The Science Club Water Test
A science club is testing three water samples from around campus. They measure the pH of each sample with a paper indicator and record: Sample A = 6, Sample B = 7, Sample C = 9. A member says, 'Sample C is the most acidic because 9 is the biggest number, and Sample A is neutral because 6 is close to 7.' The club also learns that the pH paper has an accuracy of ±1 pH unit and that the samples were left open to the air for two hours before testing. The club wants to decide which sample, if any, is safe to describe as 'acidic' in their report.
1. Correct the member's statements about acidity and neutrality, and explain what pH 6, 7, and 9 actually mean.
Read the worked answer
The member has the scale backwards. The pH scale runs from 0 to 14, and lower pH = more acidic, higher pH = more basic/alkaline, with pH 7 neutral (at standard conditions).
- Sample A (pH 6): slightly acidic - below 7, so acidic, not neutral.
- Sample B (pH 7): neutral.
- Sample C (pH 9): basic/alkaline, not acidic.
So 'Sample C is the most acidic because 9 is the biggest number' is exactly wrong: C is the least acidic (most basic) of the three. And 'Sample A is neutral because 6 is close to 7' is also wrong - 6 is on the acidic side; closeness to 7 does not make it neutral.
2. Given the ±1 accuracy of the paper and the two-hour exposure to air, which sample(s) can the club confidently describe as acidic? Justify your answer.
Read the worked answer
Apply the uncertainty to each reading:
- A: 6 ± 1 → range 5 to 7. This range straddles neutral, so A might be neutral. Cannot confidently call it acidic.
- B: 7 ± 1 → range 6 to 8. Straddles neutral; cannot confidently call it acidic or basic.
- C: 9 ± 1 → range 8 to 10. Entirely on the basic side; cannot be acidic.
Now the exposure issue: samples left open to air for two hours can absorb CO₂, which forms carbonic acid and lowers pH (drifts toward acidic). This means any reading may understate the true pH - it biases readings downward.
Conclusion: on the evidence given, the club cannot confidently describe any sample as acidic. A's range includes 7, and the air-exposure drift only adds uncertainty in the acidic direction, so even A's reading is not reliable evidence of acidity. The honest report should say 'no sample can be confidently classified as acidic given measurement uncertainty and air exposure,' and recommend retesting with a calibrated meter and sealed samples.