Cambridge IGCSE Physical Science 0652 — 2022 Oct/Nov Paper 6 · Variant 2

1 A student performs an experiment to find the value of x in the formula of sodium carbonate crystals, Na2CO3.xH2O. A weighed sample of sodium carbonate crystals is heated until all of the water has been removed. Na2CO3.xH2O Na2CO3 + xH2O The anhydrous sample (sample with no water) is weighed. The value of x can be calculated using the equation shown. amount H2O x = amount Na2CO3 (a) • She weighs the empty evaporating basin and records the mass in Table 1.1. • She places a sample of sodium carbonate crystals into the evaporating basin. She records the mass of the evaporating basin and sodium carbonate crystals in Table 1.1. • She heats the sodium carbonate crystals with a Bunsen burner for 5 minutes. • She allows the evaporating basin to cool for a few minutes. • She weighs the evaporating basin and anhydrous sodium carbonate, Na2CO3, and records this mass in Table 1.1. Fig. 1.1 shows the balance readings for the mass of the evaporating basin and sodium carbonate crystals before heating and the mass of the evaporating basin and anhydrous sodium carbonate after heating. Read the masses and record them to the nearest 0.1 g in Table 1.1. [2] evaporating basin g g evaporating basin and evaporating basin and sodium carbonate crystals anhydrous sodium carbonate Fig. 1.1 Table 1.1 mass of empty evaporating basin 26.8 g mass of evaporating basin and sodium carbonate crystals (Na2CO3.xH2O) ........................ g mass of evaporating basin and anhydrous sodium carbonate (Na2CO3) ........................ g (b) (i) Calculate the mass of anhydrous sodium carbonate, Na2CO3. Use the equation: mass anhydrous mass of evaporating basin and = – mass of empty evaporating basin sodium carbonate anhydrous sodium carbonate mass of anhydrous Na2CO3 = .......................................................g [1] (ii) Calculate the amount (number of moles) of Na2CO3. Use the equation: mass anhydrous Na2CO3 amount anhydrous Na2CO3 = 106 amount of anhydrous Na2CO3 = ......................................................... [1] (iii) Calculate the mass of water, H2O, given off. Use the equation: mass of evaporating basin and mass of evaporating basin and mass water = – sodium carbonate crystals anhydrous sodium carbonate mass of H2O = ...................................................... g [1] (iv) Calculate the amount (number of moles) of H2O. Use the equation: mass H2O amount H2O = 18 amount of H2O = ......................................................... [1] (v) Calculate the value of x in Na2CO3.xH2O. Use the answers to (b)(iv), (b)(ii) and the equation: amount H2O x = amount anhydrous Na2CO3 x = ......................................................... [1] (c) (i) Explain in detail why repeating the experiment and calculating the average would increase the accuracy of the value of x. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Identify two other major sources of error in this experiment. For each source of error suggest how the experiment can be improved to make the value of x more accurate. The changes suggested must be possible in a school or college laboratory. error 1 ................................................................................................................................ ........................................................................................................................................... improvement 1 .................................................................................................................. ........................................................................................................................................... error 2 ................................................................................................................................ ........................................................................................................................................... improvement 2 .................................................................................................................. ........................................................................................................................................... [2] [Total: 10]

Mark scheme: Question Answer Marks 1(a) 38.9 ; 2 31.4 ; 1(b)(i) (31.4 – 26.8 = ) 4.6 ; 1 1(b)(ii) (4.6 / 106 = ) 0.043 ; 1 1(b)(iii) (38.9 – 31.4 = ) 7.5 ; 1 1(b)(iv) (7.5 / 18 = ) 0.42 ; 1 1(b)(v) (0.42 / 0.042=) 10 ; 1 1(c)(i) identifies anomalies / outliers / reduces (effects of ) error ; 1 1(c)(ii) Any 2 from 2 not all water evaporated and heat for longer / to higher temp. / to constant mass ; spitting of the solid / sodium carbonate and add lid ; mass / loss water, etc. inaccurate and use a larger mass of crystals ;

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2 A student performs tests to identify three colourless solutions, A, B and C. (a) • He pours about 1 cm depth of solution A in a test-tube. • He adds universal indicator and records the colour in Table 2.1. • He repeats with solutions B and C. • He places about 1 cm depth of solution A into two test-tubes. • He adds a few drops of dilute nitric acid followed by a few drops of aqueous silver nitrate to one test-tube and records his observations in Table 2.1. • He adds a few drops of dilute nitric acid followed by a few drops of aqueous barium nitrate to the second test-tube and records his observations in Table 2.1. • He repeats with solutions B and C. Solution A gives a positive test for chloride ions. All of the other tests involving nitric acid are negative for A, B and C. Record the observations for each test in Table 2.1. [2] Table 2.1 test placed in solution A placed in solution B placed in solution C colour of red purple blue universal indicator dilute nitric acid and aqueous silver nitrate dilute nitric acid and aqueous barium nitrate (b) Identify solution A. solution A is ........................................................................................................................ [1] (c) The student now performs a flame test on each of the solutions and records his observations in Table 2.2 on page 6. Describe in detail how to perform a flame test and explain how contamination of the flame colour is avoided. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (d) The student thinks that solution B is aqueous sodium hydroxide and that solution C is aqueous ammonia. Explain how the test results in Table 2.1 and the flame test results in Table 2.2 suggest that the student is correct. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] (e) The student pours about 2 cm depth of solution B into a test-tube. • He adds aqueous copper(II) sulfate until it is in excess. • He repeats this for solution C. Record in Table 2.2 the observations if solution B is aqueous sodium hydroxide and solution C is aqueous ammonia. [2] Table 2.2 test solution B solution C flame colour yellow none add aqueous copper sulfate until it is in excess (f) The student says that solution C contains ammonium ions. Describe a test and give the result which confirms that solution C contains ammonium ions. test ............................................................................................................................................ positive result ............................................................................................................................ ................................................................................................................................................... [2] [Total: 10]

