One component system

A rule relating the possible numbers of phases,
constituents, and degrees of freedom in a chemical system.
Equation
F=C-P+2
Phase Rule

The Phase Rule Equation
F • Degree of
system
C • Component
of a system
p • Number
of phase

The least number of variable factor (pressure,
temperature)which must be specified so that the remaining
variable fixed automatically and the system is completely
defined
Example
A system with F=1 is univariant having one degree of
system.
Degree of Freedom

The least number of independent chemical constituents in
terms of which the component of every phase can be
expressed by means of a chemical equation
Example
Water system has one component i.e. water
component

Any homogenous part of system having all physical and
chemical properties the same through out.
Example
Liquid
Vapour
Phase

Three cases may arise
When only
one phase
present
When two
phase are in
equilibrium
When three
phase are in
equilibrium

When one phase present
 System is bivariant.
 System is completely defined by two variable...
When two phase present
System is monovariant .
When three phase is present.
System is non variant .
This condition is attained at definite
temperature and pressure.
Usually shown by triple point .

A phase diagram is a plot showing the condition of pressure
and temperature under which two or more physical states
can exist together in a state of dynamic equilibrium.
Phase diagram

Salient feature of phase diagram
Curves
Triple
pointareas

Phase diagram is divided into three areas .
• Solid COB
• Liquid AOC
• Vapour AOB
• Each single phase has two degree of freedom.
• Each area of phase diagram represent a bivariant
system.
AREAS

Melting
curve
• solid converted to
liquid.
• Have one degree of
freedom.
Vapour
pressure
curve
• Liquid converted to
Vapour.
• Have one degree of
freedom.
Sublimation
curve
•solid converted to
Vapour.
•Have one degree of
freedom.
Curves

The three boundary lines enclosing the three areas on the
phase diagram intersect at a common point called the triple
point.
• At triple point all the three phases can co exist in
equilibrium.
• Solid liquid vapour
• Degree of freedom is zero at triple point.
Triple point

Metastable equilibrium
• TheVapour pressure curve OA terminate at O when the
liquid freezes.
• By carful cooling of the liquid under condition under
condition that crystal are not form the curve OA can be
extended to A’.
• The liquid can be cooled far below than it’s freezing point
with out separation of crystal.
• On slightly disturbance as introduction of a seed crystal
,the entire liquid solidifies.
• Dashed curve OA’ Represent metastable equilibrium.

• Super cooled liquidVapour Vapour
• This system at once reverts to true stable system
• Solid Vapour
• The vapour pressure of metastable phase is always higher
than that of the stable phase at same pressure.

POLYMORPHISM
ALLOTROPY AND ITSTYPES

The occurrence of the same substance in more than one
crystalline form is known as polymorphism.
This phenomena is shown by both element and
compound. In case of elements the term allotropy is often
used .They have similar chemical properties but different
physical properties .
Polymorphism

Existence of an element into more than one physical forms is
known as allotropy.
ALLOTROPY
AllotrophicFormofCarbon
Coal
Diamond
Graphite
Example

Allotropy has three types which are as follows.
ALLOTROPY
ENANTIOTROPY
DYNAMIC
ALLOTROPY
MONOTROPY
Types of allotropy

When the change of one form to the other at the transition
temperature is reversible the phenomenon is called
enantiotropy.
Example
Enantiotropy
α Sulphur β Sulphur
(Rhombic Sulphur) (Monoclinic Sulphur)
Thus alpha sulphur and beta sulphur are enantiotropy.

These crystal which are short and flat diamond like
structure.they are formed under 96 degree centigrade.
Monoclinic sulphur
These crystal which are long and thin rectangular.They
are formed above 96 degree centigrade.
Monotropy
It occurs when one form is stable and the other is
metastable.
Rhombic Sulphur

Graphite
Diamond
Graphite is stable and diamond is metastable.
Dynamic allotropy
The separate forms usually have different molecular
formula but the same empirical formula.This form of
allotropy known as dynamic allotropy.
Example

EXPERIMENTAL DETERMINATION OF
TRANSITION POINT

Definition
The temperature at which a polymorphic substance
changes from one form to another is known as transition
temperature or transition point.
EXAMPLE
Rhombic sulphur is converted to monoclinic sulphur at
95.6ºC.
Experimental determination of
transition point

Determine
of transition
point
method
Color
Change
Solubility
change
Cooling
curve
method
Density
change
Determine by measuring change in
physical properties.

