# How would you explain the phase diagram of sulphur?

Apr 8, 2016

#### Explanation:

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 $\text{S}$: $\text{S(rhombic)" ⇌ "S(g)}$
• ① to② — the sublimation curve of monoclinic $\text{S}$: $\text{S(monoclinic") ⇌"S(g)}$
• ② to upper right — the vapour pressure curve of liquid $\text{S}$: $\text{S(l)" ⇌ "S(g)}$
• ① to ③ — the transition curve for $\text{S(rhombic)" ⇌ "S(monoclinic)}$
• ② to ③ — the melting point curve for $\text{S(monoclinic) ⇌ S(l)}$
• ③ to top — the melting point curve for $\text{S(rhombic) ⇌ S(l)}$

There are three triple points:

• ① ($\text{95.4 °C", 1 × 10^"-4"color(white)(l) "atm}$) — rhombic $\text{S}$ is in equilibrium with monoclinic $\text{S}$, and both have the same vapour pressure.
• ② ($\text{119 °C", 5× 10^-4color(white)(l) "atm}$) — monoclinic $\text{S}$ melts; this is the triple point for $\text{S"_"m" ⇌ "S"_"l" ⇌ "S"_"g}$.
• ③ ($\text{151 °C, 1288 atm}$) — rhombic, monoclinic, and liquid $\text{S}$ are at equilibrium.

The critical point — where liquid and gaseous $\text{S}$ have the same density — is off to the right at $\text{1041 °C and 203.3 atm}$.