Why triple point of water is higher than freezing point

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SUMMARY

The triple point of water is defined at 0.01°C and is not at atmospheric pressure, which is a common misconception. At atmospheric pressure, liquid water freezes at 0°C, but the triple point allows for the coexistence of ice, liquid water, and vapor at a specific pressure and temperature. This phenomenon is unique to water and a few other substances with a melting curve that has a negative slope. Understanding the triple point is crucial for grasping the thermodynamic properties of water.

PREREQUISITES
  • Understanding of thermodynamic principles
  • Familiarity with phase diagrams
  • Knowledge of the properties of water
  • Basic concepts of pressure and temperature relationships
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  • Research the phase diagram of water and its implications
  • Study the concept of negative slope melting curves in materials
  • Explore the effects of pressure on boiling and freezing points
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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.
 
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