Q.What is Fractional distillation? When is it used?
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Separation Techniques: The Intuition
Imagine you're making chai. You boil tea leaves in water, then pour the liquid through a strainer. The strainer catches the leaves; the tea flows through. You just performed a separation technique — you took a mixture (tea leaves + water) and isolated one component (the liquid tea) from the other (the solid leaves).
Every separation technique answers one question: How do I pull apart things that are mixed together, using their differences?
The key insight: You cannot separate things that are identical. Separation works only because the components of a mixture differ in at least one physical or chemical property — size, density, boiling point, solubility, magnetic behaviour, or something else.
Separation techniques are physical processes. They do not change the chemical identity of the substances. The tea leaves remain tea leaves; the water remains water. No chemical reaction occurs.
The Precise Statement
Separation techniques are methods used to isolate individual components from a mixture by exploiting differences in their physical or chemical properties. The goal is to obtain one or more pure substances from a mixture, without altering the chemical nature of the components.
The Core Idea in One Sentence
Every separation technique works because the components of the mixture differ in at least one measurable property. The method you choose depends entirely on which property differs.
A Quick Map of Common Separation Techniques
Here is how the choice of technique follows from the property difference:
| Property Difference | Technique | Example |
|---|---|---|
| Particle size | Sieving, Filtration | Separating stones from sand; tea leaves from water |
| Density | Centrifugation, Decantation, Sedimentation | Separating cream from milk; mud from water |
| Boiling point | Distillation | Separating alcohol from water; crude oil into fractions |
| Solubility | Evaporation, Crystallisation | Obtaining salt from seawater |
| Magnetic property | Magnetic separation | Separating iron filings from sulphur powder |
| Volatility | Sublimation | Separating camphor from sand |
| Attraction to a stationary phase | Chromatography | Separating pigments in ink or plant leaves |
How to Think About Any Separation Problem
When you see a mixture and need to decide how to separate it, ask yourself:
- What are the components? (List them.)
- What property is different between them? (Size? Density? Boiling point? Solubility? Magnetism?)
- Which technique exploits that difference? (Match the property to the method above.)
That three-step chain — mixture → property difference → technique — is the entire logic of separation.
A Concrete Walkthrough
Mixture: Sand and iron filings.
- Components: Sand (silicon dioxide) and iron (metal).
- Property difference: Iron is magnetic; sand is not.
- Technique: Magnetic separation. Pass a magnet over the mixture. Iron filings stick to the magnet; sand is left behind.
Mixture: Salt dissolved in water.
- Components: Salt (sodium chloride) and water.
- Property difference: Water boils at 100∘C and evaporates; salt does not boil or evaporate — it remains as a solid.
- Technique: Evaporation (or distillation, if you also want to collect the water). Heat the solution. Water turns to vapour and leaves; salt crystals remain in the dish.
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Fractional distillation is a purification technique used to separate a mixture of two or more miscible liquids whose boiling points are close to each other, using a fractionating column. …
Fractional distillation separates miscible liquids with close boiling points, using a fractionating column that provides repeated vaporisation-condensation.
Ordinary/simple distillation works well only when the boiling points of the two liquids in a mixture differ by a large amount (more than about 25 K). When the boiling points are close, simple distillation cannot cleanly separate them, so fractional distillation is used instead.
In fractional distillation, the mixture is heated in a flask fitted with a fractionating column (packed with glass beads or having bubble-cap trays) placed between the flask and the condenser. As vapour rises through the column, it repeatedly undergoes partial condensation and re-vaporisation at each stage of the column: the vapour is progressively enriched in the more volatile (lower boiling point) component, while the less volatile component condenses back down. By the time vapour reaches the top of the column and enters the condenser, it consists almost entirely of the lower-boiling component, which distils out first; the higher-boiling component is collected later (or remains in the flask).
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- CBSE 2026Set ANNUAL1 markQ.Fill in the blank: The word 'Chromatography' is derived from the Greek word ______.
›Reveal solutionSolution
'Chromatography' comes from Greek 'chroma' (colour) + 'graphein' (to write), because the technique was first developed to separate coloured substances.
Chromatography is a widely used technique for separating, purifying, and identifying the components of a mixture, based on differences in how strongly they interact with a stationary phase versus a mobile phase. The name reflects its historical origin: the Russian botanist Mikhail Tsvet used the technique in the early 1900s to separate coloured plant pigments (like chlorophylls and xanthophylls) as distinct coloured …
- CBSE 2026Set ANNUAL1 markQ.Some solid substances change from solid to vapour state without passing through liquid state. Write the name of above purification method.
›Reveal solutionSolution
Sublimation is both the phenomenon of solid-to-vapour transition (without a liquid stage) and the purification technique that exploits it.
Certain solids, such as camphor, naphthalene, anthracene, benzoic acid, and iodine, possess a significant vapour pressure even in the solid state and convert directly to vapour on heating, bypassing the liquid state entirely; on cooling, the vapour re-solidifies directly (sublimes back). This property is used as a purification technique called sublimation: a mixture containing a sublimable solid and non-sublimab …
- CBSE 2026Set ANNUAL1 markQ.Ethyl alcohol and methyl alcohol can be separated by ____________ method.
