3.1 800 1.90 0.54 3.5 900 1.95 0.50 3.9 1000 2.05 0.46 4.5 [2] (ii) The student repeats the procedure for lengths l = 500 mm, 700 mm, 800 mm, 900 mm and 1000 mm. The results are shown in Table 4.1. Suggest why it is good experimental technique to open the switch between each reading. ........................................................................................................................................... ..................................................................................................................................... [1] (b) Calculate the resistance R of the 400 mm length of resistance wire. Use the equation shown. V R = I Record the value of R in Table 4.1. [2] (c) (i) Plot a graph of resistance R (vertical axis) against length of wire, l. Start your axes at (0,0). [3] (ii) Draw the best-fit straight line. [1] (iii) Use your graph to estimate the value of R at length l = 600 mm. R = ..................................................... Ω [1] (iv) Calculate the gradient G of the line. Indicate on your graph the points that you use to calculate the gradient. G = ......................................................... [2] (d) Calculate the cross-sectional area A of the resistance wire. Use the equation shown. 0.00049 A = G If you do not have a value for G in (c)(iv) use G = 0.005 here. This is not the correct value for G. A = ................................................ mm2 [1]

Mark scheme: 4(a)(i) 1.45 (V) ; 0.82 (A) ; 2 4(a)(ii) prevent the wire heating (which could affect resistance) ; 1 4(b) 1.8 (Ω) ; to 1 decimal place ; 2 4(c)(i) axes correct way round and labelled with units ; suitable linear scale chosen ; all points correctly plotted ; 3 4(c)(ii) good best-fit line judgement ; 1 4(c)(iii) reading of R correct from candidate’s graph ± ½ small square ; 1 4(c)(iv) indication on graph of points chosen extending over at least half line length ; correct gradient calculation ; 2 4(d) correct calculation of A (mm2) ; 1

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