Faculty Specific (Science): Thermometer Calibration and Conclusion
A Grade 12 candidate in a science faculty test investigates a claim printed on a bottle of local mineral water: 'Naturally alkaline; boils at a lower temperature than ordinary water.' She works at an altitude of about 1,700 m. She calibrates a mercury thermometer in an ice-water slurry: it reads 0.3 °C instead of 0.0 °C. She then boils distilled water at the same altitude and records 98.4 °C. She boils the mineral water three times and records 98.7 °C, 98.6 °C, and 98.8 °C. The label does not define 'ordinary water' or state the expected boiling point. The candidate concludes: 'The mineral water boils 0.3 °C higher than distilled water, so the alkaline minerals are proven to raise the boiling point.' Her teacher asks her to correct the readings and evaluate her own conclusion.
1. Apply the thermometer calibration to both average readings. Does the mineral water boil at a higher temperature than distilled water at that altitude? Show your arithmetic.
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The ice-point reading is 0.3 °C too high, so subtract 0.3 °C from each boiling reading, assuming the error is constant over this range. Distilled water: 98.4 − 0.3 = 98.1 °C. Mineral water average: (98.7 + 98.6 + 98.8) ÷ 3 = 98.7 °C; corrected = 98.7 − 0.3 = 98.4 °C. The corrected difference is 98.4 − 98.1 = 0.3 °C. So yes, within this experiment the mineral water boiled about 0.3 °C higher than distilled water at the same altitude. The correction does not change the size of the difference because the same offset applies to both readings; it changes the absolute values.
2. The candidate says the label's claim that the water is 'naturally alkaline' is proven. Evaluate this conclusion. Distinguish what the data show from what they do not show.
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The data show a small, consistent elevation of boiling point relative to distilled water at the same altitude. They do not show alkalinity. Alkalinity is a chemical property related to acid-neutralising capacity, not to boiling point. A higher boiling point is consistent with dissolved solids, but it does not identify which solids are present or whether the water is alkaline. The label's phrase 'boils at a lower temperature than ordinary water' is also not tested, because the comparison here is with distilled water at altitude, not with 'ordinary water' at the same conditions. The conclusion should be narrowed: 'Under these conditions, the mineral water boiled about 0.3 °C higher than distilled water; further chemical tests are needed to support the alkalinity claim.'
3. Identify one variable the candidate controlled and one improvement that would make the comparison more trustworthy.
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A controlled variable is altitude: both liquids were boiled at the same location, so atmospheric pressure was the same. Another is the thermometer: the same instrument was used for both, so the calibration offset cancels in the difference. An improvement is to repeat the distilled-water measurement three times as well and report averages and ranges, rather than a single distilled reading. That would show whether the 0.3 °C difference is larger than the natural variation in the mineral-water readings (98.6 to 98.8, a range of 0.2 °C). A second improvement is to use a calibrated digital thermometer with a finer scale, or to check the thermometer at a second known point, to confirm that the offset is constant.