DBQ on p. 105 – January 9th, 2013
1) Why was pH monitored during this experiment?
The pH was monitored during this experiment to show the change in oxygen
concentration. Carbon dioxide is an acid, so as the pondweed uses it up to produce
oxygen, the pH of the water increases (because it becomes less acidic).
2) Identify the independent and the dependent variable.
The independent variable is temperature and the dependent variable is the pH.
3) Deduce which variable should go on the x-axis and which variable on the y-axis if a
graph of the results is plotted.
Time should go on the x-asis and pH should go on the y-axis.
4) Discuss what conclusions can be drawn about the optimum temperature for
photosynthesis in the green alga Chlorella.
The optimum temperature for photosynthesis in the green alga Chlorella is the
temperature at which the change in pH is the greatest. Water at 22.5˚C had a change in
pH of 0.25, at 25.0˚C it had a change in pH of 0.28, at 27.5˚C it had a change in pH of
0.26, at 30.0˚C it had a change in pH of 0.19, at 32.5˚C it had a change in pH of 0.13.
Therefore, the optimum temperature around 25.0˚C because the change in pH was
greatest. However, when taking the ±0.05 uncertainty into consideration, the actual
optimum temperature could be different because change in pH for most of the
temperatures were very close to each other. Thus it can be concluded that the
optimum temperature is around 25.0˚C.
5) If this experiment is repeated using a different alga, the results may not be the
same. Suggest reasons for differences in the results.
Results could be different for different alga because the optimum temperature for
photosynthesis could be different, or because the concentration of the alga is different.
6) Evaluate the methods used in the experiment and suggest improvements.
The experiment controls the initial concentration of carbon dioxide and the
temperature, which is good because they affect the rate of photosynthesis (and thus
would affect the change in pH). However, the experiment doesn’t control some factors
that could affect the change in pH such as light intensity and the concentration of the
alga. Therefore, a certain light intensity should be set and maintained throughout the
duration of the experiment and a certain alga concentration should be set for all trials
of the experiment.