Cambridge IGCSE Science - Combined 0653 — 2023 May/June Paper 5 · Variant 1

1 (a) You will estimate the concentration of vitamin C in a sample of fruit juice. When iodine solution is added to a mixture of starch and fruit juice the starch turns the iodine blue‑black. The vitamin C turns the iodine colourless. The amount of iodine needed to keep the blue‑black colour is an indicator of the concentration of vitamin C. You are provided with four different concentrations of vitamin C and a fruit juice with an unknown vitamin C concentration. Procedure Step 1 Label four boiling tubes (large test‑tubes) 1.00, 0.75, 0.50 and 0.25. Step 2 Use a measuring cylinder to add 10 cm3 of the 1.00% vitamin C solution to boiling tube 1.00. Step 3 Use the 1 cm3 syringe to add 0.5 cm3 of starch solution to the boiling tube and swirl to mix. Step 4 Fill the 20 cm3 syringe with 20 cm3 of iodine solution. Step 5 Add a drop of the iodine solution to the mixture in the boiling tube and stir gently to mix. A blue‑black colour appears briefly which then disappears when mixed. Step 6 Continue adding drops of iodine solution and mixing until the blue‑black colour remains. (i) Record in Table 1.1, the volume of iodine solution remaining in the 20 cm3 syringe. Table 1.1 percentage concentration volume of iodine solution volume of iodine solution of vitamin C remaining / cm3 added / cm3 1.00 0.75 0.50 0.25 [1] Procedure continued Step 7 Refill the syringe to contain 20 cm3 of iodine solution. Step 8 Repeat Step 2 to Step 7 using the other three concentrations of vitamin C and the appropriately labelled boiling tubes. (ii) Record in Table 1.1, the volumes of iodine solution remaining in the 20 cm3 syringe for each concentration of vitamin C. [3] (iii) Calculate the volume of iodine solution added to each boiling tube. Use the equation shown. volume of iodine solution volume of iodine solution = 20 – added remaining Record these values in Table 1.1. [1] (iv) On the grid, plot a graph of the volume of iodine solution added (vertical axis) against the percentage concentration of vitamin C. [3] (v) Draw the line of best‑fit. [1] (vi) Repeat Step 2 to Step 6 of the procedure in (a) using a clean boiling tube and 10 cm3 of fruit juice, instead of the vitamin C concentrations. Record the volume of iodine solution remaining and the volume of iodine solution added. volume of iodine solution remaining = ........................................................ cm3 volume of iodine solution added = ........................................................ cm3 [1] (vii) Use your graph to estimate the percentage concentration of vitamin C in the fruit juice. percentage concentration of vitamin C in fruit juice = ......................................................... [1] (b) Repeating the procedure increases confidence in the results. (i) Suggest one other reason why the procedure for each concentration should be repeated. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Suggest one other way to improve confidence in your estimate in (a)(vii). ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 13]

Mark scheme: 1(a)(i) volume of iodine solution remaining result recorded for 1.00 % ; 1 1(a)(ii) all results recorded for volume of iodine solution remaining ; values to nearest 0.1 cm3 ; lower volume of iodine remaining with higher vitamin C concentration ; 3 1(a)(iii) correct calculations for volume of iodine solution added ; 1 1(a)(iv) y axis: (volume of) iodine solution added / cm3 and x axis: percentage concentration of vitamin C / concentration of vitamin C / % ; linear scale, plotted points so that points occupy at least half of the grid ; points plotted correctly,  half a small square ; 3 1(a)(v) line of best-fit drawn ; 1 1(a)(vi) both values recorded (in cm3) ; 1 1(a)(vii) correct estimate from graph ; 1 1(b)(i) to identify / check for, anomalous results / errors ; 1 1(b)(ii) do experiments with more values of vitamin C concentration / use smaller intervals of concentration ; 1

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2 You will investigate the components of a fertiliser. The fertiliser contains an insoluble substance and a soluble salt. You are provided with a sample of the fertiliser. Procedure • Pour all of the fertiliser into a glass beaker. • Add about 20 cm3 of distilled water to the glass beaker. • Stir the mixture in the beaker with a glass rod for about one minute. • Filter the mixture into a boiling tube (large test‑tube). • Keep the residue on the filter paper. • Keep the filtrate in the boiling tube. (a) Describe the appearance of the residue and the filtrate. residue ...................................................................................................................................... filtrate ........................................................................................................................................ [2] (b) Explain why the mixture is stirred. ................................................................................................................................................... ............................................................................................................................................. [1] (c) Procedure • Put a wooden splint into the filtrate and leave it there for a few minutes. While you wait complete part (d). • Place the wooden splint into the top of the blue flame of a Bunsen burner. • Record the immediate colour of the flame. • If you do not see a colour, put the splint back into the filtrate and then back into the flame. (i) State the immediate colour of the flame. ..................................................................................................................................... [1] (ii) Identify the cation (positive ion) in the filtrate. ..................................................................................................................................... [1] (d) Procedure • Pour about 2 cm depth of the filtrate into a test‑tube. • Add about 1 cm depth of dilute nitric acid to the filtrate in the test‑tube. • Add about 1 cm depth of aqueous barium nitrate to the mixture in the test‑tube. (i) Describe your observations. ..................................................................................................................................... [1] (ii) Identify the anion (negative ion) present in the filtrate. ..................................................................................................................................... [1] Go back and complete the flame test in (c). [Total: 7]

