What is Fractional Distillation? Definition, Principle, and Examples
Fractional distillation is essential in chemistry and helps students understand various practical and theoretical applications related to this topic. It is a core separation technique used in the lab and in numerous industries, from petroleum to the purification of chemicals.
What is Fractional Distillation in Chemistry?
A fractional distillation refers to a method for separating a mixture of liquids based on differences in their boiling points. When the boiling point difference between the liquids is less than 25°C, fractional distillation is preferred over simple distillation.
This concept appears in chapters related to separation techniques, physical chemistry, and industrial chemistry, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
Fractional distillation involves mixtures, not a single compound, so there is no molecular formula. It commonly separates mixtures like ethanol and water, acetone and methanol, or the many hydrocarbon components in crude oil. Each mixture contains compounds with different boiling points and molecular weights.
Preparation and Synthesis Methods
- In laboratory fractional distillation, a mixture is placed in a distillation flask equipped with a fractionating column and heated.
- The process ensures the separation of liquids by collecting fractions as they vaporize and condense at different levels.
- Industrially, large fractionating towers are used—such as in the refining of crude oil or the separation of liquid air into oxygen and nitrogen.
Physical Properties of Fractional Distillation (Optional)
Fractional distillation relies on the physical property of boiling point. The boiling point is the temperature at which a liquid turns into vapor. The method also uses the volatility of components, condenser cooling, and the efficiency of the fractionating column to achieve high-purity separation.
Chemical Properties and Reactions
Fractional distillation does not cause chemical reactions; it is a physical process used to isolate or purify liquids. The chemical structure of each compound remains unchanged by the separation process.
Frequent Related Errors
- Confusing fractional distillation with chemical change or reaction.
- Using simple distillation for mixtures with close boiling point differences.
- Not considering the need for a fractionating column for effective separation.
- Assuming fractional distillation can separate azeotropes (special mixtures sometimes need further steps).
- Misinterpreting the role of column trays or packing material.
Uses of Fractional Distillation in Real Life
Fractional distillation is widely used in the separation of crude oil into gasoline, diesel, kerosene, and other petrochemicals. It is also used to purify ethanol in labs, separate air into oxygen and nitrogen, and produce industrial solvents and perfumes.
Everyday examples include the distillation of alcoholic spirits and the production of high-purity chemicals for pharmaceuticals.
Relation with Other Chemistry Concepts
Fractional distillation connects directly to boiling point, mixture separation, and vapour-liquid equilibirium. It is also related to purification, physical changes versus chemical changes, and the study of purification of substances.
Step-by-Step Reaction Example
1. Place the mixture (e.g., ethanol and water) in a distillation flask.2. Attach a fractionating column on top of the flask. Add a thermometer and connect to a condenser.
3. Heat the mixture slowly. The vapor of the more volatile component (with a lower boiling point) rises first.
4. As vapors travel up, they repeatedly condense and vaporize in the column, enriching in the more volatile component.
5. The vapor passes into the condenser, cools down, and collects as the distillate in the receiving flask.
6. The component with the higher boiling point remains in the original flask or distills off at a higher temperature.
7. Final Answer: Ethanol (boiling point 78°C) is separated from water (boiling point 100°C) as the distillate.
Try This Yourself
- Draw a labelled diagram of a fractional distillation apparatus.
- List three mixtures used in fractional distillation and identify the component obtained first.
- State why a fractionating column is used instead of a simple condenser for some mixtures.
- Explain the importance of boiling points in mixture separation.
Final Wrap-Up
We explored fractional distillation—its setup, principle, process, and real-world significance in fields like petroleum refining and air separation. For deeper learning and exam prep, you can find more examples and practical videos on Vedantu’s Chemistry notes and live sessions.
Understanding fractional distillation equips you with knowledge for both academics and industry.
