Assertion (A): Components of a mixture of red and blue inks can be separated by distributing the components be

NCERT Exemplar · Q60

Q. Assertion (A): Components of a mixture of red and blue inks can be separated by distributing the components between stationary and mobile phases in paper chromatography. Reason (R): The coloured components of inks migrate at different rates because paper selectively retains different components according to the difference in their partition between the two phases.

(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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Paper chromatography separates ink components by exploiting their different partition coefficients between a stationary phase (paper) and a mobile phase (solvent). Both the assertion and reason are correct, and the reason directly explains why the separation occurs.

Paper chromatography is a separation technique that works on a beautifully simple principle: different substances have different affinities for two competing phases. When you spot a mixture on chromatography paper and allow a solvent to climb up through capillary action, each component in the mixture faces a tug-of-war between staying stuck to the paper (the stationary phase) and dissolving into the moving solvent (the mobile phase).

The key concept is partition coefficient. Each substance distributes itself between the two phases in a characteristic ratio. A component that loves the solvent more than the paper will spend more time dissolved and moving upward. A component that binds strongly to the paper will lag behind. This difference in migration rates is what pulls a mixture apart into its individual components.

For the red and blue inks, each dye molecule has a unique chemical structure that determines how it interacts with both the cellulose fibres in the paper and the solvent molecules. These interactions—hydrogen bonding, van der Waals forces, polarity matching—create different partition coefficients, and therefore different speeds of travel up the paper.

Let's examine the two statements:

Assertion (A) analysis: The assertion claims that red and blue ink components can be separated by distributing them between stationary and mobile phases in paper chromatography. This is precisely what happens. The paper acts as the stationary phase, the solvent (often water or alcohol) acts as the mobile phase, and the different dyes partition between them, leading to separation. The assertion is correct.
Reason (R) analysis: The reason states that coloured components migrate at different rates because paper selectively retains different components according to differences in their partition between the two phases. This is the mechanistic explanation for chromatographic separation. The word "selectively retains" captures the idea that each component has a different affinity for the paper, and "partition between the two phases" is the fundamental principle. The reason is correct. …

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