Cambridge IGCSE Biology (9-1) 0970 — 2025 May/June Paper 5 · Variant 1

1 You are going to investigate the effect of concentration on diffusion. You will use cubes of agar which contain universal indicator. You will place the cubes of agar into different concentrations of hydrochloric acid solution and measure the time taken for the acid to diffuse to the centre of each cube. Read all the instructions but DO NOT DO THEM until you have drawn a table for your results in the space provided in 1(a)(i). You should use the safety equipment provided while you are doing the practical work. Step 1 Label four large test-tubes 0.4, 0.6, 0.8, 1.0. Put the test-tubes in the test-tube rack. Table 1.1 shows the volumes of 1.0 mol per dm3 hydrochloric acid solution and distilled water needed to make the different concentrations of hydrochloric acid solution. Table 1.1 concentration of volume of 1.0 mol per dm3 volume of distilled water hydrochloric acid solution hydrochloric acid solution / cm3 / mol per dm3 / cm3 0.4 4 6 0.6 6 4 0.8 8 2 1.0 10 0 Step 2 Use a 10 cm3 syringe to put the volumes of 1.0 mol per dm3 hydrochloric acid solution shown in Table 1.1 into the large test-tubes labelled 0.4, 0.6, 0.8, 1.0. Step 3 Use a clean 10 cm3 syringe to put the volumes of distilled water shown in Table 1.1 into the large test-tubes labelled 0.4, 0.6, 0.8, 1.0. Step 4 You are provided with a block of agar which measures approximately 1 cm × 2 cm × 2 cm. The dotted lines in Fig. 1.1 show where the block should be cut. Use the knife to cut four cubes from the large block of agar. Each cube should be approximately 1 cm × 1 cm × 1 cm. 1 cm agar block 1 cm white tile 1 cm not to scale Fig. 1.1 Step 5 Raise your hand when you are ready for warm water to be added to the beaker labelled water-bath. Step 6 Place all the large test-tubes into the water-bath. Step 7 Put one agar cube into each of the large test-tubes and immediately start the stop-clock. Observe the colour of the agar cubes. You may find it easier to observe the colour if you hold the white card behind the test-tubes. Record in your table in 1(a)(i) the time taken, in seconds, for each agar cube to become completely red. If an agar cube is not completely red after 10 minutes, stop timing and record > 600 for that concentration of hydrochloric acid solution in your table in 1(a)(i). (a) (i) Prepare a table and record your results. [4] (ii) State a conclusion for your results. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iii) State the independent and dependent variables in this investigation. independent variable ......................................................................................................... ........................................................................................................................................... dependent variable ............................................................................................................ ........................................................................................................................................... [2] (iv) State two sources of error in the method, other than not repeating the investigation. 1 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... 2 ......................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (v) Identify two hazards in this investigation. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (b) Plan an investigation to determine the effect of temperature on osmosis in potato plant tissue. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [6] [Total: 16]

Mark scheme: Question Answer Marks Guidance 1(a)(i) table drawn with a minimum of two columns and a header line ; 4 appropriate column / row headings with units for concentration and time ; recording of four correct concentrations linked to any four times ; expected trend: 1.0 took less time than 0.4 ; 1(a)(ii) expected conclusion: as the concentration (of hydrochloric acid) 1 the conclusion must match the candidate’s increases, the rate of diffusion increases / ora ; results 1(a)(iii) independent: concentration (of hydrochloric acid) ; 2 dependent: time (for the whole cube to change colour / for diffusion) ; 1(a)(iv) any two from: 2 agar cubes not of equal size / AW ; difficult to determine the end-point / AW ; idea of difficult to time all at the same time ; not putting cubes in at same time / AW ; not controlling temperature / not using thermostatically controlled water-bath ; 1(a)(v) any two for one mark ; 1 • using a knife / cutting • acid • (universal) indicator 1(b) independent variable 6 1 (at least) two different temperatures ; dependent variable 2 (measuring) length / mass / volume, of the potato pieces before and after ; 3 & 4 & 5 details of method ;;; • drying / removing excess water, before weighing • ref. to use of a cork borer / potato chipper, to prepare the piece of potato • ref. to peeling the potato / using potato without skin • putting potato pieces in (a beaker containing) water / sugar solution / salt solution / solution • ref. to making sure potato is covered by the solution / submerged • ref. to, equilibration time, AW, before adding potato (if putting the potato into a beaker that is then put into the water-bath) • method of maintaining at least one of the temperatures 6 & 7 variables kept constant ;; • type / species / age, of potato / plant tissue • initial, surface area / dimensions / mass • concentration of solution / same water potential • time (in solution) / stated time, e.g. left for one hour 8 at least three trials / repeat investigation (at least) twice more ; 9 relevant safety measure ;

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2 (a) A student used DCPIP solution to compare the concentration of vitamin C in five solutions. The student used this method: • Put 1 cm3 of 1% DCPIP solution into a test-tube. • Use a syringe to add drops of 125 mg per dm3 vitamin C solution to the test-tube. • Keep adding drops of vitamin C solution until the DCPIP solution becomes colourless. • Record the total volume of the 125 mg per dm3 vitamin C solution added. This method was repeated with four other concentrations of vitamin C solution. (i) State the expected colour of the DCPIP solution before the vitamin C solution was added. ..................................................................................................................................... [1] (ii) State two variables that were kept constant by the student in this investigation. 1 ........................................................................................................................................ 2 ........................................................................................................................................ [2] The results of the investigation are shown in Table 2.1. Table 2.1 concentration of vitamin C total volume of vitamin C / mg per dm3 solution added / cm3 125 1.6 250 1.4 500 1.1 750 0.8 1000 0.5 (b) (i) Plot a line graph on the grid of the data in Table 2.1. [4] (ii) The student tested a sample of orange juice. They found that 1.2 cm3 of orange juice was needed to react with 1 cm3 of 1% DCPIP solution. Use your graph to estimate the vitamin C concentration of the orange juice. Show on the graph how you obtained your estimate. ............................................ mg per dm3 [2]

Mark scheme: 2(a)(i) blue ; 1 2(a)(ii) volume of DCPIP ; 2 concentration of DCPIP ; 2(b)(i) axes labelled with units ; 4 suitable linear scale and data occupies at least half the grid in both directions ; five points plotted accurately  half a small square ; suitable line drawn ; 2(b)(ii) intercept indicated on the candidate’s graph at 1.2 cm3 ; 2 concentration correct for intercept at 1.2 cm3  half a small square ; 2(c)(i) storing decreases the vitamin C content (of apple juice) ; 1 2(c)(ii) a result that does not fit the, pattern / trend / AW ; 1 2(c)(iii) 170 (circled) ; 1 2(c)(iv) − 61.8 ;;; 3 MP1 correct selection of data from table 272 and 104 OR 168 MP2 correct calculation of percentage decrease indicated with a negative sign to any number of decimal places MP3 correct rounding to 1 decimal place ecf from previous step 2(d)(i) Outline is a single clear line with no shading ; 4 Size – at least half the available space used ; Detail 1: six sepals and a stalk ; Detail 2: (at least) five white stripes on the lower side, radiating out and shown as double lines and four seeds shown as circles / ovals ; 2(d)(ii) PQ = 61  1 (mm) ; 3 MP1 correct measurement of line PQ 33.9 (mm) ;; MP2 correct calculation of the actual size to any number of significant figures MP3 correct rounding to three significant figures ecf from previous step PQ of 60 mm = 33.3 (mm) PQ of 62 mm = 34.4 (mm) 2(e) add Benedict’s (reagent / solution) ; 2 heat ;

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