Procedure:
1. Using a graduated cylinder, measure out 100 mL of bromothymol blue solution for each of the 4 beakers.
2. Insert one end of a drinking straw into the bromothymol blue in one of the flasks. Blow until there is a change in the appearance of the bromothymol blue solution. Repeat this procedure with the other flasks.
3. Place a sprig of Elodea into 2 flasks. Stopper all 4 flasks with the parafilm. The flasks must be completely sealed.
4. Place one flask with the Elodea and 1 flask without Elodea in the dark for 24 hours. Place the other flasks (1 with Elodea and 1 without) on a sunny windowsill for the same amount of time.
5. After 24 hours, examine each flask. Note changes of appearance
What is the control group?
In the given procedure, the control group can be identified as the flask without Elodea that is placed in the dark for 24 hours.
A control group serves as a baseline for comparison in an experiment. It is a group where the independent variable (in this case, the presence of Elodea and light) is not applied or manipulated. By comparing the control group's results with the experimental group's results, one can determine the effect of the independent variable on the dependent variable (in this case, the changes in appearance of the bromothymol blue solution).