Question

why are carbon dioxide levels changing in the tube with elodea?

a. carbon dioxide levels are dropping because carbon dioxide is dispersing into the atmosphere.
b. carbon dioxide levels are dropping because carbon dioxide is reacting with the phenol red.
c. carbon dioxide levels are dropping because carbon dioxide is a product in photosynthesis.
d. carbon dioxide levels are dropping because carbon dioxide is a reactant in photosynthesis.

Explanation:

Identify the biological process

Using the Photosynthesis Reactants knowledge point

The aquatic plant Elodea undergoes photosynthesis when exposed to light. During this process, the plant actively consumes dissolved carbon dioxide from its surrounding water, causing the overall carbon dioxide levels in the tube to drop.

Analyze the chemical equation

Using the Photosynthesis Chemical Equation knowledge point

The balanced chemical equation for photosynthesis is:

$$ 6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 $$

This equation demonstrates that carbon dioxide (\(\text{CO}_2\)) is a reactant (starting material) consumed during the reaction, rather than a product.

Evaluate the given options

• Option A is incorrect because the tube is typically sealed in these experiments to prevent gas exchange with the atmosphere.
• Option B is incorrect because phenol red is a pH indicator that changes color in response to pH changes caused by \(\text{CO}_2\) concentration, but it does not consume or chemically destroy the \(\text{CO}_2\).
• Option C is incorrect because carbon dioxide is consumed, making it a reactant, not a product.
• Option D is correct because carbon dioxide levels drop as it is consumed as a reactant during photosynthesis.

Answer:

• (A) Carbon dioxide levels are dropping because carbon dioxide is dispersing into the atmosphere.
• (B) Carbon dioxide levels are dropping because carbon dioxide is reacting with the phenol red.
• (C) Carbon dioxide levels are dropping because carbon dioxide is a product in photosynthesis.
• (D) Carbon dioxide levels are dropping because carbon dioxide is a reactant in photosynthesis. (Correct answer)