Fractional distillation is a core separation technique in IB Chemistry Topic 1 (Stoichiometry) and Topic 11 (Measurement & Analysis). It is used to separate mixtures of liquids with different boiling points, especially when those boiling points are close together. Fractional distillation is essential for purifying chemicals, refining crude oil, producing fuels, and manufacturing solvents. Understanding how it works helps students master practical chemistry and industrial processes.
What Is Fractional Distillation?
Fractional distillation is a separation technique that uses differences in boiling points to separate a mixture of liquids.
It is essentially an extended or improved form of simple distillation, designed for mixtures where boiling points are close.
Fractional distillation uses:
- A fractionating column
- Repeated cycles of vaporization and condensation
- Temperature gradients
These features produce much better separation than simple distillation.
Why Fractional Distillation Is Needed
Simple distillation works when components differ greatly in boiling point (e.g., 30°C or more).
However, when boiling points are close:
- Vapors may contain multiple components
- Separation becomes poor
- Distillate is impure
Fractional distillation solves this problem by using multiple vaporization–condensation cycles to isolate each substance more cleanly.
How Fractional Distillation Works
The apparatus typically includes:
- A heat source
- A distillation flask
- A fractionating column
- A condenser
- A receiving container
Here’s what happens step-by-step:
1. The mixture is heated
Liquids begin to evaporate based on their boiling points.
2. Vapors rise into the fractionating column
The column is filled with glass beads, plates, or metal surfaces to create surface area.
3. Repeated condensation and vaporization occur
As vapors rise:
- Higher-boiling-point vapors condense and fall back
- Lower-boiling-point vapors continue upward
This process is called fractionation.
4. The component with the lowest boiling point reaches the top first
It enters the condenser, cools, and collects as a purified liquid.
5. Increase temperature gradually
Other components boil off in order of increasing boiling point.
Each purified component collected is called a fraction.
Key Concept: Temperature Gradient
The fractionating column maintains a temperature gradient:
- Hot at the bottom
- Cooler at the top
This gradient ensures that:
- Only the most volatile vapors reach the top
- Less volatile vapors condense before rising too high
This is why fractional distillation produces good separation.
Fractional Distillation in Industry
The most important industrial application is crude oil refining.
Crude oil contains hundreds of hydrocarbons with different boiling ranges.
Fractional distillation separates it into useful fractions:
Common fractions (approximate boiling ranges):
- Refinery gas (below 20°C)
- Gasoline/Petrol (30–200°C)
- Naphtha (100–250°C)
- Kerosene (150–300°C)
- Diesel (250–350°C)
- Fuel oil (300–400°C)
- Residue/Bitumen (above 400°C)
Each fraction is used to produce fuels, plastics, solvents, or industrial materials.
Laboratory Fractional Distillation
In the lab, fractional distillation is used to:
- Purify organic liquids
- Remove impurities
- Separate ethanol from water
- Recover solvents
IB Chemistry students often practice with ethanol–water mixtures.
Why Fractional Distillation Matters in IB Chemistry
Students learn fractional distillation because it demonstrates:
- Intermolecular forces
- Boiling point trends
- Purification methods
- Real-world chemical processing
- Energy and resource management
It directly connects theory to industrial chemistry.
Common IB Misunderstandings
“Fractional distillation works only for crude oil.”
It works for any liquid mixture with close boiling points.
“All liquids come off at once.”
Each fraction boils off in order of increasing boiling point.
“Simple distillation can separate close-boiling liquids.”
Not effective—fractionating column is required.
“Vapor composition stays constant.”
Composition changes continuously as boiling progresses.
FAQs
Why does the fractionating column improve separation?
Because repeated condensation/vaporization cycles allow only the most volatile molecules to rise to the top.
Can fractional distillation separate azeotropes?
Not completely—azeotropes behave as single substances and require special methods.
Is fractional distillation reversible?
Yes—the components can be recombined.
Conclusion
Fractional distillation is an essential technique for separating liquid mixtures with similar boiling points. By using a fractionating column and repeated vaporization–condensation cycles, it produces highly purified fractions. This technique is crucial in crude oil refining and laboratory purification, making it a key concept in IB Chemistry.
