Question 20N.3.HL.TZ0.3b.ii
| Date | November 2020 | Marks available | [Maximum mark: 3] | Reference code | 20N.3.HL.TZ0.3b.ii |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Explain | Question number | b.ii | Adapted from | N/A |
Sodium alginate beads were prepared containing a suspension of Chlorella algae. These beads were placed in nine small jars containing hydrogencarbonate indicator solution. Hydrogencarbonate indicator is commonly used to detect changes in pH.
The jars were placed at different distances from a lamp, which was the only light source. The distances ranged from 40 to 130 centimetres. Jars were placed one at a time to avoid the jars from shading each other. Colour changes were observed.
[Source: © International Baccalaureate Organization 2020.]
Explain the scientific reasoning for your hypothesis.
[3]
a. high light (intensity) increases photosynthesis;
b. photosynthesis consumes CO2;
c. (more photosynthesis/less CO2) increases pH/decreases acidity;
d. less light means more respiration (than photosynthesis);
e. respiration produces CO2 AND lowers pH/increases acidity;
Allow inverse for all answers.
Do not accept reference to colour only instead of pH.
Most candidates realised that increased light intensity leads to an increase rate of photosynthesis, which consumes CO2, or vice versa. Most related the change in colour to a change in CO2 concentration, but didn't link it to a change in pH, for which the indicator is specific. Stronger candidates understood the experiment and were able to analyse its results effectively, adding that the effect of respiration increased over that of photosynthesis as light intensity decreased, thus a lower pH.