Mark scheme: 2(a) filtrate: colourless solution ; 2 2(b) to dissolve (all of) the salt ; 1 2(c)(i) lilac ; 1 2(c)(ii) potassium / K+ / K ; 1 2(d)(i) white ppt. ; 1 2(d)(ii) sulfate / SO42 ; 1

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3 Cetyl alcohol is a white solid with a melting point of 49 °C. If salt is mixed with cetyl alcohol the melting point decreases. Some of the apparatus needed to determine the melting point of cetyl alcohol is shown in Fig. 3.1. large test-tube water gauze cetyl alcohol Bunsen burner with a small blue flame tripod Fig. 3.1 Plan an investigation to find the relationship between the mass of salt added and the decrease in the melting point of cetyl alcohol. You may use any common laboratory apparatus in your plan. You are provided with: • powdered cetyl alcohol • powdered salt • the apparatus shown in Fig. 3.1. You are not required to do this investigation. In your plan include: • the additional apparatus needed • a brief description of the method and an explanation of any safety precautions you will take • what you will measure • which variables you will keep constant • how you will process your results to draw a conclusion. You may include a table that can be used to record the results. You do not need to include any results in the table. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [7]

Mark scheme: 3 One marking point from each section and any three others (if one section is missing max 6 etc.): 1 Apparatus balance ; thermometer ; 2 Brief description of method and safety precautions add different masses of salt to cetyl alcohol ; use at least five different masses of salt ; place test-tube into hot water until mixture melts then allow mixture to cool (until it turns solid) ; wearing safety goggles to keep (cetyl) alcohol / hot water out of eyes / keep alcohol away from Bunsen flame / take precautions against injury with hot apparatus, e.g. test-tube holder ; 3 Measurements and control variables measure the masses of salt / states values for the masses of salt ; measure the temperature when mixture turns (back to) solid ; use the same / constant / quoted mass of cetyl alcohol ; 4 Process results to draw a conclusion repeat for each mass of salt and calculate an average ; plot graph of melting point against mass of salt added ;

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4 You will investigate the resistance of different combinations of two identical lamps L and M. (a) Fig. 4.1 shows a circuit using lamp L. The circuit is assembled for you. power supply A L V Fig. 4.1 Close the switch. Lamp L will light up. (i) Record the potential difference VL across lamp L. VL ......................................................... [1] (ii) Record the current IL in the circuit. IL ......................................................... [1] (iii) Open the switch. Calculate the resistance RL of lamp L. Use the equation shown. V L RL = I L RL ..................................................... Ω [1] (b) Procedure • Disconnect the voltmeter. • Add a second identical lamp M in series with lamp L as shown in Fig. 4.2. • Reconnect the voltmeter to measure the combined potential difference across lamps L and M. (i) Complete the circuit diagram in Fig. 4.2 to show how you connect the voltmeter. power supply A L M Fig. 4.2 [1] (ii) Close the switch. Both lamps will light up. Record the combined potential difference VS across lamps L and M. Include the unit in your answer. VS ........................................ unit ............... Record the current IS in the circuit. Include the unit in your answer. IS ........................................ unit ............... [1] (iii) Open the switch. Calculate the combined resistance RS of lamps L and M in series. Use the equation shown. V S RS = I S RS ..................................................... Ω [1] (c) Procedure • Disconnect the voltmeter. • Disconnect the lamps. • Change the circuit to connect lamp L in parallel with lamp M. • Connect the ammeter to measure the total current through the circuit. • Connect the voltmeter to measure the potential difference across the parallel lamps. (i) Complete the circuit diagram in Fig. 4.3 to show the parallel circuit. The power supply and switch are already shown. power supply Fig. 4.3 [2] (ii) Close the switch. Record the combined potential difference VP across lamps L and M. VP ............................................................... Record the current IP in the circuit. IP ............................................................... [1] (iii) Open the switch. Calculate the combined resistance RP of lamps L and M in parallel. Use the equation shown. V P RP = I P Give your answer to two significant figures. RP ..................................................... Ω [1]

Mark scheme: 4(a)(i) VL in range 2.0 to 3.5 (V) ; 1 4(a)(ii) IL in range 0.20 to 0.30 (A) ; 1 4(a)(iii) correct calculation of RL seen ; 1 4(b)(i) voltmeter shown connected in parallel across lamps L and M ; 1 4(b)(ii) VS and IS present and units included ; 1 4(b)(iii) RL  RS  1.8  RL ; 1 4(c)(i) voltmeter and lamps shown in parallel ; ammeter in common part of circuit ; 2 4(c)(ii) VP and IP present and IP  0.30 (A) ; 1 4(c)(iii) RP correct and to two significant figures ; 1 Question Answer Marks 4(d) so that the, lamp / wires / circuit does not overheat / to conserve, energy / battery ; 1 4(e) (expect no because) combined resistance / RS is (much) less than twice resistance of, L / RL (or reverse argument) ; calculation to support statement / values outside the limits of experimental accuracy ; 2

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