Cambridge IGCSE Science - Combined 0653 — 2020 Oct/Nov Paper 5 · Variant 2

1 (a) You are going to investigate the amount of vitamin C in orange juice. You are provided with half an orange. Make a large drawing of the cut surface of the orange. [3] (b) You will test the juice from the orange with DCPIP. DCPIP is a dark blue solution that turns colourless when vitamin C is added. If the orange juice contains a lot of vitamin C, less orange juice is needed to turn the DCPIP colourless. (i) Procedure • Squeeze the orange and collect the juice in a large beaker. • Use a pipette to put 2 drops of DCPIP into each of three test-tubes. • Use a clean pipette to add drops of the squeezed orange juice to one of the test-tubes containing DCPIP until the solution is orange. You may need to swirl the test-tube. Count the number of drops of orange juice as you add them. • Record in Table 1.1 the number of drops of orange juice added. This is experiment 1. • Repeat for the other two test-tubes containing DCPIP (experiment 2 and experiment 3) and record your results in Table 1.1. Table 1.1 experiment number of drops of orange juice added 1 2 3 average [2] (ii) Calculate the average number of drops of orange juice added for the three experiments. Record your answer in Table 1.1. [1] (iii) Suggest why using this method to find the amount of vitamin C in orange juice may not be accurate. ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 7]

Mark scheme: 1(a) more than 50% of box / more than 50 mm at widest point ; continuous line around outside, pith and segments shown ; segments within pith and centre shown; 3 1(b)(i) all results recorded ; range of values < 3 ; 2 1(b)(ii) correct average ; 1 1(b)(iii) volume of a drop not, known / measured / drop size may vary / may need to add a ½-drop ; 1

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2 A manufacturer makes two drinks, A and B, as shown in Fig. 2.1. • Drink A contains sugar and fat. • Drink B is low in sugar and contains no fat. Drink Drink B A sugar Low free Fat Fig. 2.1 Plan an investigation to compare the sugar and fat content of the two drinks. You are not required to carry out this investigation. In your answer, include: • your predictions for results for drink A and drink B • the apparatus and chemicals you will need • a brief description of the method, including how you will treat variables and any safety precautions you will take • the observations you will make and how they will help you compare drink A and drink B. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [7]

Mark scheme: 2 1 mark from each section and then additional marks up to max 7 1 prediction: Drink A tests positive for fat and sugar test AND drink B tests negative fat and low sugar ; 2 apparatus test-tube / beaker / boiling tube / conical flask / pipette ; measuring cylinder / balance ; heat source / water bath (for sugar test) ; 3 chemicals (fat test) ethanol ; (sugar) Benedict’s ; 4 method, variables, safety heat / warm for Benedict’s test ; keep ethanol away from naked flame / identifies ethanol is flammable / identifies heating hazard for Benedict’s test / Benedict’s is, harmful / toxic ; same, volume / mass of drink tested ; same, volume / concentration, of reagents / same number of drops ; 5 observations (fat) positive / drink A gives (white) emulsion / milky OR negative / drink B gives no (white) emulsion ; (sugar) positive / drink A Benedict’s goes orange / red ; (sugar) low sugar / drink B Benedict’s goes yellow / green ;

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3 You are going to investigate the effect of concentration of hydrochloric acid on the rate of a reaction. M is a unit of concentration. The higher the number the more concentrated the acid. A sample of 2.0 M hydrochloric acid is two times more concentrated than 1.0 M hydrochloric acid. (a) Procedure • Put 3 cm depth of 1.5 M hydrochloric acid into a test-tube. • Add a 3 cm piece of magnesium ribbon. • Test the gas given off in order to identify it. State the test which identifies the gas. Give the result of the test and identify the gas. test ............................................................................................................................................ result ......................................................................................................................................... identity of gas ........................................................................................................................... [1] (b) (i) Procedure • Measure 25 cm3 of 2.5 M hydrochloric acid with a measuring cylinder and pour it into a conical flask. • Add a 1 cm piece of magnesium ribbon to the hydrochloric acid and start the stop-clock. • Stop the stop-clock as soon as all of the magnesium has fully reacted. • Record this time in Table 3.1 to the nearest second. • Empty and rinse the flask with water. Repeat the procedure using 2.0 M, 1.5 M and 1.0 M hydrochloric acid instead of 2.5 M hydrochloric acid. Table 3.1 concentration of acid time for magnesium to / M fully react / s 2.5 2.0 1.5 1.0 [2] (ii) Plot a graph of time for magnesium to fully react (vertical axis) against concentration of acid. [3] (iii) Draw the best-fit line. Label the line B. [1] (iv) Describe the relationship between the concentration of acid and the time for magnesium to fully react. ........................................................................................................................................... ..................................................................................................................................... [1] (v) Use your graph to determine the time it takes for a 1 cm piece of magnesium ribbon to fully react with 1.8 M hydrochloric acid. time = ...................................................... s [1] (vi) Suggest one improvement to this experiment to make the results more accurate. ........................................................................................................................................... ..................................................................................................................................... [1] (vii) A student repeats the experiment in (b)(i). She uses the same concentrations of hydrochloric acid at a temperature of 50 °C instead of room temperature. Everything else is kept the same. The student finds the reactions happen more quickly. On the same grid used in (b)(ii), draw the line you would expect to get at 50 °C. Label the line C. [1] (c) The rate of the reaction of magnesium and dilute hydrochloric acid can be determined by collecting the gas given off. The volume of gas collected in a given time is measured. Draw a labelled diagram of the assembled apparatus used to react magnesium with hydrochloric acid and collect and measure the volume of the gas given off. You are not required to carry out this experiment. [2] [Total: 13]

