An aquatic plant was placed in a test tube containing water. The tube was then stopered and left outdoors for 24 hours. The pH value of water was measured at regular intervals. What must have been the result?

Options

A. pH value was the lowest just before sunrise
B. pH value was the highest iust before sunrise
C. pH value was the lowest at noon
D. pH value was the lowest just before sunset

Answer

To solve the question regarding the pH value of water in a test tube containing an aquatic plant left outdoors for 24 hours, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Photosynthesis**:
- Aquatic plants perform photosynthesis, which is the process of converting carbon dioxide and water into glucose and oxygen using sunlight. This process affects the pH of the water.

2. **Initial Conditions**:
- When the aquatic plant is placed in the test tube, it is likely to have some dissolved carbon dioxide (CO2) in the water, which can lower the pH, making it more acidic.

3. **Daylight Influence**:
- During the day, particularly after sunrise, the plant will utilize sunlight to perform photosynthesis. This process consumes carbon dioxide, which will lead to a decrease in the concentration of CO2 in the water.

4. **pH Changes Over Time**:
- Just before sunrise, the plant has not yet begun photosynthesis, and the concentration of CO2 is relatively high, resulting in a lower pH (more acidic).
- After sunrise, as photosynthesis begins, CO2 is consumed, leading to an increase in pH (making it less acidic or more alkaline).

5. **Conclusion**:
- Therefore, the pH of the water will be lowest just before sunrise due to the accumulation of CO2 overnight. As photosynthesis occurs during the day, the pH will rise.

### Final Answer:
The pH value of the water was lowest just before sunrise.