Cambridge IGCSE Sciences - Co-ordinated (Double) 0654 — 2023 Feb/March Paper 6 · Variant 2

1 A student investigates the nutrient content of two food solutions, A and B. Procedure The student: • pours some of solution A into each of three test-tubes • adds an equal depth of Benedict’s solution to one test-tube • adds an equal depth of biuret solution to another test-tube • adds a few drops of iodine solution to the third test-tube • repeats the procedure with solution B instead of solution A. (a) State which one of the three testing solutions needs to be heated. ............................................................................................................................................. [1] (b) State the initial colours of the testing solutions. Benedict’s solution ................................................................................................................... biuret solution ........................................................................................................................... iodine solution ........................................................................................................................... [1] (c) Table 1.1 shows the final colours the student observes. Table 1.1 food Benedict’s biuret solution iodine solution nutrient(s) present solution solution A red purple brown B blue purple blue-black Complete Table 1.1 to state the nutrient(s) present in each food solution. [3] [Total: 5]

Mark scheme: Question Answer Marks 1(a) Benedict’s ; 1 1(b) blue and 1 blue and brown ; 1(c) A and B contain protein ; 3 A also contains reducing sugar ; B also contains starch ;

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2 A student investigates the vitamin C content of four different fruit juices, E, F, G and H. DCPIP indicator is a blue solution that decolourises (turns colourless) when vitamin C is added to it. Solutions containing a high concentration of vitamin C need only a few drops to turn the DCPIP indicator colourless. Solutions containing a low concentration of vitamin C need more drops to turn the DCPIP indicator colourless. (a) Procedure The student: • uses a dropping pipette to place two drops of DCPIP into a well of a white spotting tile • uses a clean dropping pipette to add drops of fruit juice E to the well containing DCPIP • counts how many drops are needed to decolourise the DCPIP • records the result • repeats with the other fruit juice samples. (i) Draw a table to record the student’s results. [2] (ii) Fig. 2.1 shows part of the student’s notebook. III Solution F H IIIII IIIII IIIII Solution III IIIII Solution G IIII Solution IIIII IIIII IIII E Number of drops Fig. 2.1 Record the student’s results from Fig. 2.1 in your results table in (a)(i). [1] (b) (i) Identify one source of error in the student’s procedure and suggest an improvement. error ................................................................................................................................... improvement ..................................................................................................................... ........................................................................................................................................... [2] (ii) Suggest why the procedure is not suitable for use with blueberry juice. ..................................................................................................................................... [1] (c) (i) A deficiency of vitamin C in the diet causes a disease called scurvy. State which juice E, F, G or H is best at preventing scurvy. Explain your answer. juice ................................................................................................................................... explanation ........................................................................................................................ ........................................................................................................................................... [1] (ii) An adult human needs 0.030 g of vitamin C every day to prevent scurvy. A 125 cm3 glass of an orange fruit juice contains 0.060 g of vitamin C. Calculate the volume of this orange juice an adult human needs to drink every day to prevent scurvy. Give your answer to two significant figures. volume = .................................................. cm3 [2] (iii) Suggest how an adult human, who drinks less than the volume of orange juice calculated in (c)(ii), can prevent scurvy. ..................................................................................................................................... [1] (d) Citrus fruits are one good source of vitamin C. Fig. 2.2 shows the cut surface of a citrus fruit. Fig. 2.2 (i) In the box, make a large, accurate drawing of one quarter of the cut surface of the citrus fruit. The quarter is shown by the dotted lines on Fig. 2.2. [3] (ii) The diameter of the citrus fruit in Fig. 2.2 is approximately 55 mm. The magnification m of Fig. 2.2 is ×0.8. Use the equation shown. diameter of citrus fruit in Fig. 2.2 m = diameter of actual citrus fruit Calculate the approximate diameter of the actual citrus fruit. diameter = .................................................. mm [2]

Mark scheme: 2(a)(i) headings for fruit juice and number of drops ; 2 minimum of cross shape to separate headings and results ; 2(a)(ii) H=3 and E=9 and F = 18 and G=14 ; 1 2(b)(i) drop size varies ; 2 measure volume ; OR there are no repeats ; exclude anomalies / take average ; OR well not large enough ; use test-tube ; 2(b)(ii) juice colour masks result / blueberry juice has same colour as DCPIP ; 1 2(c)(i) H because it requires fewest drops / highest vitamin C concentration; 1 2(c)(ii) 125 divided by 2 OR 62.5 ; 2 63 (cm3) ; 2(c)(iii) by eating/drinking other juices/food containing vitamin C ; 1 2(d)(i) clear and continuous lines ; 3 at least half the box and ¼ of circle ; detail – skin, pith and between 2 and 4 segments ; 2(d)(ii) 55 ÷ 0.8 ; 2 69 (mm);

