Distillation is generally effective for separating mixtures of liquids with different boiling points.
However, two substances that cannot be separated by distillation are those with identical boiling points or those that form azeotropes.
Azeotropes are mixtures that boil at a constant temperature, and the composition of the vapor is the same as the liquid.
4 Key Points to Understand Why Some Substances Cannot Be Separated by Distillation
1. Substances with Identical Boiling Points
If two substances have exactly the same boiling point, they cannot be separated by distillation.
This is because they will vaporize and condense together.
The vapor pressure of each component contributes equally to the total vapor pressure at the boiling point.
So, they cannot be separated based on volatility.
This scenario is rare but can occur with isomers or closely related compounds.
2. Azeotropes
Azeotropes are mixtures that behave as a single component because they have a constant boiling point.
The composition of the vapor is identical to the liquid.
So, even though distillation is applied, the mixture remains unchanged.
There are two types of azeotropes: minimum boiling azeotropes and maximum boiling azeotropes.
Minimum boiling azeotropes have a boiling point lower than any of their components.
Maximum boiling azeotropes have a boiling point higher than any of their components.
An example of a minimum boiling azeotrope is the mixture of ethanol and water.
This mixture boils at 78.15°C, a temperature lower than the boiling point of pure ethanol (78.3°C) or water (100°C).
This makes it impossible to distill ethanol to 100% purity by simple distillation.
Summary
Distillation fails to separate substances with identical boiling points because they vaporize and condense together.
It also fails with azeotropes because the vapor and liquid compositions remain the same throughout the distillation process.
These limitations highlight the importance of understanding the properties of substances before attempting to separate them by distillation.
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