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Is the triple point of water exactly $0.01 \deg$C, or is this an empirical value/is there some uncertainty to it? If so, how do we know it is exact, and why?

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    $\begingroup$ The triple point of (pure) water is at 0.01$^\circ$ because the Celsius scale is defined by this value. Just like the speed of light is exactly 299,792,458 m/s because the meter is defined by this value. $\endgroup$
    – The Photon
    Feb 12, 2019 at 2:29
  • $\begingroup$ @ThePhoton isn’t the definition of Celsius based on melting and boiling point? Does that mean the triple point is definitely 0.01 degrees above that? $\endgroup$
    – user107224
    Feb 12, 2019 at 2:32
  • $\begingroup$ Not in the SI system: en.wikipedia.org/wiki/Celsius. $\endgroup$
    – The Photon
    Feb 12, 2019 at 2:34
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    $\begingroup$ @user107224, I don't mean to be picky, but you need to also specify a pressure with the triple point temperature. $\endgroup$ Feb 12, 2019 at 3:16
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    $\begingroup$ @DavidWhite, By specifying the measurement is at the triple point, you have specified the pressure. $\endgroup$
    – The Photon
    Feb 12, 2019 at 15:29

2 Answers 2

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As of today, February 11, 2019, the triple point of (pure) water is exactly 0.01 $^\circ$C because the Kelvin scale is defined by this value, and the Celsius scale is defined by its relation to the Kelvin scale.

This is similar to how the speed of light is exactly 299,792,458 m/s because the meter is defined to be the distance traveled by light in vacuum in 1/299,892,458 s.

On May 20, the definition of the Kelvin scale is to be changed so that the Boltzmann constant will be defined to be $1.380649×10^{−23}\ {\rm J⋅K}^{−1}$. After this date it is possible that more precise measurements than have previously been done will find that the triple point of water is not at exactly 0.01 $^\circ$C

isn’t the definition of Celsius based on melting and boiling point?

The problem is that the melting point of water changes slightly depending on the surrounding pressure of water vapor. It was found easier to reproduce the conditions of the triple point (and to know they were achieved by demonstrating the coexistence of three phases of water) than to accurately produce a 1 atm pressure as required for the old definition (1743-1954) which set the melting point of water at 0 $^\circ$C.

But the triple point is actually the melting point with 61.657 Pa partial pressure of water vapor. It just happens also to be the boiling point at that pressure. So in a sense the Celsius scale is (for a few more months) defined by the melting point of water, just at a different pressure than previously.

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  • $\begingroup$ We've seen several of these kinds of questions and answers lately, and they are right as far as they go. But to my mind the more interesting question is "How close to the currently defined value was this quantity when we were using historical definitions?" And I suppose the answer is mostly "Within the then achievable tolerance.", but those would be the interesting details. $\endgroup$ Feb 12, 2019 at 7:01
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From physical point of view the temperature in this situation can not be "exact" because exactly slighty higher energy levels in different parts of the water solution will be causing the transitions from solid to liquid and from liquid to gas and slighty lower energy levels in other parts will be causing transition from gas to liquid and from liquid to solid, but from mathematical point of view this is the exact number as the number represents the average energy for all the water molecules in this thermodynamic equilibrium.

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