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3 A student investigates the neutralisation reaction of dilute hydrochloric acid and aqueous sodium hydroxide. The reaction is exothermic. Thermal energy (heat) is given out and the temperature of the mixture increases. When the reaction is finished no more thermal energy is given out. The student assembles the apparatus shown in Fig. 3.1. burette dilute hydrochloric acid thermometer beaker plastic cup 25 cm3 aqueous sodium hydroxide Fig. 3.1 Procedure The student: • puts a plastic cup into a glass beaker • uses a measuring cylinder to put 25 cm3 of aqueous sodium hydroxide into the plastic cup • records in Table 3.1 the temperature of the aqueous sodium hydroxide in the plastic cup • adds 5.0 cm3 of dilute hydrochloric acid from a burette to the plastic cup • stirs the mixture and records the temperature in Table 3.1 • continues adding 5.0 cm3 portions of dilute hydrochloric acid, stirring and recording the temperature of the mixture until 45.0 cm3 of dilute hydrochloric acid have been added. The student keeps the contents of the plastic cup for Question 4. Table 3.1 total volume of dilute temperature hydrochloric acid added / °C / cm3 0.0 21.5 5.0 24.0 10.0 15.0 29.0 20.0 30.5 25.0 34.0 30.0 33.0 35.0 29.5 40.0 45.0 23.0 (a) (i) Fig. 3.2 shows the thermometer readings for 10.0 cm3 and 40.0 cm3 of dilute hydrochloric acid added. °C °C 30 30 25 25 20 20 10.0 cm3 of 40.0 cm3 of dilute hydrochloric acid added dilute hydrochloric acid added Fig. 3.2 Record in Table 3.1 these temperatures to the nearest 0.5 °C. [2] (ii) State the name of a piece of apparatus suitable for measuring the volume of aqueous sodium hydroxide more accurately than the measuring cylinder. ..................................................................................................................................... [1] (iii) The plastic cup is placed into the glass beaker to stop it tipping over when it is stirred. Suggest another reason why the plastic cup is placed into a glass beaker. ..................................................................................................................................... [1] (b) (i) On the grid, plot temperature on the vertical axis against volume of dilute hydrochloric acid added. Do not start the temperature axis at 0 °C. The temperature scale needs to be approximately 5 °C higher than the highest temperature recorded in Table 3.1. [3] (ii) Complete the following steps and draw two lines of best fit on the graph. • Draw the best-fit straight line through the increasing temperatures. • Extrapolate the line past the highest point. • Draw the best-fit straight line through the decreasing temperatures. • Extrapolate the line past the highest point. The maximum temperature Tmax reached by the reaction is where the two lines intersect (cross). Deduce the maximum temperature reached. Show your working on the graph. Tmax = .................................................... °C [4] (iii) Where the lines intersect (cross) on the graph also shows the volume of dilute hydrochloric acid that exactly neutralises the 25 cm3 of aqueous sodium hydroxide. This is the point where the reaction is finished. Use your graph to state the volume of dilute hydrochloric acid V that exactly neutralises the 25 cm3 of aqueous sodium hydroxide. V = .................................................. cm3 [1] (c) Calculate the increase in temperature, ΔT, for the reaction. Use the equation shown. ΔT = Tmax – T0 T0 is the temperature when 0.0 cm3 of dilute hydrochloric acid is added. ΔT = .......................................................... °C Calculate the thermal energy H given out during the reaction. Use the equation shown. H = (25 + V ) × 4.2 × ΔT where V is your answer in (b)(iii). H = ............................................................ J [1] (d) Thermal energy is lost to the air during the experiment. Suggest one change to the apparatus that reduces the amount of thermal energy lost. ................................................................................................................................................... ............................................................................................................................................. [1] (e) After the reaction, the plastic cup is left on the bench for two hours. Suggest the temperature of the mixture in the plastic cup after two hours. temperature ..................°C Explain how you arrived at your answer. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 15]

Mark scheme: 3(a)(i) 26.5 ; 2 26.0 ; 3(a)(ii) burette / (volumetric / graduated) pipette ; 1 3(a)(iii) reduces heat loss / (plastic or glass) is a poor conductor ; 1 3(b)(i) axes correct way round and labelled with quantity and unit ; 3 sensible linear axes with plotted points = 4 °C covering ⩾ half of the grid ; points plotted correctly ; 3(b)(ii) best fit line increasing temperatures ; 4 best fit line decreasing temperatures ; extrapolated to cross ; temperature at intersection  ½ small square and shown on graph ; 3(b)(iii) V from graph  ½ small square 1 3(c) correct calculation ; 1 3(d) put lid on cup / use polystyrene / insulate the beaker or cup; 1 3(e) 21.5 °C 1 AND cools to room T

