Azeotropic mixture
An azeotropic mixture is a mixture of two liquids that are boiled at a constant temperature or constant boiling point mixture and have a constant composition. The azeotropic mixture phenomenon occurs when the vapor phase of the mixture has the same composition as the liquid phase. And these are not separated by simple distillation.
This Story also Contains
- Azeotropic mixture
- Some Solved Examples
- Example.1
- Summary
After Raoult's work on the solutions and colligative properties, the idea of azeotropes was further studied by some other scientists in the early 20th century. These Scientists or researchers such as E. R. M. D. T. W. W. L. exact names can build upon Raoult's work to understand the thermodynamics of azeotropic mixture. The study of the azeotropic mixture has led to advancement in various fields as it is useful in the purification of solvents, petrochemical refining, and in the production of highly purified chemicals the chemical which needs to be highly pure azeotropes used to produce them. As the azeotropes are not separated by simple distillation new separation techniques are developed for their separation such as extractive distillation
Azeotropic mixture
An azeotropic mixture is a solution of two liquids having a certain composition in which both the gas phase and the liquid phase composition are the same i.e. XA=YA and XB=YB. These solutions distill over without changes in composition and hence, these cannot be separated by distillation. These solutions are formed by non-ideal solutions which show a large deviation from ideality.
These solutions boil at one particular temperature like a pure liquid and distill over in the same composition and hence are also referred to as constant boiling mixtures.
Types of Azeotropic Mixtures
It is of the following types:
Minimum Boiling Azeotropes
Non-ideal solutions showing large positive deviation from Raoult's law form minimum boiling azeotropes which boil at a temperature lower than the boiling point of either of the components 'A' or 'B'.
In the figure given below, point M represents the azeotropic composition. At this point, vapor pressure is maximum and therefore the solution has lowest boiling point.
![]()
e.g., Ethanol water mixture on fractional distillation gives a solution containing approximately 95 % by volume of ethanol. Once this composition is achieved, no further separation occurs.
Maximum Boiling Azeotropes
Non-ideal solutions showing large negative deviations from Raoult's law form maximum boiling azeotropes that boil at a temperature higher than the boiling point of either of the components 'A' or 'B'.
In the figure given below, the point B represents the azeotropic composition. At this point, vapor pressure is minimal and therefore the solution has the lowest boiling point.
![]()
e.g. Nitric acid and water are examples of this class of azeotrope. This azeotrope has the approximate composition, 68% nitric acid and 32% water by mass
Recommended topic video on ( Azeotropic mixture)
Some Solved Examples
Example.1
1. A liquid mixture that boils without changing constituents is called:
1)Stable structure complex
2)Binary liquid mixture
3)Zeotropic liquid mixture
4) (correct)Azeotropic liquid mixture
Solution
The azeotropic mixture is a mixture of two liquids that boil at on particular temperature like a pure liquid and distill over in the same composition.
Hence, the answer is the option (4).
Example.2
2. Which of the following mixtures form maximum boiling azeotrope?
1)H2O+ Benzene
2)Benzene + Toluene
3)C2H5OH
4) (correct)H2O+HNO3
Solution
Solution showing negative deviation from Raoult's law form maximum boiling azeotropes. The mixture of H2O and HNO3 form a non-ideal solution which shows a negative deviation from Raoult's law.
Hence, the answer is the option (4).
Example.3
3. Which of the following cannot form an azeotropic mixture?
1)H2O + C2H5OH
2) (correct)n-Hexane + n-heptane
3)HNO3 + H2O
4)None
Solution
Ideal Solutions do not form azeotropes.
a mixture of n-hexane and n-heptane is an ideal solution.
Hence, the answer is the option (2).
Example.4
4.The azeotropic mixture of water (b.p.100o C) and HCl (b.p.85o C) boils at 108.5oC. When this mixture is distilled it is possible to obtain:
1)Pure HCl
2)Pure water
3)Pure water as well as pure HCl
4) (correct)Neither HCl nor H2O in their pure states
Solution
An azeotropic mixture is a constant boiling mixture, it is not possible to separate the components of an azeotropic mixture by boiling.
Hence, the answer is the option (4).
Example.5
5. Which of the following mixtures form minimum boiling azeotrope?
1)HCl+H2O
2)H2O+HNO3
3)n-Hexane + n-Heptane
4) (correct)H2O+C2H5OH
Solution
Solutions showing Positive deviation from Raoult's law form minimum boiling azeotropes.
Since a mixture of H2O and C2H5OH shows positive deviation, it forms minimum boiling azeotropes.
Hence, the answer is the option (4).
Example.6
6. Among the following mixtures, dipole-dipole as the major interaction, is present in
1)Benzene and ethanol
2) (correct)acetonitrile and acetone
3)KCl and water
4)benzene and carbon tetrachloride.
Solution
Molecule Type of interaction
Benzene Van der Waals forces
Ethanol dipole-dipole interaction
Acetone dipole-dipole interaction
Acetonitrile dipole-dipole interaction
KCI Ionic bond
H2O dipole-dipole interaction
CCI4 Van der Waals forces
Between acetonitrile and acetone, the force of interaction is dipole-dipole Interaction.
Dipole-dipole interactions occur among the polar molecules. Polar molecules have permanent dipoles. The positive pole of one molecule is thus attracted by the negative pole of the other molecule. The magnitude of dipole-dipole forces in different polar molecules is predicted based on the polarity of the molecules, which in turn depends upon the electronegativities of the atoms present in the molecule and the geometry of the molecule ( in the case of polyatomic molecules, containing more than two atoms in a molecule ).
Hence, the answer is the option (2).
Summary
An azeotropic mixture is a special type of mixture that are also called a constant boiling point mixture because the boils at a constant temperature and has unique distillation Characteristics. Also, They have a constant composition means the vapor produced during boiling has the same ratio of components as the liquid. This occurs due to the intermolecular interactions between the components. The benefit of the azeotropic mixture is its ability to maintain a stable boiling point for the whole distillation process. With this stability, the separation of components is simple which allows the consistent boiling point. and this feature is very important in the industries in the production of ethanol, an ethanol-water azeotropes are formed which boil at a lower temperature than pure ethanol.