If a little mercury(II) iodide is placed in a melting point
attached to thermometer and heat in some form of
apparatus.
Red mercury(II) iodide changes to yellow form.
Colour Change

When rhombic sulphur changes to monoclinic sulphur
there is decrease in density and increase of volume.
The increase in volume is employed to measure transition
temperature by using apparatus Dilatometer.
Density Change

Some powered sulphur is introduced in glass bulb and
liquid paraffin above the sulphur.
The apparatus is placed in water bath and temperature
is raised.
The scale reading and temperature is recorded every
minute.
A plot of liquid level in capillary against temperature
gives a curve.
On cooling of the dilatometer reverse changes take
place.
The transition temperature is taken as the mean of
respective temperature.
Method

Two forms of same substance have different solubilities but
at the transition point they have identical solubility.
If solubility temperature graph plotted it consist of two parts
with a sharp break.
At the meeting point of two curves the solubility of two form
is same and it indicates the transition temperature.
Example
Na2SO4 and Na2SO4.10H20
Solubility change

When one form passes to another there is evolution or
adsorption of heat.
Let A converted to B on heating.
Let B to cooled and a curve obtained by plotting
temperature against time.
Steady curve has a distinct break at a temperature
corresponding to the transition point.
Cooling curve method

Suitable for determining the transition temperature.
Between different hydrates of salt or between a
hydrate and anhydrous salt or for different forms of
metal.
Importance

Water is the three phase one component system the three
phase involved are
Water
phase
Ice
VapourLiquid

The number of phases which can exist in equilibrium
any time depends on condition of temperature and
pressure
SALIENT FEATURE OF PHASE DIAGRAM
The curves 0A,OB,OC.
The triple point O.
The areas AOC,AOB,BOC.

It represent the vapour pressure of liquid water at
different temperature.
Water and vapour co-exist in equilibrium along this
curve.
The curve OA terminates at A the critical
point(218atm,374ºC).
CURVE OA, theVapour Pressure Curve of
Water

It shows the vapour presssue of solid ice at different
temperature.
The two phase solid ice and vapour co-exist in
equilibrium along this curve.
At the lower limit the curve 0B terminate at obsolute
zero(-273ºC)where no vapour exist.
Curve OBThe Sublimation Curve of Ice

The Phase Diagram of the SystemWater

It depicts the effect of pressure on the melting point of
ice.Here ice and water co exist in equilibrium.
The fact OC slopes to the left indicates that the melting
point of ice decreases with increase of pressure.
Curve OA,OB,OC there are two phase in equilibrium and
one component.
F=C-P+2
1-2+2=1
Curve OC,The Fusion Curve of Ice

Water/water vapour represented by OA .
Ice/water vapour represented by OB.
Ice/water represented by OC.
It is mono variant.
The triple point “O”
The curves OA OB and OC meet at triple point O where
all three phases are in equilibrium.
Here are three phases and one component system.
F=C-P+2
1-3+2=0

AreaAOC
represent
• Condition
of water
AreaAOB
represent
• Water
vapour
Area BOC
represent
• ice
Area AOC, AOB, BOC
 The system is bivariant i.e has 2 degree of freedom.
 The system at “O“ is non variant thus if pressure or temperature is
changed the three phases would not exist and one of the phase
disappear.