Mark scheme: 2(a) white ppt / solid for A and silver nitrate ; 2 all others colourless (solution) / no change / no reaction / no ppt ; 2(b) hydrochloric acid / HCl ; 1 2(c) Dip (nichrome) wire / (soaked) splint in solution and place in blue / roaring (bunsen) flame ; 2 clean (wire) between tests with HCl / acid (and heat in bunsen) OR use different splint ; 2(d) both alkali / base and 1 since no flame colour not likely to be a metal ion / ammonium has no flame colour / sodium has yellow flame ; 2(e) Solution B: (pale) blue ppt / solid ; 2 Solution C: deep blue (solution) (then (pale) blue ppt / solid) ; 2(f) sodium hydroxide and heat / warm ; 2 (gas / ammonia) turns (red) litmus blue ;

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3 A student measures the resistance of an unknown resistor X. Fig. 3.1 shows the circuit used. A voltmeter has been omitted from the diagram. power supply crocodile slide wire A resistor X clip Fig. 3.1 (a) Complete the circuit diagram in Fig. 3.1 by adding the symbol for a voltmeter connected in the correct position to measure the potential difference across resistor X. [1] (b) The student connects the crocodile clip at different positions on the slide wire to change the potential difference V across the resistor. He measures the current I and potential difference V for each position. Fig. 3.2 shows the ammeter reading for the value V = 2.6 V. Read the value of I and record it in Table 3.1. Record the value to 2 significant figures. [2] 0.4 0.6 0.2 0.8 0 1.0 A Fig. 3.2 Table 3.1 V / V I / A 0.2 0.09 0.8 0.16 1.4 0.26 2.0 0.35 2.6 (ii) The resistance of resistor X, Rx, is equal to 1 / G. Use your value of G from (d)(i) to calculate Rx. Give your answer to a suitable number of significant figures. Rx = ..................................................... Ω [2] (iii) Resistor X was chosen from a selection of resistors with values 4.7 Ω or 5.1 Ω. Use your value of Rx to identify the actual resistance of resistor X from the list. Tick the box to indicate your choice. 4.7 Ω 5.1 Ω either of these neither of these Explain your choice with reference to your calculated value for Rx. ........................................................................................................................................... ........................................................................................................................................... [1] (e) The resistance of resistor X can be determined by taking a single pair of values of current I, and potential difference V from Table 3.1, and using the equation R = V / I. Suggest one reason why plotting a graph gives a more accurate value of resistance. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 13]

Mark scheme: 3(a) correct symbol for voltmeter in parallel with resistor ; 1 3(b) 0.43 ; 1 2 sig fig ; 1 3(c)(i) graph: 1 axes labelled with quantity and unit ; 1 1 appropriate sensible linear scales (plots occupying at least grid) ; 2 1 1 plots all correct to small square ; 2 3(c)(ii) single line of best fit ; 1 3(d)(i) over half of the line of best fit used (shown by triangle method or co-ordinates) ; 1 correctly calculated gradient ; 1 3(d)(ii) Correct 1 / G ; 1 2 / 3 sig fig ; 1 3(d)(iii) Check candidates value for Rx 1 If one value chosen, justification should indicate a smaller difference between calculated value and chosen value; If ‘either of these’ chosen, justification should indicate that the difference between the calculated value and (both) 4.7 and 5.1 is too / very similar ; If ‘neither of these’ chosen, justification should indicate the calculated value is too / very different from (both) 4.7 and 5.1 / given value 3(e) anomalies / outliers are identified (and excluded) ; 1

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4 A student suggests that the starting temperature of hot water affects its rate of cooling. The following equipment is available to the student: a supply of water an electric kettle thermometer 250 cm3 beaker 250 cm3 measuring cylinder stopwatch clamp, boss and stand. Plan an experiment to investigate the relationship between the starting temperature of water and its rate of cooling. Your plan should include: • a brief description of the method, including how you will obtain a range of starting temperatures • the measurements you will make • the variables to control • the table you will draw to record your results, with column headings (you are not required to enter any readings in the table) • an explanation of how you would use your results to reach a conclusion. A diagram is not required but you may draw one if it helps to explain your plan. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [7]

Mark scheme: 4 Candidates must score 1 mark from each of the headed sections, and then any other 2 marks (from any section) up to a 7 maximum of 7 apparatus ( within context ) measuring cylinder/burette to measure the volume/amount (of water) ; (stop)watch/timer/clock to measure the cooling period ; thermometer to measure the temperature (of water) ; method heat water to different (desired) starting temperatures/boil water and allow water to cool to (desired) starting temperatures ; heat water to a measured temperature and record the temperature at (different) time intervals ; measure the time to cool for the same temperature drop / measure the temperature drop in the same time of cooling ; repeat at each start temperature ; controls same temperature drop / same cooling time same volume of (hot) water same beaker/material/shape etc same room temperature / other environmental conditions table (starting) temperature (of water) with units and time (to cool) with units or temperature decrease/change with units (depending on chosen dependent variable) ; processing and use of results compare readings to find out which starting temperature produced the greatest temperature drop or took the least time ; plot a graph of temperature against time ; steepest gradient gives the fastest rate of cooling ; evidence of averaging of repeated measurements ; evidence of calculating a rate ;

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