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4 The student investigates the substances from Question 3. When an acid is added to an alkali a neutralisation reaction occurs. When just enough acid is added to neutralise the alkali a neutral substance is formed. A neutral substance turns universal indicator green and has a pH of 7. Procedure The student: • adds universal indicator to separate samples of dilute hydrochloric acid, aqueous sodium hydroxide and the contents of the plastic cup from the end of the procedure in Question 3 • adds a small piece of magnesium ribbon to separate samples of dilute hydrochloric acid, aqueous sodium hydroxide and the contents of the plastic cup from the end of the procedure in Question 3. Some of the results are shown in Table 4.1. Table 4.1 colour with universal observation with substance pH indicator magnesium dilute hydrochloric acid red 1 gas given off aqueous sodium hydroxide dark blue 12 no reaction contents of the plastic cup from the end of the procedure in question 3 (a) State the test that confirms the gas given off with dilute hydrochloric acid is hydrogen. Include the observation for a positive result. test ............................................................................................................................................ observation ............................................................................................................................... [2] (b) (i) Complete Table 4.1 by predicting the results of the procedure with the contents of the plastic cup at the end of the procedure in Question 3. [2] (ii) Explain your predictions. ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 5]

Mark scheme: 4(a) Iighted splint ; 2 pops ; 4(b)(i) red and pH 1–3 ; 2 gas given off ; 4(b)(ii) more acid added after reaction finished OWTTE ; 1

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5 A student investigates the resistance of series and parallel combinations of identical resistors. The student assembles the circuit shown in Fig. 5.1. Resistors R1 and R2 are connected in series. (a) On Fig. 5.1, draw the symbol for a voltmeter and show how it is connected to measure the potential difference between point X and point Y. power source switch A R1 R2 X Y Fig. 5.1 [2] (b) Procedure The student: • connects the voltmeter to measure the potential difference between point X and point Y • closes the switch • takes the readings on the voltmeter and the ammeter • records the readings • opens the switch Fig. 5.2 shows the readings on the voltmeter and the ammeter. 1 2 V 0 3 0.4 0.6 0.2 0.8 0 1 A Fig. 5.2 (i) Record the potential difference V and the current I. V = ........................................................... V I = ............................................................ A [2] (ii) Calculate the total resistance RS between point X and point Y. Use the equation shown. V RS = I Give the unit for your answer. RS = ............................ unit = ................... [2] (iii) Explain why it is good practice to open the switch between taking and recording readings from the ammeter and voltmeter. ........................................................................................................................................... ..................................................................................................................................... [1] (c) The student needs more values of potential difference V, and current I, to calculate an average value for the resistance RS. They connect an extra component in series with the power source, to vary the current. State the name and draw the symbol for this component. name ......................................................................................................................................... symbol [2] (d) Procedure The student: • disconnects the voltmeter • connects the circuit as shown in Fig. 5.3. Resistors R1 and R2 are now connected in parallel. power source switch A R1 X Y R2 Fig. 5.3 The student: • reconnects the voltmeter to measure the potential difference between point X and point Y • closes the switch • records the potential difference V and the current I • opens the switch. The student’s readings are shown below. V = 1.4 V I = 0.58 A Calculate the total resistance RP between point X and point Y for the parallel circuit. Use the equation shown. V RP = I

Mark scheme: 5(a) correct symbol ; 2 correct parallel connection in correct place ; 5(b)(i) 1.5 (V) ; 2 0.16 (A) ; 5(b)(ii) 9.4 ; 2  / ohms / ohm ; 5(b)(iii) so that cell does not run down / resistors or circuit or wires do not overheat ; 1 5(c) variable resistor / rheostat ; 2 correct symbol ; 5(d) 2.4 () ; 1 5(e) attempt at calculation of 4 times the value of RP ; 2 correct matching statement (yes/no – expect yes) with 10% calculation and suitable comment e.g., values are too different / not close enough / >10% different ; 5(f) inspect the filament of each lamp / check if the ammeter gives a reading ; 1

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6 Water is heated from room temperature to its boiling point in a metal cooking pot. Plan an experiment to investigate if the time taken for water to reach its boiling point depends on the metal from which the cooking pot is made. You are provided with: • a supply of cold water • a set of three cooking pots of the same size, one made of aluminium, one made of copper and one made of steel • a Bunsen burner, a tripod and gauze • a measuring cylinder. You may use any other common laboratory apparatus. In your plan include: • any other apparatus needed • a brief description of the method, including what you will measure and how you will make sure your measurements are accurate • the variables you will control • a results table to record the measurements (you do not need to make any measurements or record any results in the table) • how you will process your results to draw a conclusion. You may include a labelled diagram if you wish. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..........................................................................................................................................................

Mark scheme: Question Answer Marks 6 additional apparatus: 7 stop-watch ; thermometer ; method: pour (cold) water into one of the pots, boil it, measure time and repeat for all 3 pots; repeats method for each pot (i.e. more than one trial for each pot) ; control variables: mass of cold water / volume of cold water ; same initial cold-water temperature ; keep Bunsen burner flame constant ; table: columns for pot / metal and time ; units for time ; processing and conclusion: plot bar chart / histogram of metal against time; if the times are different then metal makes a difference / if the times are the same then the metal does not make a difference / lowest time best heat conductor ; calculate average identifying / excluding anomalies ;

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