Cambridge IGCSE Science - Combined 0653 — 2018 Oct/Nov Paper 5 · Variant 2

1 You are going to investigate the nutrient content of two solutions, A and B. (a) You are provided with Benedict’s solution, biuret solution and iodine solution. 1. Add about 1 cm depth of solution A to each of three test‑tubes. 2. Add the same depth of Benedict’s solution to one of these test‑tubes and place in a hot water‑bath for at least 3 minutes. You may carry out steps 3 and 4 while you are waiting. 3. Add the same depth of biuret solution into the second test‑tube. 4. Add a few drops of iodine solution to the final test‑tube. Record, in Table 1.1, your observations of the final colours for each of the three test‑tubes. [3] Table 1.1 observation with observation with observation with solution Benedict’s solution biuret solution iodine solution A B (b) Using clean test‑tubes repeat the procedure in (a) with solution B instead of solution A. Record your observations in Table 1.1. [2] (c) Use your observations in Table 1.1 to state the nutrients present in each solution. Solution A contains ................................................................................................................... Solution B contains ................................................................................................................... [2] (d) State and explain a safety precaution you used when carrying out the tests. safety precaution ...................................................................................................................... explanation ............................................................................................................................... ................................................................................................................................................... [1] (e) Describe the method used to test a liquid for the presence of fat. Indicate the observation for a positive result. method ...................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... observation for a positive result ................................................................................................ ................................................................................................................................................... [2]

Mark scheme: 1(a) A is blue (with Benedict’s solution); purple / lilac (with biuret solution); blue-black (with iodine solution); 3 1(b) B is orange / red (with Benedict’s solution); blue (with biuret solution) and brown / orange / yellow (with iodine solution); 2 1(c) A contains protein and starch; B contains reducing sugar; 2 1(d) wear goggles because of hot water / handling chemicals; OR use test-tube holders / gloves because of hot water; 1 1(e) alcohol; water and cloudy / emulsion; 2

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2 Notes for use in Qualitative Analysis for this question are printed on page 8. You are going to investigate the thermal decomposition of metal carbonates. (a) (i) • Add about 3 cm depth of limewater to a test‑tube. • Connect the bung of the delivery tube to the hard‑glass test‑tube containing copper carbonate. • Place the delivery tube into the limewater, as shown in Fig. 2.1. delivery tube limewater copper carbonate heat Fig. 2.1 • Heat the test‑tube containing copper carbonate with a hot flame and start the stopclock. • When the limewater becomes milky, stop the stopclock, remove the delivery tube from the limewater to avoid suck back and stop heating. Record, in Table 2.1, the time to the nearest second as well as any colour change in the solid. [1] Table 2.1 name of metal carbonate time / s colour change copper carbonate magnesium carbonate zinc carbonate (ii) Repeat (a)(i) with the magnesium carbonate instead of copper carbonate. Use fresh limewater. Stop the stopclock when the limewater has the same milky colour as in (a)(i). [1] (iii) Repeat (a)(ii) with zinc carbonate instead of magnesium carbonate. [1] (iv) Place the three metal carbonates in order of rate of turning limewater milky (speed of thermal decomposition). 1. ........................................................ fastest 2. ........................................................ 3. ........................................................ slowest [1] (v) Name the gas produced by the thermal decomposition of a metal carbonate. .......................................................................................................................................[1] (b) (i) Suggest two reasons why the results in this experiment may be inaccurate. 1. ....................................................................................................................................... ........................................................................................................................................... 2. ....................................................................................................................................... ........................................................................................................................................... [2] (ii) The rate of the gas production when metal carbonates are heated can be measured without the use of limewater. Suggest an alternative method of measuring the rate of gas production. Include a diagram to show what replaces the test‑tube of limewater and state what should be measured. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [3]

