Question

In what region of the phase diagram would a substance certainly be a liquid? `a)` Past the triple point temperature `b)` Any spot past the melting point `c)` Between the melting point and critical temperature `d)` Between the melting and boiling points

Answer 1

The best answer choice is `(d)`, but it is still not a satisfactory answer because it says nothing about the pressure. There are multiple melting points and boiling points corresponding to different pressures.

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Consider a general phase diagram:

The liquid phase has two degrees of freedom: we can change temperature or change pressure, and we still remain with a liquid.

Anything at reasonably high pressure (between `P_(cr)` and `P_(tp)`), higher than the melting point, and less than the boiling point and critical point temperatures (simultaneously) is good enough.

• It is not enough to be past the triple point temperature, because being left of the solid green curve gives you a solid. Therefore, it is NOT `(a)`.
• "Any" is immediately a sign that `(b)` is false. Clearly, if we are past the melting point, we could very well be past the boiling point, and thus we could have a gas, not a liquid.
• Being between the melting point and critical temperature is almost correct... but it is easy to do that while still having too low a pressure, thus giving a gas. `(c)` is NOT a good answer but is on the right track...
• Being between the melting and boiling temperature is OK at the appropriate pressures (between `P_(tp)` and `P_(cr)` is a guarantee). `(d)` is the most accurate answer out of all of them, but it is still not a complete enough answer.