B. Sc. S. Y.

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Phase Equilibrium

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Dr. S. M. Reddy

Associate Professor

Phase rule

1. The phase rule deals with the behaviors of heterogeneous systems.
2. Using phase rule, it is possible to predict qualitatively by means of a diagram, the effect of changing pressure, temperature and concentration of a heterogeneous system in equilibrium.
3. The phase rule is first discovered in 1874 by J.W. Gibb’s.
4. Therefore it is also called as Gibb’s phase rule.

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Mathematically it states that

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F = C – P + 2

F → is the number of degrees of freedom,

C → is the number of components,

P → is the number of phases.

Phase ( P )

A phase is defined as any homogeneous part of a system having all physical and chemical properties same through out.

A system may consist of one or more than one phases.

It is denoted by P.

1) A system containing only one phase is called as 1- phase system ( P = 1 ). Ex :-

• Pure substance made of one chemical species only is considered as 1- phase. Thus oxygen (O₂), benzene (C₆H₆), ice (H₂O) are all 1- phase systems.

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• Mixture of gases :- All gases mix freely to form homogeneous mixture. Mixture of O₂ and N₂ is a 1- phase system.

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• . Miscible liquids :- Two completely miscible liquids yield a uniform solution. Thus a solution of ethanol and water is a 1- phase system

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• Aqueous solution of a solid substance such as NaCl is uniform through out. It is a 1- phase system.

2) A system containing two phases is called as two phase system (P = 2). Ex :-

• Non miscible liquids :-
• A mixture of two non miscible liquids on standing forms two separate layers.
• Ex:- mixture of chloroform and water constitute a two phase system.

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3) A system containing three phases is called as three phase system.

• Ex:- A mixture of two or more chemical substances contains as many as phases .
• Each of these substances having different physical and chemical properties makes a separate phase .
• Ex:- Decomposition of CaCO₃

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A mixture of CaCO₃ and CaO constitute two phases .Thus there are two solid phases and one gaseous phase. Hence it is a three phase system.

Components (C)

The term component is defined as the least number of independent chemical constituents in number of which the composition of every phase can be expressed by means of a chemical equation. Ex.

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• Water system has three phases ice, water, and water vapour. The composition of all these three phases is expressed in terms of one chemical individual H₂O. Thus water is one component only.

• Sulphur system has four phases, rhombic sulphur, monoclinic sulphur, liquid sulphur and sulphur vapour. The composition of all these phases can be expressed by one chemical individual sulphur (S). Thus sulphur is a one component system.

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• NaCl solution :- A solution of NaCl in water is 1-phase system. It’s composition can be expressed in terms of two chemical individuals NaCl and water. Therefore it is a two component system.

Phase Component

Aqueous solution of NaCl = x NaCl + y H₂O

Degree of freedom ( F )

Degree of freedom is defined as the least number of variable factors ( concentration, pressure and temperature ) which must be specified sothat the remaining variables are fixed automatically and the system is completely defined.

A system with F = 0, is known as non-variant or having no degree of freedom.

A system with F = 1, is known as uni-variant or having one degree of freedom.

A system with F = 2, is known as bi-variant or having two degree of freedom.

A system with F = 3, is known as tri-variant or having three degree of freedom.

Example :-

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For a given pure gas, PV = RT, if the values of P, T be specified, the volume V can have only one definite value or the value of V is fixed automatically. Hence a system containing a pure gas has two degree of freedom F = 2.

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Derivation of phase rule

1. Consider a heterogeneous system in equilibrium having C - components and P - phases.

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• To determine the degree of freedom of this system ie the number of variables which must be arbitrarily fixed in order to define the system completely.

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• Since the state of the system will depend upon the temperature and the pressure, these two variables are always there.

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• The concentration variables, however, depend upon the number of phases.

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• In order to define the composition of each phase it is necessary to specify the concentration of (C – 1) constituents of each phase.

6. The concentration of the remaining components can be determined by difference.

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• For P- phases, therefore, the total number of concentration variables will be P (C – 1) and these along with the two variables eg. temperature and pressure make the total number of the variables of the system equal to [ P ( C – 1 ) + 2 ].

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• On thermodynamic considerations when a system is in equilibrium, the partial molal free energy of each constituent of a phase is equal to the partial molal free energy of same constituent in every other phase.