Mark scheme: 2(a)(i) time for copper carbonate and black; 1 2(a)(ii) time for magnesium carbonate less than 60 s and stays white; 1 2(a)(iii) time for zinc carbonate less than 60 s and yellow (when hot); 1 2(a)(iv) correct order of speed; 1 2(a)(v) carbon dioxide / CO2; 1 Question Answer Marks 2(b)(i) any two from difficult to consistently determine ‘milky’ point of limewater; error reading / stopping the stop-clock; too much variation in amounts of carbonates; variation in volume of limewater; variation in heat; escape of gas from bung; Max 2 2 2(b)(ii) measure the volume of gas; in a certain time; suitable diagram; 3

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3 You are going to investigate how the power produced in a resistance wire XY depends upon its length. The circuit shown in Fig. 3.1 has been set up for you. resistance wire metre rule l X Y 0 C 100 cm sliding contact V A power supply switch Fig. 3.1 (a) (i) • Close the switch. • Place the sliding contact C on the resistance wire at a distance of 10.0 cm from end X. Record, in Table 3.1, the current I flowing through the wire. [1] (ii) Record, in Table 3.1, the potential difference (p.d.) V across the wire. • Open the switch. [1] Table 3.1 length l / cm current I / A p.d. V / V power P / W 10.0 20.0 40.0 80.0 (iii) Calculate the power P produced in the 10.0 cm length of the wire using the equation shown: P = V × I Record P in Table 3.1. [1] (iv) Repeat (a)(i), (a)(ii) and (a)(iii) for values of l of 20.0 cm, 40.0 cm and 80.0 cm. Record, in Table 3.1, your values of I, V and P. [4] (v) State why it is important to open the switch between taking readings. ........................................................................................................................................... .......................................................................................................................................[1] (b) Use your results in Table 3.1 to suggest the relationship between the power P produced in the wire and its length l. Explain your answer. relationship ............................................................................................................................... ................................................................................................................................................... explanation ............................................................................................................................... ................................................................................................................................................... [2] NOTES FOR USE IN QUALITATIVE ANALYSIS Tests for anions anion test test result carbonate (CO3 2–) add dilute acid effervescence, carbon dioxide produced chloride (Cl –) acidify with dilute nitric acid, then white ppt. [in solution] add aqueous silver nitrate nitrate (NO3 –) add aqueous sodium hydroxide, ammonia produced [in solution] then aluminium foil; warm carefully sulfate (SO4 2–) acidify with dilute nitric acid, then white ppt. [in solution] add aqueous barium nitrate Tests for aqueous cations cation effect of aqueous sodium hydroxide effect of aqueous ammonia ammonium (NH4 +) ammonia produced on warming – copper(II) (Cu2+) light blue ppt., insoluble in excess light blue ppt., soluble in excess, giving a dark blue solution iron(II) (Fe2+) green ppt., insoluble in excess green ppt., insoluble in excess iron(III) (Fe3+) red-brown ppt., insoluble in excess red-brown ppt., insoluble in excess zinc (Zn2+) white ppt., soluble in excess, giving white ppt., soluble in excess, giving a colourless solution a colourless solution Tests for gases gas test and test results ammonia (NH3) turns damp red litmus paper blue carbon dioxide (CO2) turns limewater milky chlorine (Cl2) bleaches damp litmus paper hydrogen (H2) ‘pops’ with a lighted splint oxygen (O2) relights a glowing splint

Mark scheme: 3(a)(i) for length = 10 cm current value recorded, < 1 A and consistently to 1 / 2 d.p.; 1 3(a)(ii) for length = 10 cm voltage value recorded and < 2 V; 1 3(a)(iii) calculation of power correct; 1 3(a)(iv) all current values approximately the same; V values increasing; all P values correct; consistent 2 / 3 d.p. for P; 4 3(a)(v) so that wire does not become hot / resistance of wire might change / cell or battery may run down; 1 3(b) (directly) proportional / greater the length the greater the power; ratio power ÷ length is constant / doubling length doubles the power, etc.; 2

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