Assertion (A): Simple distillation can help in separating a mixture of propan-1-ol (boiling point 97°C) and pr

NCERT Exemplar · Q55

Q. Assertion (A): Simple distillation can help in separating a mixture of propan-1-ol (boiling point 97°C) and propanone (boiling point 56°C). Reason (R): Liquids with a difference of more than 20°C in their boiling points can be separated by simple distillation.

(i) Both A and R are correct and R is the correct explanation of A.

(ii) Both A and R are correct but R is not the correct explanation of A.

(iii) Both A and R are not correct.

(iv) A is not correct but R is correct.

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Concept understanding — Separation Techniques

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.

Note

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

Important

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 $100^\circ\text{C}$ 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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Simple distillation works when the boiling-point difference is large enough — here the difference is $41^\circ\text{C}$ , so both Assertion and Reason are correct, and the Reason correctly explains the Assertion.

The key idea is that simple distillation separates liquids based on boiling-point differences. When one component vaporises much more readily than the other, the vapour can be condensed and collected as a pure distillate. The rule of thumb — a difference of more than $20^\circ\text{C}$ — ensures negligible overlap in boiling ranges, so the separation is clean.

Let’s check the numbers. Propanone boils at $56^\circ\text{C}$ and propan-1-ol at $97^\circ\text{C}$ . The difference is $97 - 56 = 41^\circ\text{C}$ , which is well above $20^\circ\text{C}$ . So the mixture satisfies the condition for simple distillation.

Why the Reason is correct.

The statement in R is a standard guideline: if two liquids differ by more than $20^\circ\text{C}$ in boiling point, simple distillation is sufficient. This is because the more volatile component (lower boiling point) will vaporise almost completely before the other begins to boil significantly. No fractional distillation column is needed.
Why the Assertion is correct.

Propanone (bp $56^\circ\text{C}$ ) is far more volatile than propan-1-ol (bp $97^\circ\text{C}$ ). On heating the mixture, propanone vaporises first and can be condensed and collected, leaving propan-1-ol behind. The $41^\circ\text{C}$ gap makes this a straightforward separation.
Connecting A and R. …

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Q.Assertion (A): Simple distillation can help in separating a mixture of propan-1-ol (boiling point 97°C) and propanone (boiling point 56°C). Reason (R): Liquids with a difference of more than 20°C in their boiling points can be separated by simple distillation.

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Q.Assertion (A): Simple distillation can help in separating a mixture of propan-1-ol (boiling point 97°C) and propanone (boiling point 56°C).
Reason (R): Liquids with a difference of more than 20°C in their boiling points can be separated by simple distillation.

Unlock everything free for 14 days