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• Since the partial molal free energy of the constituents of a phase is a function of the temperature, pressure and (C – 1) concentration variables, it follows that if there is one component in two phases, it is possible to write one equation amongst the variables and if there is one component in three phases, this fact may be written with the help of two equations.

10. Therefore, in general, when P-phases are present ( P – 1 ) equations are available for each component and for C components, the total number of equations or variables are C ( P – 1).

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• Since the number of equations is equal to the number of variables, the number of unknown variables or degrees of freedom F will be

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F = Number of variables - Number of equations

F = [ P (C – 1) + 2 ] – [ C(P – 1) ]

F = PC – P + 2 – PC + C

F = C – P + 2

WATER SYSTEM

• The water system has three phases, liquid water, ice, water vapour.
• All these phases can be represented by one chemical individual H₂O.
• Hence it is one component system.
• The number of phases which can exist in equilibrium any time depends on the

conditions of temperature and pressure.

• The phase diagram or PT diagram of the water system is shown in figure

1. Curves OA, OB, OC.

These three curves meet at point ‘O’ and devide the diagram into three regions or areas.

• The curve OA terminates at A, the critical point ( 218 atm, temperature

374 ºC) when the liquid and water vapour are indistinguishable from

each other and there is left one phase only.

• When the vapour pressure is equal to 1 atm, the corresponding

temperature is the boiling point (100 ºC) of water.

a) Curve OA (Vapour pressure curve of water)

• It represents the vapour pressure of liquid water at different temperatures.
• The two phases water and water vapour coexist in equilibrium along this curve.

b) Curve OB (Sublimation curve of ice)

• The curve OB shows the vapour pressure of solid ice at different

temperatures.

• The two phases solid ice and vapour coexist in equilibrium along this

curve.

• At lower limit the curve OB terminates at absolute zero (- 273 ºC )

where no vapour exists.

c) Curve OC (Fusion curve of ice)

• The curve OC represents the effect of pressure on the melting point of

ice.

• Along this curve ice and water coexist in equilibrium.
• The curve OC slopes to left indicates that the melting point of ice

decreases with increase in pressure.

• The 1 atm line meets the fusion curve at 0 ºC which is the normal

melting point of ice.

Along the curves OA, OB and OC there are two phases in equilibrium and one component.

F = C – P + 2

F = 1 – 2 + 2

F = 1

Thus each two phase systems represented by OA, OB and OC have one degree of freedom ie monovariant.

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• At triple point, the system is non variant.

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• Thus if either pressure or temperature is changed, the three phases would not exist and one of the phases would disappear.

2. The triple point ‘O’

• The curves OA, OB, OC meet at a point ‘O’ where all three phases

liquid water, ice, vapour are in equilibrium.

• The point ‘O’ is called as triple point. This occurs at 0.0076 ºC and

vapour pressure 4.58 mm Hg. Since there are three phases and one

component

F = C – P + 2

F = 1 – 3 + 2

F = 0

The areas AOC, AOB and BOC represents the conditions for the one phase system water, water vapour and ice respectively.

Thus, in these areas there is one phase and one component.

F = C – P + 2

F = 1 – 1 + 2

F = 2

Thus each system has 2 degrees of freedom ie the system is bivariant

3. Areas AOC, AOB, BOC

The areas between the curves show the conditions of temperature and pressure under which a single phase ice, water or vapour is capable of stable existence.

• The supercooled water/vapour system is metastable (unstable).
• It at once reverts to the stable system ice/vapour on the slightest

disturbance or introducing a crystal of ice.

4. Metastable system

• The vapour pressure curve of water AO can be continued past the triple point as shown by dashed line OA' ie water can be supercooled by carefully eliminating solid particles.

Carbon dioxide system

The phase diagram of CO₂ system is as shown in figure.

The CO₂ system has 1 – component and 3 – phases.

The silent features of the phase diagram are

• Curves AB, BC and BD,
• Triple point B,
• Areas ABC, ABD and CBD.

• The curve BD represents the vapourisation curve of liquid CO₂.
• Along this curve, liquid CO₂ and gaseous CO₂ coexist in equilibrium.
• The curve BC represents the fusion curve of solid CO₂.
• Along this curve solid CO₂ and liquid CO₂ coexist in equilibrium.
• Along all these three curves, the system has two phases and one

component.

