Question

why is a 3% sodium bicarbonate (nahco3) solution added to each test tube?
select all that apply.
a. its volume changes due to photosynthesis.
b. it is a source of water.
c. it is a source of carbon dioxide.
d. it is a ph indicator.
e. it is a source of oxygen.

Explanation:

Identify the experimental context

Using the Photosynthesis Reactants knowledge point

In photosynthesis experiments (such as measuring the rate of oxygen production by an aquatic plant like Elodea), plants require light, water, and carbon dioxide to perform photosynthesis:

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

Determine the role of sodium bicarbonate

Using the Photosynthesis Reactants knowledge point

Sodium bicarbonate (\(\text{NaHCO}_3\)) dissolves in water and releases bicarbonate ions (\(\text{HCO}_3^-\)), which exist in equilibrium with dissolved carbon dioxide (\(\text{CO}_2\)):

$$ \text{NaHCO}_3 ightarrow \text{Na}^+ + \text{HCO}_3^- $$

$$ \text{HCO}_3^- + \text{H}^+ ightleftharpoons \text{H}_2\text{CO}_3 ightleftharpoons \text{CO}_2 + \text{H}_2\text{O} $$

This provides an abundant, constant source of carbon dioxide (\(\text{CO}_2\)) to ensure that carbon dioxide availability does not limit the rate of photosynthesis.

Evaluate the options

Using the Photosynthesis Reactants knowledge point

• A. Its volume changes due to photosynthesis. Incorrect. The volume of the solution itself does not change significantly; rather, gas production (oxygen) is measured.
• B. It is a source of water. Incorrect. While the solution is aqueous, the specific addition of sodium bicarbonate is not for the water itself, but for the solute.
• C. It is a source of carbon dioxide. Correct. Bicarbonate acts as the carbon source.
• D. It is a pH indicator. Incorrect. Sodium bicarbonate is a weak base/buffer, not an indicator.
• E. It is a source of oxygen. Incorrect. Oxygen is a product of photosynthesis, not a reactant supplied by bicarbonate.

Answer:

• A. Its volume changes due to photosynthesis.
• B. It is a source of water.
• C. It is a source of carbon dioxide. (Correct answer)
• D. It is a pH indicator.
• E. It is a source of oxygen.