Elena set up an experiment to investigate how light intensity affects the net gas exchange of an aquatic pondweed, Elodea canadensis.
She prepared four boiling tubes, each containing 15 cm3 of green bromothymol blue indicator. She placed a healthy, 8 cm sprig of Elodea into tubes A, B, and C. Tube D was left with only the indicator solution and no plant. Each tube was sealed with a rubber bung.
The tubes were then incubated under the following conditions for two hours:
Suggest a hypothesis for Elena's investigation.
Give two variables that Elena should keep constant in her investigation.
State the purpose of Tube D in the investigation.
Elena recorded her results in the table below:
| Tube | Colour of indicator at start | Colour of indicator after two hours |
|---|---|---|
| A | green | blue |
| B | green | yellow |
| C | green | green |
| D | green | green |
Explain the change in colour of the indicator in Tube A.
Explain the change in colour of the indicator in Tube B.
Suggest why the indicator did not change colour in:
Tube C
Suggest why the indicator did not change colour in:
Tube D
Limewater is an indicator that can be used to show an increase in the level of carbon dioxide. Suggest why it would not be a suitable indicator for use in this investigation.