F = C – P + 2

F = 1 – 2 + 2

F = 1

The system along these curves is monovariant.

1. Curves AB, BC and BD

• The curve AB represents the sublimation curve of solid CO₂.
• Along this curve, solid CO₂ and gaseous CO₂ coexist in equilibrium. When the pressure is 1 atm, the temperature is – 78 ºC.

2. Triple point B

• The point B is the triple point at which all the three phases of

CO₂ coexist in equilibrium with one another.

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• All three curves meet at point B. At this point B, the temperature

of the system is – 57 ºC and pressure is 5∙2 atm.

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• A slight variation in temperature or pressure at this point may

result in the disappearance of one of the two phases.

3. Areas

The area ABD, ABC and CBD represent the conditions for single phase system of gaseous CO₂, solid CO₂ and liquid CO₂ respectively.

In these areas there is one phase and one components.

F = C – P + 2

F = 1 – 1 + 2

F = 2

Thus along these areas the systems are bivariant.

Sulphur system

• The sulphur system has four phases, rhombic sulphur (S_R), monoclinic sulphur (S_M), sulphur liquid (S_L), sulphur vapour (S_V).
• All these phases are represented by the chemical individual sulphur. Hence it is one component system.

• The two crystalline forms of sulphur S_R and S_M exhibit

enantiotrophy with a transition point 95∙6 ºC.

• Below this temperature, S_R is stable while above it S_M is stable variety.
• At 95∙6 ºC, each form can be gradually transformed to the other and

the two are in equilibrium. At 120 ºC, S_M melts.

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The silent features of the phase diagram are :-

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1. Six curves AB, BC, CD, BE, CE, EG.

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2. Three triple points B, C, E.

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3. Four areas ABG, BEC, GECD and ABCD.

a) Curve AB ( Vapour pressure curve of S_R)

• It shows the vapour pressure of solid rhombic sulphur ( S_R) at

different temperatures.

• Along this curve the two phases S_R and S_V are in equilibrium.
• The system S_R/ S_V has one degree of freedom ie monovariant.

F = C – P + 2

F = 1 – 2 + 2

F = 1

1. Curves AB, BC, CD, BE, CE and EG.

These six curves divide the diagram into four areas.

b) Curve BC ( Vapour pressure curve of S_M )

• It shows the vapour pressure of monoclinic sulphur ( S_M) at different

temperatures.

• Along this curve, the two phases S_M and S_V are in equilibrium.
• The system S_M / S_V has one degree of freedom ie monovariant.

F = C – P + 2

F = 1 – 2 + 2

F = 1

c) Curve CD ( Vapour pressure curve of S_L )

• It shows the vapour pressure of liquid sulphur ( S_L) at different temperatures.
• Along this curve the two phases S_L and S_V are in equilibrium.
• 1 atm line meets this curve at a temperature (444∙6 ºC) which is the boiling point

of sulphur.

• The system S_L/ S_V has one degree of freedom ie monovariant.

F = C – P + 2

F = 1 – 2 + 2

F = 1

• It shows the effect of pressure on the transition temperature for S_R and

S_M. As two solid phases are in equilibrium along this curve, the system

S_R/ S_M is monovariant.

• The transformation of S_R and S_M is accompanied by increase of volume and

absorption of heat.

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d) Curve BE ( Transition curve )

• Thus the increase of pressure will shift the equilibrium to the left and thus the transition temperature will be raised.
• This is why the line BE slopes away from the pressure axis showing thereby that the transition temperature is raised with increase of pressure.

e) Curve CE ( Fusion curve of S_M )

• It shows the effect of pressure on the melting point of S_M.
• The two phases S_L and S_M are in equilibrium along this curve.
• The system S_M / S_L is monovariant. As the melting or fusion of S_M is
• accompanied by a slight increase of volume, the melting point will rise by

increase of pressure. Thus the curve CE slopes slightly away from the pressure

axis.

• The curve ends at E because S_M ceases to exist beyond this point.

Curve EG ( Fusion curve for S_R )

• Along this curve the two phases S_R and S_L are in equilibrium.
• The system S_R / S_L is monovariant.

