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Is it necessary for all compounds to have a triple point? Also, how is the triple point determined?

Suppose a substance does not have a triple point, so how do researchers agree on the impossibility of existence of a triple point in such a case? Also, can there be multiple triple points for a compound?

Also, since the vapour equilibrium curve ends at point E, what is the phase of water beyond that point?

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    $\begingroup$ All compounds except helium have a triple point, unless they decompose before reaching it. As for the second question, the phase beyond E is called supercritical fluid. $\endgroup$
    – Ivan Neretin
    Commented Mar 25, 2021 at 9:43
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    $\begingroup$ The phase is supercritical fluid. To call it a liquid would be wrong. To call it a gas would be wrong too. $\endgroup$
    – Ivan Neretin
    Commented Mar 25, 2021 at 9:52
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    $\begingroup$ Imagine analogy: There is a beach(liquid) and a steep cliff plateau above it (gas). when you walk along the shore ( heating up), the beach climbs up and the cliff plateau goes down, each becoming more similar to the other. At some point ( critical point ) they meet and become the same. Is it a cliff or a beach ? $\endgroup$
    – Poutnik
    Commented Mar 25, 2021 at 9:53
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    $\begingroup$ It is evidence you have not got it yet. Their ALL property values converge to each other. Imagine a fog and a foam, each under intensive mixing, where you gradually add water or air respectively, at some point, there is no difference between fog and foam. $\endgroup$
    – Poutnik
    Commented Mar 25, 2021 at 9:59
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    $\begingroup$ @buckthorn Not, you are not. :-) It was said the beach slowly climbing ( or is prolonged ) and cliff does the otherwise. As some point, there is no cliff in the sense of an edge. $\endgroup$
    – Poutnik
    Commented Apr 12, 2021 at 12:54

2 Answers 2

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The essential condition is the compound thermal stability. If it decomposes below its melting point, it does not have a triple point. If it decomposes before properties of gaseous and liquid phases converge to each other, it has just estimated, extrapolated triple point.

Beyond CP we talk about supercritical fluid, it kind of shares many properties of both liquid and gaseous worlds.

It is usually determined experimentally, or as an extrapolation.

See the picture of liquid/gas phase system below, at and above the critical point on Wikipedia: Critical_point

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You must first understand what happens when water is heated along the boiling curve. Start from the point $\pu{100°C}$and $\pu{1 atm}$. At this point there are two phases, one liquid whose density is nearly $\pu{1 g/cm^3}$ and exactly $\pu{0.96 g/cm3}$. And the vapor has a density of $\ce{0.0006 g/cm^3}$. If now you heat this system in a closed volume, the temperature and the pressure will increase like in a steamer. But the densities of the two phases vary in opposite directions. The liquid expands, and the vapor contracts.

At $\pu{180°C}$ , the liquid has a density of $\pu{0.87 g/cm3}$ and the vapor $\pu{0.0079 g/cm3}$

At $\pu{312°C}$, the liquid has a density of $\pu{0.71 g/cm3}$ and the vapor $\pu{0.046 g/cm3}$

At $\pu{374.8°C}$, both the liquid and the vapor have a density of $\pu{0.32 g/cm3}$

If you look at the surface of the liquid just before $\pu{374.8°C}$, the surface of the liquid gets less and less visible. It is transformed into a sort of haze, and slowly disappears. Of course the pressure is extremely high ($\ce{217 atm}$) and the curve pressure vs temperature stops at $\pu{374.8°C}$.

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