›Reveal solutionSolution
A mixture of ethyl alcohol and methyl alcohol is separated by fractional distillation.
Ethanol (b.p. ≈ 78 C) and methanol (b.p. ≈ 65 C) are completely miscible liquids whose boiling points differ by only about 13 C. Two miscible liquids with boiling points fairly close together are separated using a fractionating column, which provides repeated vaporisatio …
- CBSE 2025Set ANNUAL1 markMCQQ.The best method for the separation of naphthalene and benzoic acid from their mixture is(a) Sublimation(b) Chromatography(c) Crystallisation(d) Distillation
›Reveal solutionSolution
Sublimation is the standard method used to separate naphthalene from benzoic acid.
Naphthalene is a volatile solid that sublimes readily on gentle heating (passing directly from solid to vapour without melting), while benzoic acid does not sublime under the same mild conditions. Heating the mixture causes naphthalene vapours to rise and be collected/condensed separately (e.g. on a cold surface), leaving the …
- CBSE 2025Set ANNUAL1 markQ.Chromatography is the technique used to separate the components of a mixture.
›Reveal solutionSolution
The statement is True: chromatography separates the components of a mixture based on their differing rates of movement between a stationary and a mobile phase.
Chromatography is a widely used separation and purification technique in which a mixture, dissolved in a suitable solvent (mobile phase), is passed over a stationary phase (such as paper, silica, or alumina). Different components of the mixture move at different rat …
- CBSE 2023Set annual21 markQ.The method used to separate a volatile solid form a non-volatile solid is called ..................... . (Fill in the blank)
›Reveal solutionSolution
The method is called sublimation, used to separate a sublimable (volatile) solid from a non-volatile solid.
Some solids (e.g. camphor, naphthalene, iodine, ammonium chloride) pass directly from the solid state to the vapour state on heating, without passing through the liquid state — this property is called sublimation.
To separate a mixture of a volatile (sublimable) solid and a non-volatile solid: the mixture is heated gently in a china dish covered with an inverted funnel (its stem plugged with cotton wool). The volatile solid sublimes and its vapours rise, striking the cooler surface of the funnel/watch glass, where they condense back into solid form (deposit as crystals). The non-volatile solid is left behind in the dis …
- CBSE 2023Set ANNUAL1 markQ.On which principle/law is paper chromatography based?
›Reveal solutionSolution
Paper chromatography works on the principle of partition — components distribute differently between the stationary phase (water in the paper) and the mobile phase (solvent).
In paper chromatography, a strip of filter paper acts as the stationary phase (its cellulose fibres hold a thin film of water), while a suitable solvent acts as the mobile phase, moving up (or along) the paper by capillary action.
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- CBSE 2020Set ANNUAL1 markQ.Fill in the blank: Paper chromatography is based on the principle of ___. (choose: adsorption / absorption)
›Reveal solutionSolution
Paper chromatography is a partition technique: the paper's cellulose fibres absorb and hold water as the stationary phase, and components of a mixture distribute (partition) between this absorbed water and the flowing solvent, based on the printed choice of 'adsorption' or 'absorption', the correct answer is absorption.
Chromatography techniques are broadly of two kinds:
- Adsorption chromatography (e.g. column chromatography with alumina/silica, thin-layer chromatography): the stationary phase is a solid surface, and separation happens because different components are adsorbed onto that solid surface with different strengths.
- Partition chromatography (e.g. paper chromatography): the stationary phase is a liquid (here, water) held by absorption within the pores/fibres of an inert support (the paper). Components separate because they distribute (partition) differently between this stationary liquid and the mobile solvent that flows over the paper by capillary action. …
- CBSE 2019Set ANNUAL1 markQ.What is the conversion of a solid substance on heating into vapours, without changing into the liquid state, known as?
›Reveal solutionSolution
Sublimation is the direct solid-to-vapour phase change (and its reverse, vapour-to-solid) without an intermediate liquid stage; it is also used as an organic-chemistry purification technique.
Most solids melt into a liquid before they can vaporise. However, certain substances (such as camphor, naphthalene, ammonium chloride, and iodine) possess a vapour pressure high enough that, on heating, they pass directly into the vapour phase without forming a liquid in between. This process is called sublimation. In organic chemistry it is exploited as a purificat …
- CBSE 2019Set ANNUAL1 markQ.What is the method of obtaining a pure substance by vaporisation of an impure liquid followed by condensation of the vapours, known as?
›Reveal solutionSolution
Distillation purifies a liquid by heating it to vaporise the liquid component and then condensing the vapours, separating it from non-volatile or higher-boiling impurities.
Distillation is one of the standard purification techniques taught for organic compounds. The impure liquid is heated in a distillation flask; the more volatile component vaporises first, travels through a condenser where it is cooled and converted back into a liquid, and is collected separately as the distillate. Non-volatile impurit …
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