2) Triple points B, C and E

a) Triple point B :-

The three curves AB, BC and BE meet at point B. The three phases S_R, S_M and S_L are in equilibrium at point B.

F = C – P + 2

F = 1 – 3 + 2

F = 0

Thus the system S_R / S_M / S_L are nonvariant. At poin B, S_R is changed to S_M and the process is reversible. Thus the temperature corresponds to B is the transition temperature ( 95∙6 ºC).

c) Triple point E:-

• The three curves CE, BE and GE meet at point E.
• The three phases S_R, S_M and S_L are in equilibrium at point E.
• The system S_R / S_M / S_L is nonvariant.
• At this point the temperature is 165 ºC and 1290 atm pressure.

b) Triple point C :-

• The three curves BC, CD and CE meet at point C.
• The three phases S_M, S_L and S_V are in equilibrium at point C. At point C, the system is nonvariant.
• The temperature corresponding to point C (120 ºC ) is the melting point of S_M.

3) Areas

The four areas ABG, BEC, GECD and ABCD represent the single phase system for solid rhombic, solid monoclinic, liquid sulphur and sulphur vapour respectively.

F = C – P + 2

F = 1 – 1 + 2

F = 2

Each of the system S_R, S_M, S_L and S_V are bivariant.

4) Meta stable equilibrium

• The change of S_R to S_M takes place very slowly.
• If enough time for the change is not allowed, then S_R is heated rapidly, it is possible to pass well above the transition point without getting S_M.
• The dashed curve BF is the vapour pressure curve of metastable S_R.
• It is a continuation of the vapour pressure curve AB of stable S_R.
• The metastable phases S_R and S_V are in equilibrium along this curve.

• The dashed curve CF, is the vapour pressure curve of supercooled S_L.
• On supercooling liquid sulphur, the dashed curve CF is obtained.
• It is the back prolongation of DC.
• The metastable phases S_L and S_V are in equilibrium along this curve.

• The dashed curve FE is the fusion curve of metastable S_R.
• The two metastable phases S_R and S_L are in equilibrium along this

curve.

• This shows that the melting point of metastable S_R is increased with

pressure.

• Beyond E, this curve depicts the conditions for the stable equilibrium

S_R/S_L as the metastable S_R disappears.

• The system along the curves BF, CF and FE is monovariant.
• The point ‘F’ is the metastbale triple point at which the metastable

phases S_R, S_L and S_V are in equilibrium.

• The corresponding temperature is the melting point of metastable

S_R (114 ºC ).

• The system at point ‘F’ has no degree of freedom.

Two component systems

For two components, we have

F = C – P + 2

F = 2 – P + 2

F = 4 – P

In any system, minimum number of phases is 1

F = 4 – P

F = 4 – 1

F = 3

• Thus in any two component system, we need three variables

(T, P and C) to describe the system.

• These three variables are temperature, pressure and

composition of one of the components.

• To describe the condition of equilibrium graphically, we need

three co-ordinate axes at right angles to one another.

• But this is not possible to represent on a plane paper.

• It requires a space model. For the sake of simplicity, such

systems are studied with the help of two variables, keeping third

variable constant.

• Generally third variable- the pressure is kept constant and the

system is studied with the help of temperature and composition

variables.

For this , the phase rule is modified as

F = C – P + 2 – 1

F = C – P + 1

This is known as reduced phase rule equation.

The silver – lead system

The silver – lead system has two components and four phases.

The phases are:

1. solid silver,
2. solid lead,
3. solution of molten silver and lead,
4. vapour.

• The boiling point of silver and lead is considerably high, the vapour phase is

practically absent.

• Thus Ag / Pb is a condensed system with three phases.
• In such case, there is no effect of pressure on the system.
• Therefore only two variables temperature ( T ) and concentration ( C ) are

considered.

The TC diagram of the system Ag / Pb is shown in figure.

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The silent features of the diagram are

a) Two curves AC and BC.

b) Eutectic point C,

c) Three areas, 1. Above ACB, 2. Below AC, 3. Below BC.

