A student wanted to investigate the effect of caffeine on the pulse rate of the aquatic worm Lumbriculus variegatus (commonly known as the blackworm).
Her results are shown in the table below:
| Blackworm | Contractions in 20 seconds in Pond Water | Contractions in 20 seconds in Caffeine solution |
|---|---|---|
| 1 | 30 | 42 |
| 2 | 32 | 45 |
| 3 | 29 | 40 |
| 4 | 31 | 43 |
| 5 | 32 | 41 |
| 6 | 28 | 39 |
| 7 | 30 | 44 |
| 8 | 31 | 41 |
| 9 | 29 | 42 |
| 10 | 28 | 43 |
Name the independent variable in this investigation.
Identify one variable that the student must control to ensure a valid investigation.
Calculate the mean contraction rate (in contractions per minute) of the blackworms in pond water. Show your working.
Describe the effect of caffeine on the pulse rate of Lumbriculus variegatus.
Explain why repeating the measurement on 10 blackworms is better than using only one.
