Fun fact: The normal boiling point of water isn't exactly 100 °C (at least not since 2019 when the definition of the Kelvin scale was changed to use the Boltzmann constant and decoupled from the triple point of water)

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Finally you can win those pointless bets at the pub with insistent victims who frantically grab their mobile phones to Google stuff to try to confirm what they think they already know, while bellowing "I'll prove it to you!"

The boiling point of water is no longer exactly 100 °C . The main reason is that in 2019 the definition of the kelvin scale (and resulting impact on the Celsius scale) was changed to be based on the Boltzmann constant, rather than depending on the solid-liquid-gas triple point of water.

And it's not just for any old water; after 2007 it was defined to be based on Vienna Standard Mean Ocean Water (VSMOW), a precisely defined water standard. I guess Austrians just love very precise things, so their water is more precise than water elsewhere.

And it also depends on exactly which calibration standard you use for your measurement equipment! If you trust Wikipedia:

Wikipedia: 'Precise measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C) when adhering strictly to the two-point definition of thermodynamic temperature.'

Wikipedia: 'When calibrated to ITS-90, where one must interpolate between the defining points of gallium and indium, the boiling point of VSMOW water is about 10 mK less, about 99.974 °C.'

CoolProp seems to like the later, it gives 99.9743 °C at 1 atmosphere (101325 Pa).

And BTW the International Temperature "Scale" of 1990 is not a scale, it's an equipment calibration standard. But you probably won't be winning many bets at the pub with that one.

If your "I'll prove it to you" betting victim has had too much to drink, you can trick them by using the standard boiling point, which is defined at exactly 100,000 Pa (1 bar), instead of normal boiling point at atmospheric pressure 101, 325 Pa.

At 100,000 Pa, CoolProp gives a standard boiling point for water of 99.6059 °C.

Now everyone knows for sure that Standard Temperature and Pressure (STP) means at 1 atmosphere (exactly 101, 325 Pa), right? Well, not quite:

Wikipedia: 'In chemistry and in various industries, the reference pressure referred to in standard temperature and pressure (STP) was commonly 1 atm (101.325 kPa) but standards have since diverged; In 1982, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the physical properties of substances, standard pressure should be precisely 100 kPa (1 bar).'

And what about "1 atmosphere" anyway? Is the pressure of the atmosphere really conveniently exactly 101, 325 Pa? Well atmospheric pressure is defined to be exactly 101, 325 Pa. The actual 'pressure of the atmosphere' at sea-level varies above and below that according to location and countless weather influences.

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Precise measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C) when adhering strictly to the two-point definition of thermodynamic temperature.

When calibrated to ITS-90, where one must interpolate between the defining points of gallium and indium, the boiling point of VSMOW water is about 10 mK less, about 99.974 °C.

In chemistry and in various industries, the reference pressure referred to in standard temperature and pressure (STP) was commonly 1 atm (101.325 kPa) but standards have since diverged;

In 1982, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the physical properties of substances, standard pressure should be precisely 100 kPa (1 bar).

As part of the 2019 redefinition of SI base units, the Boltzmann constant is one of the seven "defining constants" that have been given exact definitions. They are used in various combinations to define the seven SI base units.

The Boltzmann constant is defined to be exactly 1.380649×10^(−23) J⋅K^(−1)

Visit also

Cool Physics links: Some fun stuff around thermodynamics