Recording and processing observations — Apply worksheet
Original practice. Write explanations that distinguish observations, evidence and claims.
Name: __________________________ Date: ______________
Compare pH readings without confusing the scales
Text description
- Sample A has repeated pH readings 4.1, 4.0 and 3.9. Sample B has readings 6.1, 6.0 and 5.9. Sample C has readings 8.1, 8.0 and 7.9. A and B are acidic; C is alkaline under these conditions. The unitless pH scale is logarithmic.
Sample A has repeated pH readings 4.1, 4.0 and 3.9. Sample B has readings 6.1, 6.0 and 5.9. Sample C has readings 8.1, 8.0 and 7.9. A and B are acidic; C is alkaline under these conditions. The unitless pH scale is logarithmic.
| Sample | Reading 1 (pH) | Reading 2 (pH) | Reading 3 (pH) |
|---|---|---|---|
| A | 4.1 | 4 | 3.9 |
| B | 6.1 | 6 | 5.9 |
| C | 8.1 | 8 | 7.9 |
Question 1
From the synthetic pH table above, calculate each sample’s mean recorded pH and range. Classify all three at 25 °C.
Question 2
Find the valid-reading mean for 8.0, missing, 10.0 mm.
Question 3
A student says pH 4 is “two times as acidic” as pH 6, and repeated pH values prove that the meter is accurate. Repair both claims.
Teacher answer key — 3 questions
Model responses identify the essential ideas. Accept scientifically correct alternatives that address the question; wording need not match exactly.
- A mean = (4.1 + 4.0 + 3.9)/3 = 4.0, range = 4.1 − 3.9 = 0.2. B mean = 6.0, range = 0.2; C mean = 8.0, range = 0.2. A and B are acidic; C is alkaline at 25 °C. pH is unitless; these summaries describe recorded pH values, not a mixture.
- 9.0 mm from two valid readings; report the missing-data reason.
- The two-pH difference represents a 100-fold hydrogen-ion activity ratio (approximately concentration for dilute solutions). Repeats show agreement under the conditions, not accuracy. Check calibration against suitable buffers. The result does not establish chemical identity or safety.