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 #(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.

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.