Application of phase rule : Sulphur system

Application of phase rule : Sulphur system

Application of phase rule : Sulphur system

  1. In today’s article, we are going to study about ‘ Application of phase rule : sulphur system ‘ How beneficial it is and what is the sulphur system. We will also read about some more topics like triple point, phase diagram of sulphur system, explanations of curves, etc in detail.  So, take your notebooks in you hand and get ready to study physics in an easy and sorted way.

Sulphur System

It is also a one component system consisting of four phases.

Rhombic sulphur Sr….Solid phase

Monoclinic sulphur Sm….solid phase

Liquid sulphur Sl….Liquid phase

Sulphur Vapour Sv….Vapour phase

The phase diagram of sulphur system is represented in fig.

Application of phase rule : Sulphur system

There are two crystalline allotropic forms of sulphur one is monoclinic and other is rhombic. They are in equilibrium at 95.6°C.

Application of phase rule : Sulphur system

This temprature 95.6°C is called transition temperature. below this temprature rhombohedral sulphur is stable and above this temprature monoclinic sulphur exists. Melting point of rhombic sulphur is 114°C st 0.03mm of Hg atm pressure whereas monoclinic sulphur melts at 120°C. Liquid sulphur vaporises at about 450°C. Therefore, there are four phases of sulphur which can exist.

If P=4 is substituted in phase rule F= C-P+2 for a one component system (C=1) F comes out to be -1 which is impracticable. Therefore, all the four phases can not co-exist together. Maximum three phase can coexist together in sulphur system.

The phase-diagram of sulphur system consists of

  1. Curves FA, AB, BE, AC, BE and CD, where two phases are in equilibrium at different temperatures and pressures. They are monovarient.
  2. Areas : The above curves divide the diagram into four areas where only one phase exists. They are bivariant.
  3. Triple point : A, B, C and O are four triple points where three phases are in equilibrium at these points. They are nonvariant.

Now we will study and discuss the characteristic of these curves, areas and points in detail.

(1) Different Curves

Curve FA : This curve is the sublimation or vapour pressure curve of rhombic sulphur (Sr). This gives the variation of vapour pressure of Sr at different temperature. Two phases rhombic sulphur and sulphur vapour are in equilibrium along this curve.

Sr ⇔ Sv

It follows from phase rule

F= C-P+2 =1-2+2 =1

Thus, the system is univarient, means there can be only one pressure at one temprature along this curve.

Curve AB- It is vapour pressure or sublimation curve of monoclinic sulphur (Sm). Following equilibrium establishes along this curve.

Sm ⇔ Sv

There are two phases along this curve at any temperature. This is also a univarient or it has one degree of freedom.

Curve BE : It is the vapour pressure curve of liquid sulphur (Sl). Sl ⇔ Sv equilibrium is established along this curve. Here also degree of freedom is one. Thus, it also univariant system.

Curve AC : This is known as the Transition curve of Sr to Sm. Two solid phases are in equilibrium along this curve.

Sr ⇔ Sm

This curve gives the effect of pressure on the Transition point. Inclination of curve shows that Transition temperature increases with pressure or volume increases when rhombic sulphur changes into monoclinic sulphur.

Curve BC– It is melting or fusion curve for monoclinic sulphur. It shows the effect of pressure on melting point of Sm which is raised by increase of pressure, the curve BC, thus slopes slightly away from the pressure axis. Monoclinic and liquid sulphur are in equilibrium along this curve.

Sm ⇔ Sl

Thus, it is also an univariant system.

Curve CD– It is the fusion curve for rhombic sulphur. Rhombic sulphur and liquid sulphur are in equilibrium along this curve.

Sr ‌⇔ Sl

Number of phases is one and the System is univarient.

(2) Areas-

Area left to FACD : This area has only rhombic sulphur. It follows that it has only one phase, so degree of freedom is now. It is a bivariant system means both temprature and pressure to be fixed to locate any point in this region.

Area below FABE- This area has only vapour sulphur (Sv). This is also a bivariant system having two degree of freedom.

Area right to EBCD : This region has only liquid sulphur (Sl). This region is bivariant having two degree of freedom. This region provides the condition for the existence of liquid sulphur.

Area surrounded by curves AB, BC and AC –

This area has only monoclinic sulphur Sm and is bivariant. This region provides the conditions for the existence of monoclinic sulphur.

3) Triple points

Point A- At the point A, rhombic sulphur changes into monoclinic sulphur. This is known as transition temperature. Three phases Sr, Sm and Sv are in equilibrium st this point.

Sr ⇔ Sm ⇔ Sv

At this point P=3, C=1 so F=0. Thus, it is a nonvariant system. Temprature (95.6°) and pressure (0.006 mm of Hg) are fixed at this point.

Point B- Curves AB, EB and XB meet at this point. Sm, Sl and Sv states are in equilibrium at this point.

Sm ⇔ Sl ⇔ Sv

This point represent the melting point of Sm (120°C) at constant pressure (0.04 mm or Hg). This is also a non variant system as degree of freedom is zero.

Point C- Curve AC, BC and DC meet at this point. Thus, it is another triple point. Which is nonvariant. At this point, three phases (Sr, Sm and Sl) are in equilibrium.

Sm ⇔ Sl ⇔ Sv

Temperature of this point is 150° and pressure is 1290 atm. This is also melting point of Sr.


So our article is finished and after completely reading this article, one can easily tell what an phase diagram of sulphur system is. And how we can explain it.

‌So one can say that they got a detailed information about phase diagram of sulphur system.

IT was an article based on ‘ Application of phase diagram : sulphur system’. Your suggestions are invited for more topics of articles.


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