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Suppose an aquatic plant is placed in a test tube containing distilled water and the tube is stoppered. The tube is left outdoors for 24 hours and the pH value of the water is then measured at regular intervals. Which of the following is the most probable result?

1. pH of water will increase
2. pH of water will decrease
3. pH will remain constant
4. pH will first increase and then decrease

Correct answer: pH of water will increase

Solution

In daylight, the aquatic plant photosynthesizes and uses up dissolved carbon dioxide. Since CO2 forms carbonic acid in water, removing it makes the water less acidic, so the pH rises. In a stoppered tube, this effect is most likely to dominate over 24 hours outdoors.

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