Did you expect a change in color in either of the tubes without Elodea? Explain your answer.
The correct answer and explanation is:
Correct Answer:
No, a change in color is not expected in the tubes without Elodea.
Explanation:
In an experiment involving Elodea and an indicator such as bromothymol blue, color changes help reveal whether carbon dioxide (CO₂) is being added to or removed from the solution. Elodea, a photosynthetic aquatic plant, plays a key role in this process depending on whether it is exposed to light or kept in darkness.
If a test tube contains water with a CO₂-sensitive indicator but no Elodea, there should be no biological process occurring that would either absorb or release CO₂. In the absence of Elodea, there are no plant cells to carry out photosynthesis or cellular respiration. Since photosynthesis removes CO₂ from the solution and respiration releases CO₂, both of these processes rely on living cells.
Without Elodea and without any external addition of CO₂, the CO₂ level in the solution remains unchanged. As a result, the pH stays constant, and so does the color of the indicator. For example, if the initial color of bromothymol blue in the solution is green, indicating a neutral pH, it will remain green because there is no biological activity to alter the carbon dioxide concentration.
Only tubes with Elodea would potentially show a color change. In light, Elodea uses photosynthesis to absorb CO₂, raising the pH and turning the indicator blue. In darkness, Elodea undergoes cellular respiration, releasing CO₂, lowering the pH, and shifting the indicator to yellow.
Therefore, in the absence of Elodea, the solution remains chemically stable. No CO₂ is consumed or released, so the indicator shows no change in color. This confirms that Elodea is essential for causing a shift in carbon dioxide levels and, consequently, the color of the solution.