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 and what is its fundamental principle?
Fractional distillation is a laboratory and industrial process used to separate a mixture of two or more miscible liquids that have different, but often close, boiling points. The fundamental principle behind it is the difference in volatility of the components. When the mixture is heated, the liquid with the lower boiling point vaporises more readily, rises through a fractionating column, cools, and condenses, allowing it to be collected separately.
2. What is the key difference between simple distillation and fractional distillation?
The primary difference lies in the apparatus and its application. Simple distillation is effective for separating liquids with a large difference in boiling points (typically >25°C) or for separating a liquid from a non-volatile solid. Fractional distillation uses a fractionating column, which provides a large surface area (through glass beads, rings, or trays) for repeated cycles of vaporisation and condensation. This makes it suitable for separating liquids with very close boiling points.
3. What are the main steps involved in the fractional distillation process?
The process of fractional distillation involves several key stages:
- Heating: The liquid mixture in the distillation flask is heated, causing the components to start boiling and turning into vapour. The most volatile component (lowest boiling point) vaporises first.
- Fractionation: The vapour mixture rises into the fractionating column. As it rises, it cools, condenses on the packing material, and re-vaporises multiple times. With each cycle, the vapour becomes progressively richer in the more volatile component.
- Condensation: The purified vapour of the most volatile component reaches the top of the column, enters the condenser, and is cooled by circulating water, turning it back into a liquid.
- Collection: This condensed, purified liquid, known as the distillate, is collected in a receiving flask.
4. How does a fractionating column work to separate liquids with close boiling points?
A fractionating column creates a temperature gradient, being hottest at the bottom and coolest at the top. The packing material inside (like beads or rings) provides a large surface area. When the vapour mixture rises, it cools and condenses on this surface. The heat from the rising vapour below causes this condensate to re-vaporise. This process of successive distillations occurs many times along the column. At each stage, the vapour becomes more enriched with the lower-boiling-point component, while the liquid flowing back down becomes enriched with the higher-boiling-point component, leading to a highly effective separation.
5. When should you use fractional distillation instead of other separation techniques?
You should use fractional distillation specifically when you need to separate a mixture of miscible liquids with boiling points that are close to each other (generally less than 25°C apart). If the boiling points are far apart, simple distillation is more efficient. If the substances are immiscible (like oil and water), a separating funnel would be used. For separating a soluble solid from a liquid, evaporation or crystallisation is the appropriate method.
6. What are some common real-world examples of fractional distillation?
Fractional distillation is a critical process in many industries. Key examples include:
- Crude Oil Refining: Separating crude petroleum into useful fractions like petrol, diesel, kerosene, and lubricating oil.
- Alcohol Production: Concentrating ethanol from the fermented mixture to produce alcoholic spirits and purified industrial alcohol.
- Separation of Air: Liquefied air is separated into its primary components—nitrogen, oxygen, and argon—which have different boiling points.
- Chemical Purification: Purifying solvents and separating products from reactants in the chemical manufacturing industry.
7. How is fractional distillation used in the petroleum industry to refine crude oil?
In an oil refinery, crude oil is heated to a high temperature (around 400°C) and piped into the bottom of a tall fractionating tower. The hot liquid and vapour mixture rises. As the vapours rise, they cool and condense at different levels (on trays) according to their boiling points. Fractions with high boiling points (like bitumen and lubricating oil) condense at the bottom, while fractions with low boiling points (like petrol and petroleum gas) rise higher before condensing.
8. What factors affect the efficiency and purity of separation in fractional distillation?
Several factors determine how well the separation works:
- Column Height and Packing: A taller column with more packing material provides a larger surface area, allowing for more vaporisation-condensation cycles (more 'theoretical plates'), which leads to purer fractions.
- Temperature Gradient: A stable and consistent temperature difference between the bottom and top of the column is crucial for effective separation.
- Pressure: Operating under a vacuum (vacuum distillation) lowers the boiling points of the components, which is essential for separating high-boiling or heat-sensitive compounds that might decompose at their atmospheric boiling point.
9. Why can't standard fractional distillation completely separate an azeotropic mixture?
An azeotropic mixture is a special blend of liquids (like 95.6% ethanol and 4.4% water) whose vapour has the same composition as the liquid. This means the mixture boils at a constant temperature without changing its composition. Because fractional distillation relies on the difference between liquid and vapour compositions to achieve separation, it cannot separate the components of an azeotrope beyond this specific concentration. Special techniques like azeotropic or extractive distillation are needed for further purification.
10. What are the essential safety precautions to follow during a laboratory fractional distillation experiment?
Safety is paramount during any distillation. Key precautions include:
- Always wear safety goggles and appropriate gloves.
- Ensure the apparatus is securely clamped and on a stable surface.
- Never heat a sealed system. The apparatus must be open to the atmosphere (e.g., at the collection adapter) to avoid pressure build-up.
- Use boiling chips or a magnetic stirrer to ensure smooth boiling and prevent bumping.
- Do not fill the distillation flask more than two-thirds full.
- Work in a well-ventilated area or a fume hood, especially when using flammable or toxic liquids.
