Why triple point of water is higher than freezing point

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Discussion Overview

The discussion centers around the relationship between the triple point of water and its freezing point, specifically why the triple point is defined at 0.01°C while the freezing point is at 0°C. Participants explore the implications of these definitions and the conditions under which ice and water coexist.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants express confusion over why the triple point of water is higher than the freezing point, questioning the apparent contradiction.
  • Others argue that the triple point occurs at conditions that are not at atmospheric pressure, which may clarify the confusion regarding the coexistence of ice and water.
  • One participant suggests that it might be naive to expect the triple point to fall between the freezing and boiling points of water, indicating that typical experiences may not apply in this context.
  • Another participant reiterates the definition of freezing point and questions how ice can exist at the triple point temperature.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of the triple point versus freezing point. Multiple competing views remain regarding the conditions under which these states exist and the definitions involved.

Contextual Notes

There are limitations in the discussion regarding assumptions about pressure conditions and the definitions of phase transitions, which are not fully resolved.

animalcroc
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Why is the triple point of water .01C while the freezing point 0C? It seems contradictory.
 
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The opposite would be more surprising to me. Why does it feel contradictory to you?
 
Water, bismuth, and a very small number of other substances, have the rare distinction of possessing a melting curve with a negative slope.
 
cesiumfrog said:
The opposite would be more surprising to me. Why does it feel contradictory to you?

Liquid water at atmospheric pressure cannot become ice until it cools to 0 C, by the definition of freezing point. So, how can ice exist at the Triple Point (above 0 C) since it has not yet reached 0 C ?

It seems that the Triple Point should be 0 C, not 0.01 C.
 
animalcroc said:
Liquid water at atmospheric pressure cannot become ice until it cools to 0 C, by the definition of freezing point. So, how can ice exist at the Triple Point (above 0 C) since it has not yet reached 0 C ?
The triple point of water is not at atmospheric pressure.

Claude.
 
animalcroc said:
Liquid water at atmospheric pressure cannot become ice until it cools to 0 C, by the definition of freezing point. So, how can ice exist at the Triple Point (above 0 C) since it has not yet reached 0 C ?

Since water normally boils at 100C, shouldn't you naively expect (based on your above argument) that the triple point (where steam and ice coexist) must be somewhere between 0C and 100C, rather than at either extreme. (Of course, normal experience isn't very helpful in extrapolating to abnormal domains..)
 
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cesiumfrog said:
Since water normally boils at 100C, shouldn't you naively expect (based on your above argument) that the triple point (where steam and ice coexist) must be somewhere between 0C and 100C, rather than at either extreme.
You'd have to be really naive! You'd be wrong if you made this kind of guess with most any other liquid.
 
Claude Bile said:
The triple point of water is not at atmospheric pressure.

Claude.

The answer I was seeking! Where the hell did I come to believe that the TP was at 1 atm? Oh well.
 
Last edited:

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