Mark scheme: 3(a) lighted splint AND pops AND hydrogen ; 1 3(b)(i) time for 2.5 M ; times increase down table ; 2 3(b)(ii) both axes labelled, time in s AND concentration in M ; time on vertical axis, scale linear and points cover more than half of the grid ; 4 points plotted correctly in linear portion of the grid ; 3 3(b)(ii) both axes labelled with quantity and unit ; scale linear and so points cover more than half of the grid ; 4 points plotted correctly ; 3 3(b)(iii) good best-fit line judgement AND clear label ; 1 3(b)(iv) as concentration increases, time (for magnesium to fully react) decreases; 1 3(b)(v) time for 1.8 M interpolated from graph; 1 3(b)(vi) measure the time taken for fixed volume / amount of gas to be produced OR measure volume / amount of gas for a fixed time / use a pipette (to meaure the acid) / measure the mass of the metal / in a water bath to keep T constant ; 1 3(b)(vii) line C underneath line B ; 1 3(c) container with contents, bung and delivery tube ; gas syringe / upturned measuring cylinder in water ; 2

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4 You are going to investigate the refraction of light in a rectangular block. (a) Fig. 4.1 is provided for reference. Use a sharpened pencil to draw thin neat lines. hole A4 sheet of F paper N θ E A B 5 cm D C eye position X Fig. 4.1 (plan view) Procedure • Place the block approximately in the centre of the blank sheet of paper provided. • Draw around the block and label the corners ABCD as shown in Fig. 4.1. • Remove the block from the paper. • Mark and label the point E on the line AB a distance of 5.0 cm from A. • Draw a normal to AB (a line at 90° to AB) at point E and label the end of the line N. • Draw an 8.0 cm line from E at an angle θ = 35° to the normal as shown in Fig. 4.1. • Label the other end of this line F. [3] (b) • Place the sheet of paper from (a) on the cork mat. • Put the block back on the paper in exactly the same position as in (a). • Push two pins, P1 and P2, into the paper approximately 5 cm apart on the line FE. • Label the positions of P1 and P2 on the paper. • View the images of P1 and P2 through the glass block. Look from the eye position X shown in Fig. 4.1. • Move your eye position until the images of P1 and P2 are in line with each other. • Line up a third pin, P3, with the images of P1 and P2 and push the pin into the paper close to side CD of the block. • Line up a fourth pin, P4, with P3 and the images of P1 and P2 and push the pin into the paper approximately 5 cm from P3. • Label the positions of P3 and P4 on the paper. • Remove the block and the pins from the paper. [2] (c) • Draw a line through P3 and P4 and extend it to meet CD. • Label the point at which this line meets CD with the letter G. • Draw a line through G at 90° to CD. Extend this line until it crosses AB. • Label the point at which this line meets AB with the letter H. • Extend the line FE until it meets the line GH. • Label the point at which this line meets GH with the letter J. • Draw a straight line joining points E and G. [2] (d) (i) Measure the length of line EG and record to the nearest 0.1 cm. EG = ................................................... cm [1] (ii) Measure the length of line EJ and record to the nearest 0.1 cm. EJ = ................................................... cm [1] (iii) Calculate the refractive index n of the glass block. Use the equation shown. Give your answer to 2 significant figures. (If you do not have values for EG and EJ, use EG = 6.5 cm and EJ = 4.1 cm. These are not the correct values.) EG n = EJ n = ......................................................... [2] (e) Suggest one precaution that you should take to ensure that your results are as accurate as possible. ................................................................................................................................................... ............................................................................................................................................. [1] (f) A teacher says that the refractive index of the block is 1.5. Compare the value of the refractive index n you calculated in (d)(iii) with the teacher’s value. State whether the two values agree within the limits of experimental accuracy. Justify your answer with reference to the values. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 13]

Mark scheme: 4(a) block drawn and normal drawn ; normal is 5 cm from A and at 90° to AB ; angle FEN = 35° ; 3 4(b) P1 and P2 shown on the paper ; P3 and P4 below the block outline ; 2 4(c) line through P3 and P4 AND points G and H correctly labelled ; lines EJ and EG both present and drawn with ruler and thin lines ; 2 4(d)(i) EG correct to nearest 0.1 cm ; 1 4(d)(ii) EJ correct to nearest 0.1 cm ; 1 4(d)(iii) n correct based on candidate values in 4(d)(i) and 4(d)(ii) ; candidate’s answer recorded to 2 sig. figs. AND with no unit present ; 2 4(e) view base of pins / close one eye ; 1 4(f) answer yes (agree) / no (disagree) suitable for candidate value of n AND values close enough to each other / too far apart ; 1

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