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The fusion curve of triple phase diagram of water and fusion curve of triple phase diagram of general substances is different. Why is it so?

Following are the fusion curves of water and general substance (actually my teacher gave me this diagram)

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    $\begingroup$ Different? To me, they look pretty much the same. $\endgroup$ Apr 15, 2021 at 17:20
  • $\begingroup$ No, they don't. Please look at the images that I have sent (the diagrams were given to me by my teacher). $\endgroup$
    – user281837
    Apr 16, 2021 at 2:43
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    $\begingroup$ Your diagrams are exaggerated a great deal. In fact, the curve in both cases is very nearly vertical. The different slope, though, is indeed a fact that requires an explanation. It has to do with the volume change. $\endgroup$ Apr 16, 2021 at 6:55
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    $\begingroup$ There is no such thing as "general substance". There might be typical behaviour and unusual behaviour but "general substance" is bad terminology and a ridiculous overgeneralisation. $\endgroup$
    – matt_black
    Apr 16, 2021 at 11:08

1 Answer 1

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The phase diagram of water is peculiar for the negative slope of the line separating the solid, and the liquid for the representation of pressure in function of temperature:

enter image description here

(slightly modified, credit).

As indicated in the comment by @Ivan Neretin, this is due to volume change. You may recall that $\pu{3.98 ^\circ{}C}$ is special for water because at any temperature above as well as below the density of water is less that at this extreme. Thus the volume expansion of ice cracks rocks; and while ice skating (i.e., putting you body weight on a pointy small surface area, thus increasing the pressure), you may generate a thin film of molten water. This applies for the transition between liquid water and hexagonal ice formed under about ambient pressure. (There are multiple other polymorphs of solid water (an entry) in addition to the one frequently seen.)

Much more frequently experienced is a positive slope for the solid-liquid phase boundary, e.g., for $\ce{CO2}$:

enter image description here

(from here).

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