Water can be super cooled by carefully eliminating solid
particle.
The super cooled water/vapour system is metastable.
It once reverts to the stable system ice/vapour on the
slightest disturbance or introducing a crystal of ice .
Metastable system

Rhombic Sulphur
Monoclinic Sulphur
Sulphur Liquid
SulphurVapour
Four Phases are as follows

• The two crystalline form of sulphur rhombic sulphur and
monoclinic sulphur exhibit enantiotropy with a transition
point 95.6 degree centigrade.
• Rhombic sulphur monoclinic sulphur liquid
sulphur (95.6) (120)

curve
Triple point
area
Salient feature of phase
diagram

Sulphur system phase diagram has four areas.
Rhombic sulphur. ABG
Monoclinic sulphur. BEC
Liquid sulphur. GECD
SulphurVapour. ABCD
• Each represent single phase.
• Degree of freedom is two i.e bivariant
Area

• Six curves AB,BC,CD,BE,CE,EG
• Curve AB Vapour pressure curve of Rhombic sulphur
• Curve BC Vapour pressure curve of monoclinic sulphur
• Curve CD vapour pressure curve of liquid sulphur
• Curve BE transition curve
• Curve CE fusion curve of monoclinic sulphur
• Curve EG fusion curve for Rhombic sulphur
Curves

• Along this curve two phase rhombic sulphur and sulphur
Vapour co exist and are in equilibrium.
• Have one degree of freedom
• Along this curve monoclinic sulphur and vapour sulphur co –
exist.
• Shows variation of vapour pressure of monoclinic sulphur with
temperature.
Curve AB
Curve BC

Shows of variation ofVapour pressure of liquid sulphur
with temperature.
Along CD liquid sulphur and sulphurVapour are in
equilibrium.
It has degree of freedom one.
1 atm line meet at temperature 444.6 which is boiling
point of sulphur
Curve CD

• Increase in pressure will shift the equilibrium to
the left (Le chatelier’s principle) and the
transition temperature will raised.
• BE slopes away from pressure axis.
• Along curve Monoclinic and liquid sulphur co
exist
• Shows increase of pressure will increase the
melting point and volume increase
• Slope CE away from pressure axis
• The curve end at E because monoclinic sulphur
ceases to beyond this limit.
Curve CE

• Show effect of pressure on transition temperature for
rhombic and monoclinic sulphur.
• Along this curve rhombic and monoclinic sulphur are in
equilibrium
• The system is monovariant
• The transformation of rhombic sulphur and monoclinic
sulphur is due to increase of volume and absorption of
heat .
Curve BE
transition curve

• Rhombic and liquid sulphur co exist in equilibrium
• Degree of freedom is two
Curve EG Fusion Curve
For
Rhombic Sulphur

• There are three triple point in sulphur system which are
as follows.
• Triple point at B.
• Triple point at C.
• Triple point at E.
Triple point

A phase that can be produced by a very rapid change in
system conditions( temperature, pressure)or in some case
The metastable state may persist indefinitely as the
movement towards equilibrium is virtually imperceptible
over long period of time.
Meta stable equilibrium

 If enough time for the change is not allowed and rhombic
sulphur is heated rapidly
 It is possible to pass well above the transition point with
out getting monoclinic sulphur.
 In that case phase diagram will consist of
 Three curves
 One triple curve
 Three areas (rhombic sulphur,liquid sulphur,sulphur
Vapour )

 Continuation of vapour pressure curveAB of stable
rhombic sulphur.
 The metastable phase rhombic sulphur and sulphur
vapour are in equilibrium.
 It has one degree of freedom.
 TheVapour pressure curve of supercooled liquid sulphur
Dashed curve BF,Vapour pressure curve
of metastable rhombic sulphur

Curve CF,Vapour pressure curve of super
cooled liquid sulphur
 On super cooling liquid sulphur, dash curve CF is obtained
 Metastable equilibrium between liquid sulphur and
sulphurVapour
Dashed curve FE, the melting curve of
metastable of Rhombic sulphur
The two metastable phase rhombic sulphur and
liquid sulphur are in equilibrium.
Melting point of rhombic sulphur increase with
pressure.

At this point, three metastable phases rhombic
sulphur,liquid sulphur andVapour sulphur are in
equilibrium.
The system is a metastable triple.
With no triple point.
Metastable triple point O

One component system

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One component system