# 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

Sep 9, 2017

The best answer choice is $\left(d\right)$, 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.

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 ${P}_{c r}$ and ${P}_{t p}$), 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 $\left(a\right)$.
• "Any" is immediately a sign that $\left(b\right)$ 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. $\left(c\right)$ 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 ${P}_{t p}$ and ${P}_{c r}$ is a guarantee). $\left(d\right)$ is the most accurate answer out of all of them, but it is still not a complete enough answer.