Pond snails and pondweed are living in water in sealed test tubes - OCR Gateway - GCSE Biology - Question 2 - 2019 - Paper 3

To determine in which test tube the water would become most acidic, we need to consider the roles of both pond snails and pondweed, particularly in the presence of sunlight.

1. Test Tube 1: Contains pond water and a pond snail. Here, the snail will respire, producing carbon dioxide, but no photosynthesis occurs, because there is no pondweed.

2. Test Tube 2: Contains pond water, a pond snail, and pondweed in sunlight. In this setup, the pondweed will photosynthesize, consuming carbon dioxide during daylight, which results in the water being less acidic compared to if only the snail were present.

3. Test Tube 3: Contains pond water and pondweed without the snail, also in sunlight. Similar to Test Tube 2, the pondweed will use up carbon dioxide through photosynthesis, and this setup also results in less acidity.

4. Test Tube 4: Contains only pond water and a pond snail in the sunlight. The snail will still release carbon dioxide due to respiration, but without the pondweed present to consume it, the acidity will increase.

Given these observations, the greatest acidity will occur in test tube 4, where the snail is active, generating carbon dioxide, and there is no pondweed to utilize it. Therefore, the answer is B: 2.