Question 127 marks
Question
Sugars are a source of energy in the diet. A student wanted to estimate the concentration of glucose in an energy drink (D). Step 1 Three test-tubes were labelled A, B and C. Step 2 Three different concentrations of glucose solution were prepared as shown in Table 1.1. (a) (i) Complete Table 1.1 by calculating and writing in the volume of 4% glucose solution used 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 2 C 2.5 7.5 1 [1] Step 3 5 cm3 of Benedict’s solution was added to each of the glucose solutions in test-tubes A, B and C. Each test-tube was gently shaken for 3 seconds to mix the contents. Step 4 5 cm3 of Benedict’s solution was added to test-tube D which contained 10 cm3 of energy drink D. The test-tube was gently shaken for 3 seconds to mix the contents. Step 5 Hot water was put into a beaker that was used as a water-bath. Step 6 The temperature of the hot water in the water-bath (initial temperature) was measured. Step 7 Test-tube A was put into the water-bath and a stop-clock was started. Step 8 Test-tube A was observed and the stop-clock was stopped when the contents of the test-tube first changed colour. Step 9 Test-tube A was removed from the water-bath. Step 10 Steps 7, 8 and 9 were repeated with test-tubes B, C and D. Step 11 The final temperature of the water in the water-bath was measured at the end of the investigation. (ii) Fig. 1.1 shows the appearance of the thermometer in step 6 and in step 11. °C °C initial temperature final temperature step 6 step 11 Fig. 1.1 Record the temperatures shown in Fig. 1.1 in Table 1.2. Table 1.2 step 6 initial temperature / °C step 11 final temperature / °C [1] (iii) Fig. 1.2 shows the stop-clock readings for each test-tube in step 8. minutes:seconds minutes:seconds minutes:seconds minutes:seconds 00:36 01:27 02:45 01:30 test-tube A test-tube B test-tube C test-tube D Fig. 1.2 Convert the times shown in Fig. 1.2 to seconds. Prepare a table and record these times in your table. [3] (iv) Estimate the concentration of glucose in energy drink D using the information in Table 1.1 and the results in your table in 1(a)(iii). ……% [1] (v) Explain how you estimated the concentration of glucose in energy drink D in 1(a)(iv). …… [1] (b) Fig. 1.1 and 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 the results and the estimation of the concentration of glucose in energy drink D. …… [1] (c) (i) State two variables that were kept constant in the investigation in 1(a). 1 …… 2 …… [2] (ii) State one reason why the student should wear eye protection when carrying out this investigation. …… [1] (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) 5.0 ; 1 1(a)(ii) (initial temperature) 80 and (final temperature) 60 ; 1 1(a)(iii) table drawn with two columns and a header row ; 3 column headings with unit: test-tube and time (taken for colour change) / s ; four correct times recorded in seconds (A:36, B:87, C:165, D:90) ; 1(a)(iv) 2(%); 1 1(a)(v) idea of comparing the time for D with A, B and C / B was the closest time to D / 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)(i) 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(c)(ii) heating liquid (that might spill out of test-tube) / use of Benedict’s solution / AW ; 1 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 ;