1. Curves

a) Curve AC ( Freezing point curve of Ag )

• The point ‘A’ shows the freezing point or melting point of solid silver ( 961 ºC ).
• The curve AC shows that the addition of lead lowers the melting point along it.
• The phases solid silver and solution of silver and lead are in equilibrium along this

curve.

F = C – P + 1

F = 2 – 2 + 1

F = 1

The system Ag / solution / Pb is monovariant.

b) Curve BC ( Freezing point curve of Pb )

• The point ‘B’ represents the melting point of solid lead ( 327 ºC ).
• The curve BC shows that the melting point is lowered by the addition of silver.
• The phases solid lead and solution are in equilibrium along this curve.
• The system is monovariant.

2. The eutectic point ‘C’

• The curves AC and BC intersect at point ‘C’ which is called as eutectic point.
• At this point ‘C’, the three phases solid Ag, solid Pb, and solution are in

equilibrium.

F = C – P + 1

F = 2 – 3 + 1

F = 0

• At point C the system Ag / Pb / solution is nonvariant.
• At C both the variables T (303 ºC) and C (97∙5 % Pb and 2∙5 % Ag ) are

fixed.

• If we raise the temperature above the eutectic temperature, the solid phases

Ag and Pb disappear and if we cool below it, the solution phase disappears.

3. The areas

The area ACB represents the single phase system, the solution of molten Ag and Pb.

F = C – P + 1

F = 2 – 1+ 1

F = 2

The system solution Ag / Pb is bivariant.

• The area below AC represents the phases Ag and solution while

below BC the phases Pb and solution.

• The area below 303 ºC represents Ag + solid Pb.
• All these areas have two phases and one degree of freedom.

F = C – P + 1

F = 2 – 2+ 1

F = 1

Pattinson’s process for the desilverisation of Argentiferous lead

• The argentiferous lead containing small amount of silver (less than 0∙1 %) is

melted well above the melting temperature of pure lead (327 ºC).

• Let, the point X represents the system molten lead on the diagram.
• It is then allowed to cool when the temperature of the melt falls along the

dashed line XY.

• As the temperature corresponding to Y on the curve BC is reached, the solid

lead begins to separate and the solution would contain relatively larger amount

of silver.

• On further cooling, more of lead separates and we travel along the curve BC

until the eutectic point C is reached.

• Lead is continuously removed by means of ladles and the % of silver in the

melt goes on increasing.

• At C, an alloy containing 97∙5 % Pb and 2∙5 % Ag is obtained. This is

treated for the recovery of silver profitably.

KI-H₂O System

• KI does not form hydrates.
• Therefore the KI-H₂O system has two components and four phases solid KI,

ice, solution of KI in water and vapour.

The silent features of KI-H₂O system are:

1. Curves AO, BO.
2. Cryohydric or Eutectic Point ‘O’
3. Areas above and below the curves AO and BO.

Curves

Curve AO:

• The curve AO represents the freezing point curve of water.
• It shows the effect of addition of potassium iodide on the freezing point of water.
• The point represents the freezing of water ie M.P. of ice.
• At this point A, three phases ice, liquid water and vapours co-exist in equilibrium.
• It is observed that when KI is added gradually to it, the freezing point of water is lowered and the curve AO bends downwards.
• However, this does not go indefinitely.
• As soon as the solution is saturated with respect to KI, the lowest temperature is reached at the point O, then further addition of KI will not further lowering in the freezing point of water.

• The point O corresponding to the concentration of 52 % KI represents the lowest

temperature that can be attained in this system.

• This eutectic temperature in this case is called as cryohydrate point and the

eutectic itself is called as cryohydrate.

• All along the curve OA, two phases namely solution and ice are in equilibrium

(neglecting the vapour phase).

Therefore, from the reduced phase rule

F = C – P + 1

F = 2 – 2+ 1

F = 1

Thus the system along the curve AO is univariant.

Curve OB:

• The curve OB is the solubility curve of KI in water at different temps.
• It represents the effect of temperature on the solubility of solid KI.
• It is observed that the solubility of KI increases with rise in temperature

and is maximum at point B., the boiling point of the saturated solution.

• However, the curve comes to an end before it can reach the 100 % KI

axis.

• Conversely, if the saturated solution of KI in water is cooled, KI

separates out and the solution becomes dilute and the curve OB is

followed till the point O is reached.

