Dr. Aris investigated the balance between photosynthesis and respiration in the aquatic plant Elodea canadensis under different light conditions.
He prepared three identical glass vials, labelled A, B, and C, each containing 8 cm3 of green bromothymol blue indicator solution.
All three vials were sealed and placed in a water bath maintained at 20∘C20^\circ\text{C}20∘C for three hours.
Bromothymol blue indicator is green at neutral pH under normal atmospheric carbon dioxide levels. It turns yellow as carbon dioxide levels rise (becoming acidic), and turns blue as carbon dioxide levels fall (becoming alkaline).
The results at the end of the three-hour period are summarised below:
| Vial | Indicator colour at start | Indicator colour at end |
|---|---|---|
| A | green | yellow |
| B | green | green |
| C | green | blue |
Explain the colour change observed in vial C.
Explain the purpose of vial B.
Dr. Aris set up a fourth vial, vial D, containing a sprig of Elodea in the green indicator solution, but placed it under a neutral density filter that severely reduced the light intensity. After three hours, the indicator solution in this vial remained green. Explain why.
Dr. Aris carried out a separate investigation to study the quantitative relationship between light intensity and the rate of photosynthesis in Elodea sprigs.
(i) Suggest how he could vary his independent variable.
(ii) Suggest how he could measure his dependent variable.
(iii) Suggest how he could ensure that his results were reliable.
