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•Vapor Pressure (of
pure solvent): The
equilibrium pressure
from the vapor of a
pure liquid in a closed
container at a given
temperature (before
mixing).
•Partial Vapor: The
pressure exerted by a
specific solvent's vapor
in a solution, which is
reduced because the
solute molecules
occupy some surface
area.
•Mole Fraction: The
ratio of the moles of the
solvent to the total
moles of all
components (solvent +
solute) in the solution,
indicating its relative
abundance.

1
Contents of ThisChapter
•
•
•
•
Ideal and realsolutions
Concepts of Raoult'slaw
Positive and negative deviations from Raoult’slaw
Concepts of partially miscible liquids and critical solution
temperature
Faculty of Pharmacy © Ramaiah University of AppliedSciences

2
Learning Objectives
• At the end of this lecture, student will be ableto
-Describe the concepts and applications of Raoult'slaw
-
-Describe
Explain the concepts, ideal and realsolutions
thepositive andnegative deviations from
Raoult’s law
-Describe the principle and applications of partially miscible
liquids and critical solutiontemperature

3
Solubility Curves
Continuous SolubilityCurves
Discontinuous SolubilityCurves

4
Miscible Liquids
•
•
Example of miscible liquids-Ethyl alcohol inwater
• Liquids that are miscible in all proportions are calledas
Miscible Liquids
This principle of solvents are applied on aerosolproducts

5
Raoult’s Law
•
• Raoult’s law states that “the partial vapour pressure of each
volatile constituents is equal to the vapour pressure of the
pure constituents multiplied by its mole fraction in the
solution at a given temperature”
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6
Raoult’s Law
•
•
Raoult's law may be mathematically expressed as
Raoult’s law is appropriately suited to describe an ideal solution
• When two liquids are mix d, the vapour pressure of each one is
reduced by the presence of other by the extent of dilution of
eachphase
Partial vapour pressure= vapour pressure of pure liquid X mole
fraction of liquid

7
Ideal Solutions
•
•
•
•
Heat is neither absorbed nor evolved during mixing
No shrinkage or expansion when liquids are mixed
Examples of ideal solutions are –methanol-water, benzene-
toluene
These liquids have similar properties, i.e., attractive forces are
in completeuniformity
Ideal solution is defined as the one in which there is no change
in the properties of the components other than dilution, when
they are mixed to form a solution

LastBenchPharmacist.blogspot.com 8
Nonideal or Real Solutions
• Most liquid mixtures show varying degree of deviation from
Raoult’s law, i.e. which does not obey Raoult’s law; these
solutions are real or non ideal solutions
• When solute-solute, solute-solvent and solvent-solvent
interactions are unequal, these deviations are observed
• Typical examples are- carbon tetrachloride and cyclohexane,
chloroform and acetone
• Equations(1)and (2)maybemodifiedas:
• αA and αB indicates are activities of components A and B
respectively

9
Deviation From Raoult’s Law-PositiveDeviation
•
•
•
In some liquid systems, the vapour pressure is greater than
the sum of the partial pressures of the individual components
Such systems exhibit positive deviation from Raoult’s law
Examples are carbon tetrachloride and cyclohexane, water
and ethanol etc.
This type of behaviour occurs when the components differ in
their polarity, length of hydrocarbon chain and degree of
association

10
Deviation From Raoult’s Law-NegativeDeviation
•
•
•
•
In some liquid systems, the vapour pressure is less than the
sum of partial pressures of the individual components
Such systems are said to exhibit negative deviation from
Raoult’s law
Examples include chloroform and acetone, pyridine and acetic
acid etc.
This type of behaviour occurs when interactions such as
hydrogen bonding, salt formation and hydration occur
between the components

11
Summary
• Mathematical expression of Raoult’s law can be given by
• Miscible liquids- Liquids that are miscible in all proportions
are called as miscible liquids
• Raoult’s law – It states that the partial vapour pressure of
each volatile constituents is equal to the vapour pressure of
the pure constituents multiplied by its mole fraction in the
solution at a given temperature

12
Summary Continuation…..
•
•
Ideal solution- It is defined as the one in which there is no
change in the properties of the components other than
dilution, when they are mixed to form a solution
Nonideal or real solution- Liquid mixtures that show varying
degree of deviation from Raoult’s law, are real or non ideal
solutions
Positive deviations from Raoult’s law- The systems in which
the vapour pressure is greater than the sum of the partial
pressures of the individual components

