Question 127 marks
Question
Sugars are a source of energy in the diet. You are going to estimate the concentration of glucose in an energy drink (D). Read all the instructions but DO NOT CARRY THEM OUT until you have drawn a table for your results in the space provided in 1(a)(iii). You should use the gloves and eye protection provided while you are carrying out the practical work. Step 1 Label three test-tubes A, B and C. Step 2 Use the volumes of 4% glucose solution and distilled water shown in Table 1.1 to make three different concentrations of glucose solution in test-tubes A, B and C. (a) (i) Complete Table 1.1 by calculating and writing in the percentage concentration of glucose solution in test-tube B. Table 1.1 volume of 4% glucose solution volume of distilled water percentage concentration test-tube / cm3 / cm3 of glucose solution A 10.0 0.0 4 B 5.0 5.0 C 2.5 7.5 1 [1] Step 3 Add 5 cm3 of Benedict’s solution to each of the glucose solutions in the test-tubes A, B and C. Shake each test-tube gently for 3 seconds to mix the contents. Step 4 Add 5 cm3 of Benedict’s solution to test-tube D which contains 10 cm3 of energy drink D. Shake the test-tube gently for 3 seconds to mix the contents. Step 5 Raise your hand when you are ready for hot water to be put into the beaker labelled water-bath. Step 6 Measure the temperature of the hot water in the water-bath (initial temperature). (ii) Record the initial temperature in Table 1.2. Table 1.2 initial temperature / °C final temperature / °C [1] Step 7 Put test-tube A into the water-bath and immediately start the stop-clock. Step 8 Observe test-tube A carefully and note the time at which the contents of the test-tube first change colour. This reaction may occur very quickly. If there is no colour change after 5 minutes record the time as >300. Record the time taken in seconds for the colour change in the table you have prepared in 1(a)(iii). Step 9 Remove test-tube A from the water-bath and put it back into the test-tube rack. Reset the stop-clock. Step 10 Repeat steps 7, 8 and 9 with test-tube B. Step 11 Repeat steps 7, 8 and 9 with test-tube C. Step 12 Repeat steps 7, 8 and 9 with test-tube D. Step 13 Measure the final temperature of the water in the water-bath. Record this temperature in Table 1.2. (iii) Prepare a table to record your results in the space provided. [4] (iv) Estimate the concentration of glucose in energy drink D using the information in Table 1.1 and your results. …… % [1] (v) Explain how you estimated the concentration of glucose in energy drink D in 1(a)(iv). …… [1] (b) Table 1.2 may indicate that there is a source of error in this investigation. (i) Identify the possible source of error and suggest one way the method could be improved to reduce this error. source of error …… improvement …… [2] (ii) Explain how the error identified in 1(b)(i) could affect your results and the estimation of the concentration of glucose in energy drink D. …… [1] (c) State two variables that were kept constant in this investigation. 1 …… 2 …… [2] (d) Barley grains are used in the food industry as a source of sugars. Barley grains contain starch. Starch can be broken down into sugars by enzymes. Students made a starch suspension from barley grains to investigate the breakdown of starch by enzymes. The starch suspension contained 100 g of starch. They mixed the starch suspension with enzymes and recorded the mass of starch remaining in the mixture at different times. The results are shown in Table 1.3. Table 1.3 time / minutes mass of starch remaining / g 0 100 5 58 10 40 20 32 40 28 (i) Plot a line graph on the grid of the data in Table 1.3. [4] (ii) Describe the trend shown in your graph. …… [1] (iii) Estimate the mass of starch remaining at 15 minutes. Show on your graph how you obtained your answer. …… g [2] (iv) Calculate the rate of starch breakdown in the first five minutes using the information in Table 1.3 or your graph. …… g per minute [1] (e) Plan an investigation to determine the effect of temperature on the breakdown of starch by enzymes. …… [6] [Total: 27]
Mark scheme
1(a)(i) 2; 1 1a(ii) initial temperature recorded ; 1 1(a)(iii) table drawn with two columns and a header row ; 4 column headings with unit: test-tube and time (taken for colour change) / s ; four times recorded in seconds ; correct trend: times show expected pattern (A faster than C) ; 1(a)(iv) value that matches candidate’s results (expect 4% / same time as test-tube A) ; 1 1(a)(v) idea of comparing the times for D with A, B and C / AW ; 1 1(b)(i) error: temperature has decreased ; 2 improvement: maintain temperature by, using a thermostatically controlled water-bath / Bunsen burner / adding hot water throughout the investigation (and monitoring) temperature / insulate beaker / AW ; 1(b)(ii) times are slower as temperature decreases / unable to identify the concentration of glucose in D with certainty / may 1 underestimate the concentration ; 1(c) any two from: 2 volume of glucose solution ; volume of Benedict’s solution ; concentration of Benedict’s solution ; time for mixing of test-tube contents ; 1(d)(i) axes correctly labelled with units: time / minutes and mass of starch remaining / g ; 4 scale evenly spaced and occupying half of the grid in both directions ; five correct plots ± half a small square ; line joining all five plot points ; 1(d)(ii) mass of starch remaining decreases and levels off / AW ; 1 1(d)(iii) mass correct from candidate’s graph ; 2 graph marked at interpolation to show where reading taken ; 1(d)(iv) (42 ÷ 5 =) 8.4 (g per minute) ; 1 1(e) method: 6 1&2 use of Benedict’s reagent to measure reducing sugar produced, at set time / at regular intervals ; ref. to use of hot water-bath / use of (thermostatically controlled) water-bath to maintain temperatures ; OR use of iodine solution to determine starch remaining ; ref. to drops on spotting tile at regular intervals ; independent variable: 3 ref. to using at least two different temperatures ; variables kept constant – max two from mp 4-8: 4 same type of enzyme ; 5 concentration of starch suspension ; 6 concentration of enzyme ; 7 same pH ; 8 incubating enzymes and starch suspension for same time ; dependent variable: 9 concentration of reducing sugar produced / time taken for drops and iodine solution to remain brown ; 10 ref. to at least two further replications / repeat investigation two more times ; 11 ref. to relevant safety precaution ;