2020 O Level 6093 Ans Key - paper 3

1.0 0 0.9 0 0.8 0 0.7 0 0.6 0 0.5 0 0.4 0 0.3 0 0.2 0 0.1 0 2020 O’ Levels 6093/3 – Recommended Answers 1a. test tube 1 – final colour: brick-red [1]. Time taken for colour change to first occur: 19s [1] test tube 2 – final colour: blue [1]. 1b. Correct headings [1] Correct unit [1] Column for time taken is on the left of final colour and concentration of glucose [1] Time taken for sample D is >120 s & Time taken for sample B is the shortest [1] Correct concentration recorded based on time taken [1] ‘Urine’ sample Time taken for colour change to first occur/ s Final colour Concentration of glucose/ g/ dm3 A 34 Red (ppt) 0.62 B 26 Red (ppt) 0.95 C 42 Red ppt in blue solution 0.45 D >120 Blue (solution) 0 E 34 Red (ppt) 0.62 [Note: 0 need not be in 2 dp] 1c. [1] each. Total: [4] - Vertical axis labelled “concentration of glucose/ g / dm 3” & horizontal axis labelled ‘urine’ sample (R: sample) - Scale: Appropriate, easy to read and graph occupies at least ½ of grid. All values on the vertical axis are in 2 dp - All bars plotted in accurately - Bars are of equal width, with equal spacing in between each bar 1d. variable: volume of ‘urine’ sample [1] explanation: to ensure that concentration of glucose is the only independent variable [1] 1e. Sample D is from a healthy person and Samples A, B, C and E are taken from unhealthy people [1]. Healthy people (where sample D is collected) do not have glucose in their urine [1]. Unhealthy people (where samples A, B, C and E are collected) are unable to reabsorb all the glucose from the glomerular filtrate at the kidneys [1]. Page 1 of 3

1f. Any 2 sets of the following. Max: [4] source of error [1] [P: if impact on results not discussed] Improvement [1] It is difficult to determine the exact time when the first sign of colour change occurs, hence the reading may be higher or lower than expected. Repeat the experiment to obtain an average time taken for first sign of colour change to improve the reliability of result. With the lack of instruction to wash the syringes, contamination of urine samples may occur, hence the reading may be higher or lower than expected. OR With limited number of syringes, dilution may occur when syringes are washed but cannot be completely dry, hence the reading may be lower than expected. Wash and dry the syringe after each use before using the syringe to collect the other urine samples. Use of the 10 cm 3 syringe to collect 2 cm 3 of Benedict’s solution may result in inaccurate volumes collected, hence the reading may be higher or lower than expected. Use the 5 cm 3 syringe to collect Benedict’s solution. 1g. [1] each. Total: [4] - independent variable – temperature of water bath + dependent variable – time taken for the first sign of colour change - ≥ two constant variables, e.g. volume of glucose solution/ deionized water, duration of incubati