VJC 2023 H2 9744_P4 Answers (Sharing)

1 Victoria Junior College Biology Department 2023 H2 Biology Prelim Paper 4 Suggested Answer Marking abbreviations: A: Accept, R: Reject, BOD: benefit of doubt, AW: alternative wording, AVP: Any valid point, NAQ: not answering question, ECF: error carried forward Question 1 (a) (i) State the volume of W that you added to the test-tube in step 16. [1] • volume recorded between 10.0 and 20.0 cm3; (ii) Record your results in an appropriate table. [4] Example: Sampling time / min Colour of bromothymol blue indicator 0 blue 1 blue 2 blue-green 3 blue-green 4 green 5 green 6 green-yellow 7 green-yellow 8 yellow 9 yellow 1. I ndependent variable on leftmost column; 2. Correct headings and no units in body of table; Column 1 heading – Accept time; Column 2 heading – Accept colour of B/ (B and W) mixture/ sample; Reject colour (observed) 3. Record using only colours from Fig. 1.1, at least 4; Reject dark green, light green 4. Trend: Colour for each minute for 10 minutes or until end-point reached for two consecutive times from left to right of colours from Fig.1.1 and time 0 is blue; (iii) This investigation used colour to indicate the concentration of carbon dioxide in the sample which is subjective. Describe two improvements to this investigation that would increase the accuracy of the results other than the use of colorimeter/ colour chart. [2] Any two from: • use smaller intervals of time e.g., 30 s to determine the end-point more accurately; • use a pH meter to determine the pH of the solution due to release of carbon dioxide which dissolves in the surrounding water; • use a carbon dioxide probe to determine the concentration of carbon dioxide released; • use set volume e.g., 0.5 cm 3 of B and/ or W instead of using drops (with a 1 cm3 micropipette or syringe);

2 (iv) A student hypothesised that glucose diffuses into the surrounding of the dialysis tube. You are provided a vial labelled R which contains the remaining contents of the boiling tube, collected after 20 minutes of the investigation. Describe and carry out an appropriate test to determine whether the student’s hypothesis can be accepted. Suggest how this would impact your results. [4] • Add 2 cm3 of sample R and 2 cm 3/ equal volume of Benedict’s solution into a boiling tube and mix well; • Put the boiling tube into a boiling water-bath for 3 minutes; • *Contents changed/ turned from a clear blue solution to a cloudy, blue mixture with suspended reddish-brown precipitate; • showing the presence of (minute quantity of) reducing sugar, thus accept/ do not reject the student’s hypothesis; • Since glucose diffuses into the surrounding of the dialysis tube, with time, less glucose is available for yeast to respire to produce carbon dioxide; • The results obtained is an underestimate of the actual carbon dioxide production from the mixture of yeast and glucose / more time is required for end-point to be reac