# 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

##### 1 Answer
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.