• Here the solution freezes as a whole with a fixed composition.
• All along the curve OB, two phases namely solid KI and solution are in

equilibrium.

• Therefore, from the reduced phase rule

F = C – P + 1

F = 2 – 2+ 1

F = 1

Thus the system along the curve OB is univariant.

If we apply the original phase rule, we find the system is invariant.

F = C – P + 2

F = 2 – 4+ 2

F = 0

• Any change in the temperature or composition will cause disappearance of

one the phase.

• The point O corresponds to a definite temperature (- 22 ºC) and composition

(52 % KI and 48 % Ice).

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Cryohydric or Eutectic Point ‘O’

• The two curves OA and BO meet at

the point O.

• The three phases ice, solid and

solution are coexist in equilibrium.

• If the vapour phase is not neglected,

then the fourth phase is also present.

• Therefore, the point O is called as

quadruple point.

Areas

Area above AOB, areas below the curves AO and BO.

The area above AOB represents the single phase system consisting of only unsaturated solution.

F = C – P + 1

F = 2 – 1+ 1

F = 2

The system above the area AOB is bivariant

The area below AO shows the existence of ice and solution while the area below BO shows the presence of solid KI and solution.

F = C – P + 1

F = 2 – 2+ 1

F = 1

The system below AO and BO is monovariant.

Effect of cooling KI solution:

• The solution phase exists in the area above AOB.
• Let the solution represented by the point x be allowed to cool gradually

along the line xy without any change in composition till the point y is

reached.

• As soon as this point is reached, ice starts separating, and on further

cooling, solution continues becoming more concentrated till the

cryohydric point O is reached where the solid KI also appears.

• Similarly if a solution of composition x” lying on the right hand of point

O is cooled then KI will begin to separate as soon as y’’ is reached.

• The composition will change with temperature along y” O and more

and more of KI will continue to freeze to separate out until point O is

reached.

• Ultimately, as before, the whole of the mixture will freeze to give

eutectic mixture.

• Now consider a solution of composition represented by the point x’

lying vertically above the cryohydric point O.

• When such a solution is allowed to cool gradually along the line x’ O

without any change in composition, the solution will solidify as a

whole ie on reaching the point O, both ice and KI starts separating out

simultaneously.

• Hence it is concluded that all solutions on cooling show no further

change in temperature at the eutectic point O.

• Moreover, on cooling a solution of eutectic composition, the solution

freezes at the eutectic temperature without any change in composition.

Solutions of liquids in liquids

The solutions of liquids in liquids may be divided into three classes.

• Liquids that are completely miscible. eg :- alcohol and water.
• Liquids that are partially miscible. eg :- ether and water
• Liquids that are practically immiscible. eg :- benzene and water

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Solubility of completely miscible liquids

• Liquids like alcohol and ether or water mix in all proportions and in this

respect they could be compared to gases.

• The properties of such solutions however, are not strictly additive and

therefore their study has not proved of much interest.

• Generally the volume decreases on mixing but in some cases it

increases.

• Sometimes heat is evolved when they are mixed while in others, it is

absorbed.

• The separation of such types of solution can be effected by fractional

distillation

Solubility of partially miscible liquids

• A large number of liquids are dissolved in one another only to a limited

extent. eg :- ether and water.

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• Ether dissolved about 1∙2 % water and water also dissolves about

6∙5 % ether.

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• Since their mutual solubilities are limited, they are only partially miscible.

• When equal volume of ether and water are shaken together, two layers

are formed, one of the saturated solution of ether in water and the other of

the saturated solution of water in ether.

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• These two solutions are called as conjugate solution. The effect of

temperature on the composition of such mixtures can be studied.

Critical Solution Temperature ( CST )

• The temperature at which the two liquids, which are otherwise partially miscible at ordinary temperature, become completely miscible is called as critical solution temperature ( CST ).

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• The temperature above which the two conjugate solutions merge into one another to form one layer is called as the Upper consulate temperature.

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• The temperature below which the two conjugate solutions merge into one another to form one layer is called as or lower consulate temperature.

• The curve represents the miscibility of phenol and water.
• The L.H.S of the parabolic curve represents one of the two conjugate

solutions which depicts the % of phenol dissolved in water at different

temperatures.

