Cambridge IGCSE Science - Combined 0653 — 2021 Feb/March Paper 5 · Variant 2

1 You are going to investigate the rate of respiration in yeast cells. Yeast is a single-celled organism similar to plant and animal cells. (a) You are provided with a yeast suspension, Y, and some yeast suspension that has been boiled, Z. As yeast cells respire they produce carbon dioxide gas. Procedure Make sure that the yeast suspension is well mixed before using it each time. Step 1 Set up and maintain a water-bath at approximately 40 °C. Step 2 Label the three small test-tubes A, B and C. Step 3 Put 1 cm3 distilled water into test-tube A. Step 4 Put 1 cm3 5% glucose solution into test-tube B. Step 5 Put 1 cm3 5% glucose solution into test-tube C. Step 6 Fill test-tube A to the top with yeast suspension Y. Step 7 Fill test-tube B to the top with yeast suspension Y. Step 8 Fill test-tube C to the top with boiled yeast suspension Z. Step 9 Place a large test-tube (boiling tube) over the top of test-tube A and quickly turn both test-tubes upside down as shown in Fig. 1.1. test-tube A large test-tube both test-tubes containing 1 cm3 placed over the quickly turned water and filled to top of test-tube A upside down the top with yeast suspension Fig. 1.1 Step 10 Place both test-tubes into the water-bath. Step 11 Repeat step 9 and step 10 with test-tubes B and C. Step 12 Start the stop-clock. Step 13 Leave the test-tubes in the water-bath for 5 minutes. (i) Suggest why the yeast suspension is mixed immediately before using it each time. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Observe what is happening to the yeast suspension in small test-tube B. Record your observations. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) After 5 minutes measure the height of the liquid, to the nearest mm, in the large test-tube (boiling tube), as shown in Fig. 1.2. height of liquid in large test-tube Fig. 1.2 Record your measurements in Table 1.1. Table 1.1 height of liquid test-tube / mm A B C [3] (iv) Test-tube B contains glucose. Test-tube A contains no glucose. Use this information to explain the difference in your results for test-tube B and test-tube A. ........................................................................................................................................... ..................................................................................................................................... [1] (v) Test-tube C is used as a type of control. Explain the purpose of this control. ........................................................................................................................................... ..................................................................................................................................... [1] (vi) Explain why it is difficult to get an accurate value for the height of the liquid in the large test-tube. ........................................................................................................................................... ..................................................................................................................................... [1] (vii) Describe a test to find out if the gas made is carbon dioxide. Include the observation for a positive result. test .................................................................................................................................... ........................................................................................................................................... observation ........................................................................................................................ [1] (b) The teacher does a similar experiment at 40 °C to investigate the effect of different concentrations of glucose on the yeast suspension. The teacher measures the volume of gas collected after 5 minutes. The results are shown in Table 1.2. Table 1.2 volume of gas percentage concentration of collected after glucose solution 5 minutes / cm3 6 3 7 8 8 13 9 17 10 17 (i) Describe the relationship between the percentage concentration of glucose solution and the volume of gas made. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Suggest a piece of apparatus suitable for collecting the gas and measuring its volume. ..................................................................................................................................... [1] (iii) The teacher repeats their investigation at 20 °C instead of 40 °C. Suggest what effect, if any, a lower temperature has on the results. ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 13]

Mark scheme: 1(a)(i) so equal amount of yeast in each test-tube ; 1 1(a)(ii) starts to, bubble / fizz / effervesce ; 1 1(a)(iii) all three values recorded ; in whole mm ; result for test-tube C is lower than B ; 3 1(a)(iv) any one from: glucose is needed for respiration ; glucose is needed for CO2 production ; glucose speeds up respiration ; 1 1(a)(v) to find out, if yeast is needed / if glucose reacts on its own ; 1 1(a)(vi) rounded bottom (of test-tube) / meniscus (of water) ; 1 1(a)(vii) limewater and goes milky ; 1 1(b)(i) higher concentration of glucose gives higher volume gas ; remains the same at 9 to 10 / above 9 ; 2 1(b)(ii) gas syringe / measuring cylinder ; 1 1(b)(iii) any one from: lower volume of gas (in 5 minutes) ; slower rate of gas made ; 1

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2 You are going to prepare a sample of the insoluble salt barium sulfate by reacting aqueous copper(II) sulfate with aqueous barium nitrate. (a) Procedure Step 1 Use a 10 cm3 measuring cylinder to measure 7.5 cm3 of aqueous copper(II) sulfate and pour it into a 100 cm3 beaker. Step 2 Use a second 10 cm3 measuring cylinder to measure 4.0 cm3 of aqueous barium nitrate and pour it into the beaker. Step 3 Stir the mixture for about 10 seconds with a glass rod. Step 4 Filter the reaction mixture and collect the filtrate in a conical flask. Step 5 Keep the filtrate and the residue. (i) Explain why a 10 cm3 measuring cylinder is used in step 1 rather than a 25 cm3 measuring cylinder. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) State one observation of the filtrate that shows it contains copper(II) ions. ..................................................................................................................................... [1] (iii) Describe the appearance of the residue on the filter paper. ..................................................................................................................................... [1] (b) The residue of barium sulfate is impure. It contains some soluble copper(II) compounds. Describe how you can purify the barium sulfate by removing the soluble copper(II) compounds from the residue. ................................................................................................................................................... ............................................................................................................................................. [1] (c) Pour 1 cm depth of the filtrate into one test-tube and another 1 cm depth of the filtrate into a second test-tube. (i) Add aqueous ammonia drop by drop to the first test-tube until there is no further change in the appearance of the reaction mixture. Record in detail your observations. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Add 15 drops of aqueous potassium iodide to the second test-tube. Record your observations. ........................................................................................................................................... ........................................................................................................................................... Then add about 3 cm depth of aqueous sodium thiosulfate. Record your observations. ........................................................................................................................................... ........................................................................................................................................... [2] [Total: 7]

