Sodium hydrogencarbonate indicator solution contains sodium hydrogencarbonate. Some students found out on the internet that this indicator solution changes colour with pH. The students also discovered that this indicator is used to determine the concentration of carbon dioxide in a solution. The students decided to use this indicator to measure the rate of photosynthesis in algal balls. Algal balls are unicellular algae that are immobilised in alginate. The students assumed that the rate of carbon dioxide uptake was proportional to the rate of photosynthesis. Fig. 1.1 shows the apparatus that the students used. [Figure 1.1] A laboratory technician prepared a series of test-tubes containing hydrogencarbonate indicator solution from pH 7.6 to pH 9.2. The colour of the indicator solution in each test-tube is described in Table 1.1. [Table 1.1]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

1. 1(a) (named) buffer ;
2. 1(b) ref. to first mixing the algae with (sodium) alginate;
3. ref. to then adding (alginate and algae) to calcium chloride ;
4. ref. to method of dropping mixture (to form beads) ; e.g. using syringe or pipette
5. 1(c)(i) independent variable: light intensity / distance of container from lamp;
6. dependent variable: colour of indicator / pH;
7. 1(c)(ii) five from:
8. 1 method to vary light intensity ; e.g. distances between tube and lamp or different strengths of neutral density filters or wattages of bulbs / variable resistor/dimmer switch / number of lamps
9. 2 five values of stated distances in the range 10 to 200 cm ; if any other method used, must refer to 5 stated values e.g. wattages / etc.
10. 3 cover tubes with metal foil / light proof cover ; A work in a dark room with only one light source
11. critical step
12. 4 expose to light and record, colour change / pH, after fixed time; A any stated time A time to a specific colour / pH
13. 5 ref. to a suitable control; e.g. hydrogencarbonate with, glass beads / balls with no algae / balls with dead algae
14. 6 same, (stated) volume / concentration, of hydrogencarbonate solution ;
15. 7 same, number / mass / volume, of algal balls;
16. 8 ref. to a minimum of two replicates and calculate a mean or identify / eliminate / remove anomalies ;
17. 9 medium risk or hazard and suitable safety precaution; e.g. sodium hydrogencarbonate indicator solution is harmful/irritant and wear gloves/goggles / mask A allergy to, alginate / algae, and wear gloves/mask/goggles
18. 1(c)(iii) place a heat filter between the lamp and container; A description of heat filter e.g. container of water / double glazed glass sheet A cold light source e.g. LED lights
19. 1(d) 1.6 × 10⁻³ / 0.0016 / 0.002 ;
20. 1(e)(i) one from: judging colour alone is subjective; different people see colours differently; idea that pH probe is more sensitive; A gives a numerical / quantitative result
21. 1(e)(ii) curve should be to the right of original curve and must be drawn from 0 to 10 minutes;
22. 1(e)(iii) indicator solution provides, hydrogencarbonate ions / carbon dioxide;
23. 1(f)(i) there is no significant difference between the (mean) pH for treatment 1 and treatment 2 / no weed killer and weed killer A ; or the difference between the (mean) pH for treatment 1 and treatment 2 / no weed killer and weed killer A is not significant
24. 1(f)(ii) one from: continuous data; data is normally distributed; standard deviations are approximately the same ; size of the two samples is less than 30; I 'compare the means'
25. 1(f)(iii) n = 10;
26. 1(f)(iv) yes for weed killers A and C / no for weed killer B ;
27. plus two from:
28. (significant for A or C) because p value / probability, is less than 0.05
29. or
30. p value shows there is less than 5% chance that results are due to chance;
31. effect of weed killer A greater than weed killer C (because less than 0.001);
32. (not significant for B) because p value greater than (0).05
33. or
34. p value shows there is less than 5% chance that results are due to weed killer B;