Comparison of Simple and Fractional Distillation
| Parameter | Simple Distillation | Fractional Distillation |
|---|---|---|
| Boiling Point Difference | More than 25°C | Less than 25°C |
| Apparatus | Only condenser and flask | Fractionating column included |
| Number of Components Separated | Usually two | Two or more |
| Purity of Separation | Moderate | High |
| Example | Water and ink | Petroleum refining, ethanol-water separation |
Common Examples of Mixtures Separated by Fractional Distillation
- Crude oil into petrol, diesel, kerosene, and other fractions
- Liquid air into nitrogen, oxygen, and argon
- Ethanol and water (alcoholic spirits)
- Acetone and methanol
- Benzene and toluene
Fractional Distillation of Petroleum: Fraction Table
| Fraction | Carbon Atoms | Boiling Range (°C) | Main Use |
|---|---|---|---|
| LPG/Gaseous Hydrocarbons | C1–C4 | Up to 40 | Cooking fuel, heating |
| Petrol (Gasoline) | C5–C10 | 40–180 | Car fuel |
| Kerosene | C10–C16 | 180–250 | Jet fuel, lighting |
| Diesel | C15–C18 | 250–350 | Trucks, trains |
| Lubricating Oil | C16–C20 | 350–450 | Machines, engines |
| Bitumen / Residue | C20+ | 450+ | Road surfacing, roofing |
Internal Links: Explore Related Chemistry Topics
FAQs on Fractional Distillation in Chemistry: Meaning, Process, and Uses
1. What is fractional distillation in Chemistry?
Fractional distillation is a separation technique that uses different boiling points to separate components of a liquid mixture. It is commonly used to separate substances like crude oil, alcohol, and liquid air into their individual parts.
2. Give 5 examples of mixtures that can be separated by fractional distillation.
Common mixtures separated by fractional distillation include:
• Crude oil (into petrol, diesel, etc.)
• Liquid air (into oxygen, nitrogen, argon)
• Alcohol and water mixtures
• Acetone and water
• Industrial solvents with varying boiling points
3. What is the main difference between simple and fractional distillation?
The key difference is:
Simple distillation separates mixtures with very different boiling points (over 25-30°C apart), while fractional distillation is used for mixtures with closer boiling points by using a fractionating column for better separation and purity.
4. What are the uses of fractional distillation in everyday life?
Fractional distillation is essential for:
• Refining petroleum into fuels and chemicals
• Producing liquefied gases like oxygen and nitrogen from air
• Separating alcohol from water in beverages and industry
• Purifying solvents used in laboratories and manufacturing
5. How does a fractional distillation column work?
A fractional distillation column allows repeated vaporization and condensation cycles. This process gradually enriches each fraction with components closest to its boiling point, improving the purity of separated substances as vapors rise and condense along the column trays or packing.
6. Why are trays or packing material used inside a fractional distillation column?
Trays or packing materials create more surface area, allowing vapors and liquids to contact and exchange heat repeatedly. This increases separation efficiency, producing purer fractions during the distillation process.
7. Can azeotropic mixtures be separated by fractional distillation?
Most azeotropic mixtures cannot be completely separated by ordinary fractional distillation because their components form a constant boiling mixture. Special methods like azeotropic or extractive distillation are needed for further separation.
8. How does pressure affect fractional distillation efficiency?
Pressure changes the boiling points of mixture components. Reducing pressure (vacuum distillation) allows separation of substances with high boiling points at lower temperatures, protecting them from decomposition and improving separation efficiency.
9. What is the impact of column height on the purity of fractions?
A taller fractionating column provides more surface for repeated vaporization-condensation cycles (theoretical plates). This results in greater purity of separated components since each vapor rises, cools, and condenses multiple times.
10. What safety precautions are necessary during laboratory fractional distillation?
Important safety steps include:
• Use heat-resistant glassware and clamps
• Do not overfill the distillation flask
• Wear safety goggles and gloves
• Ensure proper ventilation
• Monitor temperature and pressure to prevent accidents
11. What is the principle behind fractional distillation?
Fractional distillation works on the principle that different liquids have different boiling points. By heating the mixture, the component with the lowest boiling point vaporizes first, and components are separated as vapors condense at various heights in the column.
12. What are the main products obtained from fractional distillation of petroleum?
Main products include:
• Petroleum gas
• Petrol (gasoline)
• Kerosene
• Diesel
• Lubricating oil
• Bitumen
Each fraction is collected at different temperatures based on boiling point differences.
