How would you explain the phase diagram of sulphur?

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

The diagram is complicated by the fact that sulfur can exist in two crystalline forms: rhombic and monoclinic.

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 triple points:

• ① (#"95.4 °C", 1 × 10^"-4"color(white)(l) "atm"#) — rhombic #"S"# is in equilibrium with monoclinic #"S"#, and both have the same vapour pressure.
• ② (#"119 °C", 5× 10^-4color(white)(l) "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"#.