• The solubility of phenol increases with temperature.

Phenol - water system

• The R.H.S. of the curve represents the other conjugate solution layer that gives the % of water in phenol.
• The solubility of water in phenol also increases with temperature.
• The two solutions meet at maxima on the temperature composition curve of the system.
• At this point, temperature is 66 ºC and composition of phenol is 33 %.
• Thus at a certain maximum temperature, the two conjugate solutions merge, become identical and only one layer is formed.

• The temperature at which the two conjugate solutions merge into one another to form one layer is called as the critical solution temperature( CST ) or Upper consulate temperature.

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• This is characteristic of a particular system and is influenced very much by the presence of impurities.

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• The determination of CST may be used for testing the purity of phenol and other substance.

• At any temperature above the critical solution temperature, phenol and water are miscible in all proportions.
• Outside the curve there is complete homogeneity of the system ie only one layer exists.
• Under the curve there may be complete miscibility but it depends upon the composition of the mixture. Below 50 ºC, a mixture of 90 % phenol and 10 % water or 5 % phenol and 95 % water will be completely miscible since the corresponding points do not lie under the curve.
• Two layers will always separate out below the curve and the curve gives the composition of two conjugate solutions containing two layers.

• At 50 ºC a mixture of equal proportion of phenol and water ( 50 % each ) will form two layers whose compositions are given by A and B.

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• The line joining the points M and N corresponding to the compositions A and B is called the tie line.

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• This line helps in calculating the relative amounts of the two layers, which is here given by the ratio MN / ML .

Trimethyl amine and water system

• The figure shows the temperature – composition curve of mutual solubility of trimethyl amine and water.
• The L.H.S. of the curve indicates the solubility curve of trimethyl amine in water while R.H.S. of the curve shows the solubility of water in trimethyl amine.
• The solubility decreases with the increase in temperature in this system.

• The two conjugate solutions mix up completely at or below 18∙5 ºC.
• This temperature is called as critical solution temperature or the lower consolate temperature.
• Any point above the horizontal line corresponds to heterogeneity of the system (two layers) while below, it is complete homogeneity (one layer).
• Thus an equi – component mixture (50 – 50) will be completely miscible at 10 ºC but at 50 ºC there will be separating out two layers having compositions corresponding to the points C and D.

Nicotine – water system

• At ordinary temperature, nicotine and water are completely miscible but at high temperature, the mutual solubility of nicotine in water decreases and two layers appear.
• But if temperature is further increased, two liquids again become miscible.
• In other words, the mutual solubility increases both on lowering as well as increasing the temperature in a certain ranges.

• Thus we have a closed solubility curve and the system has two critical

solution temperatures, the upper 208 ºC and the lower 61ºC.

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• The effect of pressure on this is that the lower critical temperature is

raised while the upper critical temperature is lowered gradually until

finally they become one.

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• At this point the liquids are miscible ar all temperature.

Effect of impurities on consulute temperature

• The presence of impurity alters the consolute temperature of a system of

two liquid components.

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• The impurity causes a change in the mutual solubility of two liquids.

For ex.

• In case of phenol-water system, if we add some hydrocarbon, which is

soluble in phenol only, we find that the consulate temperature is raised

from 66 ºC to 68.4 ºC.

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• Similarly, if we add KCl to the same system, again we observed that the

consulate temperature is raised.

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• KCl is soluble in water only. We can therefore generalize that if the

impurity is soluble in only one of the two liquids, it results into rise of

consulate temperature.

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• The magnitude of change is proportional to the relative amount of the

impurity.

• Addition of succinic acid which has considerable solubility in both

components of phenol-water system, lowers the consolute temperature

of phenol-water system.

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• Similarly effect is observed with soap in phenol-water system.

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• This property is used in the preparation of commercial disinfectant

solutions.

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• Thus, if the impurity is soluble in both the liquids, the consolute

temperature of the liquid mixture is lowered.

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• Addition of salts, which are soluble in water but insoluble in organic

components, raises the consolute temperature as in ex 1.

• However, addition of lithium iodide to aniline-water system depresses the

consolute temperature.

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• This is because lithium iodide is partially ionic and partially covalent.

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• In small amounts, it dissolves both in aniline and water.

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