The table below shows the mean oxygen concentration in sealed bottles containing Elodea, a water plant. Each bottle was kept at 20 degrees C for 30 minutes....
Question 1 Report
The table below shows the mean oxygen concentration in sealed bottles containing Elodea, a water plant. Each bottle was kept at 20 degrees C for 30 minutes. A lamp was placed at different distances from the bottles. Oxygen concentration was measured before and after the test. Fig. 1 is a line diagram of one bottle arrangement. The student used the same mass of plant in every bottle and repeated each distance three times. Carbon dioxide dissolved in the water was available for photosynthesis.
| Lamp distance / cm | Change in oxygen / mg dm-3 |
|---|---|
| 10 | +3.8 |
| 20 | +2.4 |
| 40 | +0.7 |
| 80 | -0.5 |
(a) What process adds oxygen to the water in this investigation? [1]
(b) Describe the relationship between lamp distance and change in oxygen concentration. [2]
(c) Suggest why oxygen concentration decreased at 80 cm. [2]
Answer Details
(a) Photosynthesis adds oxygen to the water [1].
(b) As lamp distance increases, the increase in oxygen concentration becomes smaller [1]. At 80 cm, oxygen concentration decreases by 0.5 mg dm-3 instead of increasing [1].
(c) At 80 cm, light intensity is too low for photosynthesis to replace the oxygen used [1]. Plant cells continue to respire and use oxygen [1], producing the net decrease.
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