The Grade 9 Science Club's Water Testing Report
A Grade 9 science club tested three water samples from around their school. Sample A came from a tap in the science lab. Sample B came from a puddle near the parking lot. Sample C came from a drainage ditch behind the cafeteria. They measured pH, clarity (how clear the water looked), and whether any small organisms were visible under a hand lens. Results: A had pH 7.2, was clear, and showed no organisms. B had pH 6.1, was slightly cloudy, and showed no organisms. C had pH 8.9, was very cloudy, and showed many small moving organisms. The club concluded that Sample C was the most polluted because it had the most organisms and was the cloudiest. Their teacher asked them to think more carefully before writing their conclusion for the school newsletter.
1. Evaluate the club's conclusion that Sample C is the most polluted. What is a reasonable alternative explanation for their observations, and what additional evidence would they need?
Read the worked answer
The club's conclusion overreaches the evidence. Cloudiness and visible organisms are not reliable single indicators of pollution.
- Sample C had many organisms and was cloudy. But a drainage ditch is a natural habitat. Algae, protozoa, and tiny invertebrates live in slow-moving water and make it look cloudy and full of life. High pH (8.9) is also consistent with algal photosynthesis removing CO₂ during the day - not necessarily with toxic pollution.
- Sample B (pH 6.1) is slightly acidic. Parking-lot runoff can carry acidic or oily residues, which is a plausible pollution signal even though it looked 'cleaner'.
Additional evidence needed:
- Chemical tests: dissolved oxygen, nitrates, phosphates, heavy metals, hydrocarbons.
- Bacterial testing (e.g., coliform counts) to check for sewage or animal waste.
- A control comparison: test undisturbed local pond water for baseline organism levels.
- Repeat sampling at different times of day (pH and oxygen change with sunlight).
Better conclusion: 'Sample C supports the most biological activity and highest pH; further chemical and bacterial testing is needed before calling it the most polluted.'
2. The club wants to present their findings as a scientific claim. Rewrite their conclusion so that it states only what their data supports.
Read the worked answer
A defensible claim separates observation from inference:
'Sample C had the highest pH (8.9), was the cloudiest, and contained the most visible organisms of the three samples. These observations are consistent with high biological activity, but they do not by themselves show that Sample C is polluted. Sample B had the lowest pH (6.1). Further chemical and bacterial testing is needed to determine water quality.'
Key moves:
- Report the measured values (pH, clarity, organisms).
- Say 'consistent with' rather than 'proves'.
- Explicitly flag the limitation of the methods used.
- Avoid ranking 'most polluted' without pollutant measurements.