How is the upper fixed point of a Celsius thermometer determined?
Question 1 Report
How is the upper fixed point of a Celsius thermometer determined?
Answer Details
The upper fixed point (100 °C) on the Celsius scale is defined as the temperature of steam above pure water boiling at standard atmospheric pressure (1 atm or 101 325 Pa).
The thermometer bulb is placed in the steam rather than in the boiling water itself, because water can superheat slightly and because dissolved impurities raise the boiling point. Steam from pure water at a fixed pressure provides a reliable, reproducible temperature.
Using salt water would raise the boiling point above 100 °C. Boiling under 'any conditions' does not control pressure, and the boiling point changes with pressure. Heating water to 'its highest possible temperature' is vague and not a defined fixed point.
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