Is it a fluke that the triple point of water is almost exactly 0°C ?

[peanutaxis]

Seems a crazy coincidence that the tiple point of water is also virtually the same temperature at which water freezes/melts. Or is it that the triple point of water was always going to be at the temperature that water freezes/melts (so those two neccessarily co-exist) and then above water there is always going to be some vapour?
Why is the triple point slightly above 0°C? (0.0100°C)

Thanks
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[vanhees71]

It's not a coincidence but it has been defined, arbitrarily, in this way by Celsius. For good reasons today the SI defines the units by fixing the values of fundamental constants, which provides much better accuracy and sustainability of the units, but of course in "redefining" the SI units one has been invested decades of painstaking effort to make them as accurately as possible to be consistent with the previous definition.

 

[DrClaude]

Look at the phase diagram of water (taken from here):

The difference between the temperature of freezing water and the triple point is only a function of the slope of the line separating solid from liquid and where it intersects with the line separating gas from liquid/solid. The fact that it is a small difference is an accident. Had we lived on Venus, where the atmospheric pressure is 75 times bigger, we would have found a greater difference between the two (since the freezing point is a function of pressure).

 

[DrClaude]

It's not a coincidence but it has been defined, arbitrarily, in this way by Celsius. For good reasons today the SI defines the units by fixing the values of fundamental constants, which provides much better accuracy and sustainability of the units, but of course in "redefining" the SI units one has been invested decades of painstaking effort to make them as accurately as possible to be consistent with the previous definition.

The difference in T between the freezing point of water at atmospheric pressure and the triple point is independent of the system of units, apart from the trivial point that had Celsius decided that the boiling point of water was at 10000 °C, then the difference would have been 1 °C instead of 0.01 °C.

 

[jbriggs444]

Why is the triple point slightly above 0°C? (0.0100°C)

This seems straightforward enough to reason out from simple principles. Let us see whether that is so.

Zero degrees is, at least traditionally, the freezing/melting point of pure water under ordinary atmospheric pressure -- dry air at 1 atm.

Under these conditions, the water/ice mixture will be out-gassing water vapor. We are not at the triple point yet.

So we allow the mixture to outgas until the ambient partial pressure of water vapor is high enough so that equilibrium is attained. And we remove the air. Now we have water, ice and water vapor. The pressure of the vapor on the water/ice mixture is far less than 1 atm.

Water expands when it freezes and contracts when it melts. One effect of that original 1 atm pressure was to force the equilibrium in the direction of melting more ice. Like the idea of ice skates melting a track in the ice. Now that we have removed the pressure, the equilibrium shifts in the direction of allowing more ice to freeze. The freezing temperature has been elevated.

If you get confused trying to figure how the freezing temperature shifts, think about the trick of using salt and ice to make ice cream in an old fashioned ice cream freezer. If you shift the equilibrium of an ice/water mixture in the direction of melting ice, you depress the freezing temperature -- so that you can make the ice cream. If you shift the equilibrium in the direction of freezing ice you do the opposite. You raise the freezing temperature.

QED.

Edit: Scooped by @DrClaude but I like to think in terms of ice skates and ice cream rather than phase diagrams.