1
Contents of ThisChapter
•
•
•
• Nicotine- water system
Triethylamine- water system
Concept of phase rule and itsapplications
Concepts of partially miscible liquids Phenol watersystem,
critical solution temperature and itsapplications

2
Learning Objectives
• At the end of this lecture, student will be ableto
-Explain phase rule and itsapplications
-Describe the principle and applications of partially miscible
liquids
-Describe the concept of phenol water system and critical
solution temperature
- Explain the applications of phenol-water system and
trietylamine-water system

3
Phase Rule
• Phase is defined as a homogenous physically distinct portion of
a system that is separated by bounding surfaces from each
other
Examples of independent variables are temperature, pressure
and concentration
Phase rule is a device for relating the effect of the least number
of independent variables upon the various phases
that can exist in equilibrium system containing a given number
of components
•
•

4
Phase Rule and ItsApplications
•
•
Where, F= number of degree of freedom
C= number ofcomponents
P= number of phases
It is known as Gibb’s phase rule,
mathematically as:
Applications of phase ruleare:
-in determining the purity of a substance
-in the solubility phenomenon
F = C –P +2……..(1)
and may be stated

5
Partially MiscibleLiquids
•
•
•
Partially miscible liquids or conjugate liquids are defined as a
two phase liquid system in which their mutual solubility in one
another is limited
As such when temperature is increased, the mutual solubility
of one liquid in another increases
Miscibility temperature is defined as the temperature at
which two conjugate solutions are mutually soluble

6
•
•
When solubility of one liquid in another liquid is plotted
against miscibility temperature, a specific pattern isobtained
These solubility temperature profiles are known as miscibility
curves or phasediagrams
The miscibility temperature is identified either by the
disappearance of turbidity or by reappearance of turbidity
•

7
• Typical examples of partially miscible liquidsare:
- phenol-water system
- trimethylamine-water system
-nicotine –water system
•Applications of phase diagramsare:
-to decide the proportion of two liquids to be taken during
formulation of solutions
-testing purity of aliquid

8
Phenol- Water System
• The miscibility pattern of phenol-water system can be shown
as below:

9
•
•
•
•
The left hand side of the curve represents the % W/Wof
phenol in water at varioustemperature
The two curves meet at a maximum temperature of66.80C
The right hand side of the curve represents % W/W of water
in phenol at various temperature
The critical solution temperature (CST) is defined as maximum
temperature at which the two conjugate solutions(layers)
merge into one layer at all proportions

LastBenchPharmacist.blogspot.com 10
•
•
•
•
•
CST of phenol water system is66.80C
Outside the curves, pheol and water aremiscible
CST is also known as upper consolutetemperature
At any temperature above CST phenol and water are miscible
in all proportions
The Tie line is represented by the line drawn parallel to the
base line from two points on the curve at any temperature in
the phase diagram of partially miscible liquids

11
Phenol- Water System-Applications
•
•
CST is a characteristic of a system and used for testingthe
purity of asubstance
The method can be used to determine the percentage
composition of added component in the conjugatesolution
The phenol-water miscibility curve suggests that 76% w/w
phenol corresponds to 80 % w/v solution, which should be
usedin dispensing

12
Triethylamine-Water System
• The temperature composition curve of trimethylamineand
water is shown below:

13
Triethylamine-Water System
The left hand side of the curve indicates the miscibility of
triethylamine in water
•
•
•
The right hand side of the curve indicates the solubility of
water in triethylamine
Lower consolute temperature is defined as the minimum
temperature at which the two conjugate solutions are
miscible in all proportions
The lower consolute temperature for triethylamine-water
system is 18.50C

14
Nicotine- Water System
• The temperature composition curve of nicotine –watersystem
is shownbelow:

15
Nicotine- Water System
•
•
•
At higher temperature, the mutual solubilitydecreases
This system exhibit both lower (60.80C) and upper (2080C)
critical solutiontemperature
At room temperature, nicotine and water are miscible in all
proportions

16
Summary
•
•
Phase rule-can be representedby:
F = C –P +2
Partially miscible liquids- Partially miscible liquids or conjugate
liquids are defined as a two phase liquid system in
whichtheirmutualsolubilityin oneanother islimited
Phase- It is defined as a homogenous physically distinct portion
of a system that is separated by bounding surfaces from each
other

17
Summary
•
• Lower consolute temperature- it is defined as the minimum
temperature at which the two conjugate solutions are miscible
in all proportions
Critical solution temperature- The critical solution temperature
(CST) is defined as maximum temperature at which the two
conjugate solutions(layers) merge into one layer at all
proportions

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