Biology · Oct / Nov 2022 · Paper 61 · Question 1
0610
As printed in the paper
The question as text
A student investigated the vitamin C concentration in three different health drinks: A, B and C. The concentration of vitamin C can be estimated in a sample of health drink by testing it with iodine solution. The test involves adding drops of iodine solution to the health drink sample until the sample remains blue-black. The greater the volume of iodine solution added, the higher the concentration of vitamin C in the health drink. The student used this method: Step 1 Label three test-tubes A, B and C. Step 2 Use a syringe to add 1 cm3 of starch suspension to each of test-tubes A, B and C. Step 3 Use a clean syringe to add 3 cm3 of health drink A to test-tube A. Step 4 Use a clean syringe to add 3 cm3 of health drink B to test-tube B. Step 5 Use a clean syringe to add 3 cm3 of health drink C to test-tube C. Step 6 Use a dropping pipette to add one drop of iodine solution to test-tube A and shake the test-tube gently to mix the contents. Step 7 Repeat step 6, counting the number of drops added, until a blue-black colour appears and stays blue-black after mixing. Step 8 Record the total number of drops of iodine solution added to test-tube A. Step 9 Repeat steps 6 to 8 with test-tubes B and C. Refill the dropping pipette with more iodine solution as necessary. Step 10 Pour the contents of test-tubes A, B and C into the waste container and rinse the test-tubes in clean water. Repeat steps 2 to 9 to obtain another set of results. Step 11 Repeat step 10 to obtain a third set of results. The student’s results are shown in Fig. 1.1. Solution A = 5 = 4 = 6 B = 17 = 19 = 15 C = 3 drops = 2 = 1 Fig. 1.1
Read automatically from the paper; the printed page above is exact.
Show the answer and mark scheme
1(a)(i)[4]
table drawn with minimum of two , columns / rows and with a line separating headings from the body of the table ; appropriate headings for each, column / row ;
nine correct results recorded ;
three correct averages ;
1(a)(ii)[1]
as an indicator / so that the iodine would change colour / to see the change / to react with the iodine
/ to give a positive result with iodine / AW ;
1(a)(iii)[1]
(health drink / solution) B
because B needs more (drops / greater volume of) iodine (solution to stay blue-black) ;
1(a)(iv)[1]
the, number / amount, of drops (of iodine solution added) ;
1(a)(v)[1]
to prevent contamination / so the solutions are not mixed / AW ;
1(a)(vi)[2]
error:
volume (of iodine) not directly measured
/ inconsistent drop size / using a (dropping) pipette ; improvement:
use a syringe / use a burette / use a graduated pipette / use a measuring cylinder / measure the (iodine) volume ; or
error:
judgement of the endpoint / endpoint is subjective / endpoint is qualitative ;
improvement:
compare to a colour standard / compare to a colour chart / compare to one that had already changed colour / colorimeter ;
or
error:
different degree of , shaking / mixing / stirring ;
improvement:
shake at a fixed speed / use a (mechanical) stirrer / AW ; or
error:
different time of , mixing / shaking ;
improvement:
shake / stir , for a fixed time ;
1(b)(i)[1]
a result that does not fit the , pattern / trend (of the other results)
or
was, very different / much higher, than the other two, results / trials 1 and 2
or
was outside the range of the other results / is an outlier / AW ;
1(b)(ii)[1]
removed it / did not include it / ignored it / excluded it / only used the other two results / AW ;
1(b)(iii)[4]
axes labelled with units that match the table headers ; linear scale and plotting area that occupies at least half the grid in both directions ;
six points plotted accurately ± half a small square ; suitable line drawn ;
1(b)(iv)[2]
indication on the graph of where reading taken ;
correct value from candidate’s graph at seven drops (± ½ a small square) ;
1(b)(v)[1]
(Benedict’s reagent / it) must be heated ;
Make a free account to save your answers and see your weak topics and ask the AI why a wrong answer is wrong.