Mark scheme: 2(a)(i) more precise ; 1 2(a)(ii) (because it is a pale) blue ; 1 2(a)(iii) white solid ; 1 2(b) wash with water / pour water through residue / pour water through filter funnel ; 1 2(c)(i) (light) blue precipitate ; AND (with excess aqueous ammonia) re-dissolves to form a dark blue colour / forms a dark blue solution ; 1 2(c)(ii) (off-)white precipitate / solid, in a brown solution ; white AND precipitate / suspension / milky appearance ; 2

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3 Calcium hydroxide is a white solid that is added to acidic soils to increase their pH. Calcium hydroxide neutralises acids such as dilute nitric acid to make a salt and water. Plan an investigation to find out how the pH of dilute nitric acid changes as calcium hydroxide is added to the acid. You are provided with: • calcium hydroxide powder • dilute nitric acid • Universal Indicator solution. 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 should 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: apparatus measuring cylinder / burette / (volumetric) pipette ; balance ; brief description and safety precautions add Universal Indicator to acid ; add calcium hydroxide (solid) to acid ; stir / complete reaction ; repeat for different masses (at least 2) or a continuous method ; repeat, each mass / whole of continuous method ; idea of using at least five, different masses / additions if continuous ; safety goggles AND protect eyes AND (from) acid / calcium hydroxide is corrosive ; measurements mass of calcium hydroxide ; volume of nitric acid ; colour / pH, of Universal Indicator in acid alone ; colour / pH, of Universal Indicator after adding calcium hydroxide ; using pH colour chart to match colour with pH ; variables constant volume of acid ; concentration of acid ; processing and conclusion take averages from repeated experiments ; plot graph of mass against pH ; does increasing mass of calcium hydroxide increase the pH ;

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4 You are going to investigate the rate at which hot water cools. (a) Procedure • Add approximately 300 cm3 of water at room temperature to the beaker containing the large test-tube (boiling tube). • Measure and record in Table 4.1 the initial temperature of the water in the beaker. • Approximately half-fill the test-tube with hot water as shown in Fig. 4.1. • Using the other thermometer, measure the initial temperature of the hot water in the test-tube and immediately start the stop-watch. • Record in Table 4.1 this temperature at time t = 0. • Record in Table 4.1 the temperature of the hot water in the test-tube and the temperature of the water in the beaker every 30 seconds for 180 s. (Use the glass rod to stir the water in the beaker before each measurement.) clamp clamp thermometer test-tube beaker hot water water at room temperature Fig. 4.1 Table 4.1 temperature of water in time t / s beaker test-tube / °C / °C 0 30 60 90 120 150 180 [3] (b) Describe one practical technique you use to ensure the reading of the water temperature in the beaker is measured accurately. ................................................................................................................................................... ............................................................................................................................................. [1] (c) Describe how the temperature of the water in the beaker changes during your experiment. Suggest a reason for your answer. description ................................................................................................................................ reason ....................................................................................................................................... ................................................................................................................................................... [1] (d) (i) Plot a graph of the temperature of the hot water in the test-tube (vertical axis) against time. Do not start the temperature scale at 0 °C. [3] (ii) Draw the best-fit curve. [1] (e) (i) Calculate the rate of cooling of the hot water in the test-tube during the first 30 s. Use the equation shown. rate of cooling during first 30 s = Give your answer to two significant figures. rate of cooling = ................................................. °C / s [1] (ii) Describe how the rate of cooling of the hot water in the test-tube changes during the 180 s. ........................................................................................................................................... ..................................................................................................................................... [1] (f) Suggest and explain one change you can make to this experiment which will cause the initial rate of cooling to be less. suggestion ................................................................................................................................ ................................................................................................................................................... explanation ............................................................................................................................... ................................................................................................................................................... [2] [Total: 13]

Mark scheme: 4(a) temperature of beaker water and hot test-tube water recorded at time = 0 s ; temperature of hot water decreasing with time over 180 s ; all temperatures recorded to same number of decimal places ; 3 4(b) any one from: ensure the thermometer is read perpendicular to the scale ; stir (using a stirring rod to ensure water has even temperature throughout) ; thermometer is in liquid / not touching beaker ; 1 4(c) description matches results AND sensible suggestion for description, e.g. increases - some (thermal) energy transferred from the large test-tube OR stays the same - water in beaker at room temperature at start / much larger volume of water in beaker than large test-tube so negligible change in temperature OR decreases - water in beaker was higher than room temperature at start ; 1 4(d)(i) axes vertical temperature AND °C AND horizontal time AND s / min ; sensible linear scale with plots covering at least half of grid ; points plotted correctly ; 3 4(d)(ii) best-fit curve ; 1 4(e)(i) correct answer on their values to two significant figures ; 1 4(e)(ii) rate decreases (over time) ; 1 4(f) suggestion replace water in beaker with warm(er) or hot water / description of improving insulation around large test-tube / AVP ; explanation smaller temperature difference between test-tube and surroundings / use of insulation helps prevent heat loss / AVP ; 2

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