A student wanted to investigate the effect of caffeine on heart rate. She decided to study this effect using a small aquatic invertebrate called Daphnia magna.
Her results are shown in the table below:
| Daphnia | Heartbeats in 20 seconds in Water | Heartbeats in 20 seconds in Caffeine solution |
|---|---|---|
| 1 | 61 | 78 |
| 2 | 63 | 81 |
| 3 | 60 | 79 |
| 4 | 64 | 82 |
| 5 | 62 | 80 |
| 6 | 59 | 77 |
| 7 | 61 | 79 |
| 8 | 63 | 83 |
| 9 | 65 | 80 |
| 10 | 62 | 81 |
(i) Name the independent variable in this investigation.
(ii) Identify one variable that the student must control to ensure a valid investigation.
Calculate the mean heart rate (in beats per minute) of the Daphnia in water. Show your working.
Describe the effect of caffeine on the heart rate of Daphnia.
Explain why repeating the measurement on 10 Daphnia is better than using only one.
