During hearing of a court case, the judge suspected that some changes in the documents had been carried out. H

NCERT Exemplar · Q7

Q. During hearing of a court case, the judge suspected that some changes in the documents had been carried out. He asked the forensic department to check the ink used at two different places. According to you which technique can give the best results?

(i) Column chromatography

(ii) Solvent extraction

(iii) Distillation

(iv) Thin layer chromatography

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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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The problem is about separating and comparing the components of two ink samples to detect forgery. Thin layer chromatography (TLC) is the best technique because it is simple, quick, and highly sensitive for separating non-volatile mixtures like inks, allowing direct visual comparison of the colour bands.

The judge’s suspicion is that the ink used in two different places on a document might not be the same — perhaps one part was written later with a different pen. The forensic lab needs a method that can separate the dye mixture in each ink and let them compare the patterns side by side. This is a classic separation problem where the components are non-volatile, coloured, and present in very small amounts.

Let’s think about why each technique is or isn’t suitable.

Column chromatography is excellent for separating mixtures on a preparative scale — you pack a column with adsorbent, run the sample through, and collect fractions. But it is time-consuming, requires more sample, and is not the most convenient for a quick side-by-side comparison of two tiny ink spots. For forensic work, you want something faster and more visual.
Solvent extraction relies on differences in solubility between two immiscible liquids. Inks are complex mixtures of dyes, pigments, and binders that are often all soluble in the same solvent. You would not get clean separation of individual components; you’d just move the whole ink from one phase to another. That tells you nothing about whether the inks are different.
Distillation separates based on boiling points. Ink components are mostly non-volatile solids (dyes) that decompose before they boil. Distillation is completely useless here — it is meant for liquids like petroleum or alcohol, not for analysing dried ink on paper.
Thin layer chromatography (TLC) is the perfect fit. You scrape a tiny bit of each ink spot, dissolve it in a suitable solvent, and spot both solutions side by side on a TLC plate coated with silica gel. The plate is placed in a jar with a shallow layer of developing solvent. As the solvent rises by capillary action, the different dye components travel at different rates, producing a pattern of coloured bands for each ink. …

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Q.During hearing of a court case, the judge suspected that some changes in the documents had been carried out. He asked the forensic department to check the ink used at two different places. According to you which technique can give the best results?

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