Grapes are fruit that contain high concentrations of soluble sugars such as sucrose, fructose and glucose. The proportions of these sugars change as the grapes mature. You will determine the concentration of reducing sugars in a sample of grape extract by using known concentrations of reducing sugar. You are provided with the materials shown in Table 1.1. Table 1.1 | labelled | contents | hazard | volume/cm³ | |---|---|---|---| | G | grape extract | none | 20 | | W | distilled water | none | 100 | | B | Benedict's solution | harmful irritant | 40 | | R | 8.0% reducing sugar solution | none | 40 | If any solution comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. You will need to carry out a serial dilution of the 8.0% reducing sugar solution, R, to reduce the concentration by half between each successive dilution. You will need to prepare four concentrations of reducing sugar in addition to 8.0% reducing sugar solution, R. After the serial dilution is completed, you will need to have 10cm³ of each concentration available to use.

1. (a)(i)Complete Fig. 1.1 to show how you will prepare your serial dilution. Fig. 1.1 shows the first two beakers you will use to make your serial dilution. You will need to draw three additional beakers. For each beaker, add labelled arrows to show: • the volume of reducing sugar solution transferred • the volume of distilled water, W, added. Under each beaker, state the concentration of reducing sugar solution.[3]
2. (a)(ii)Record your results in an appropriate table for the known concentrations of reducing sugar.[5]
3. (a)(iii)Identify one source of error in step 7.[1]
4. (a)(iv)To determine the concentration of reducing sugar in grape extract G, you will need to test a sample of the extract. State the volume of grape extract G that you will use to test for reducing sugars. volume = ........................................................[1]
5. (a)(v)Record the time taken for the first colour change in test-tube G. time taken = ........................................................[1]
6. (a)(vi)The concentration of reducing sugars in G can be estimated from a graph of your results. Draw a graph of the results you recorded in (a)(ii) on the grid in Fig. 1.2, using a line of best fit. The axes have been labelled for you. Use a sharp pencil.[2]
7. (a)(vii)Use your graph to estimate the percentage concentration of reducing sugars in G. Show on your graph how you determined your answer. percentage concentration of reducing sugars in G .. ............................................................[2]
8. (a)(viii)Suggest how you would modify this investigation to obtain a more accurate estimate for the concentration of reducing sugars in sample G.[2]
9. (b)(i)The concentration of reducing sugars in grapes changes as the grapes age (get older). Table 1.2 shows the concentration of reducing sugars for grapes of different ages. Table 1.2 | age of grapes/days | percentage concentration of reducing sugars | |---|---| | 14 | 1.1 | | 28 | 1.9 | | 42 | 2.6 | | 56 | 3.9 | | 70 | 7.5 | | 84 | 11.3 | Plot a graph of the data shown in Table 1.2 on the grid in Fig. 1.3.[2]
10. (b)(ii)Use your estimate from (a)(vii) and your graph in (b)(i) to estimate the age of the grapes that were used to make grape extract G. age of grapes ..................................................................... days[1]
11. (c)Grapes contain starch as well as reducing sugars. In a study, the concentration of amylase in grapes was measured as the grapes aged. The results of the study are shown in Fig. 1.4. [Figure 1.4] Use the data in Fig. 1.3 and Fig. 1.4 to suggest a possible explanation for the change in the concentration of reducing sugars in grapes as they age.[3]

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