Phase Diagram : -

A phase diagram is a chart that shows the conditions of pressure and temperature at which distinct phases occur and coexist at equilibrium.

The lines on a phase diagram divide into regions – solid, liquid, and gas.

The phase diagram of sulfur is -

Let's look first at the four areas:

• Pink — only rhombic sulfur

• Brown — only monoclinic sulfur

• Green — only liquid sulfur

• Blue — gaseous sulfur

The corresponding curves are:

• lower left to ① - the sublimation curve of rhombic S: S(rhombic) ⇌ S(g)

• ① to② - the sublimation curve of monoclinic S: S(monoclinic) ⇌ S(g)

• ② to upper right - the vapour pressure curve of liquid S: S(l) ⇌ S(g)

• ① to ③ - the transition curve for S(rhombic) ⇌ S(monoclinic)

• ② to ③ - the melting point curve for S(monoclinic) ⇌ S(l)

• ③ to top - the melting point curve for S(rhombic) ⇌ S(l)

There are three triplets : -

• ① (95.4 °C, 1 ×$10^{-4}$ atm) - rhombic S is in equilibrium with monoclinic S, and both have the same vapour pressure.

• ② (119 °C, 5 ×$10^{-4}$ atm) - monoclinic S melts; this is the triple point for $S_m$ ⇌ $S_l$ ⇌ $S_g$

• ③ (151 °C, 1288 atm) - rhombic, monoclinic, and liquid S are at equilibrium.

The critical point where liquid and gaseous S have the same density is off to the right at 1041 °C and 203.3 atm.