This happens when the vapor pressure equals the external pressure. In other words, the solid-liquid line is very nearly vertical. There is one unique point at which the solid, liquid, and vapor all have the same chemical potential. This is known as the triple point . The triple point is a fact of the nature of the substance, beyond out control.
Triple points can also occur which involve two solid phases as well.
The phase diagram for water is nearly unique in that the solid-liquid line has a negative slope near 1 atm. This means that the liquid is more dense than the solid, and that a solid sample will liquify if sufficient pressure is introduced. B. Carbon Dioxide CO 2 has a more normal phase diagram. The phase diagram for carbon is not fully known. D. Helium Helium is unique in its low temperature, low pressure regime.