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Explain phase diagram of sulphur
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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 -

enter image description here


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.

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