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Purification questions ask one thing in many forms: which method would you use, and why? If you learn each technique as a list of steps, the questions feel random. If you learn the one physical difference each method uses, they become quick. This post goes through the main methods in NCERT, matches each to the mixtures it suits, works an Rf calculation and lists the practical-exam slips.
What does the syllabus ask for?
The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus list "crystallization, sublimation, distillation, differential extraction and chromatography - principles and their applications". The CBSE Chemistry curriculum for 2026-27 has crystallisation of an impure sample in the Class 11 practical and paper chromatography with Rf values in Class 12. Check the current documents for your exam.
Which difference does each method use?
From the NCERT chapter Organic Chemistry: Some Basic Principles and Techniques:
Sublimation: separates a sublimable compound from non-sublimable impurities.
Crystallisation: uses a solvent in which the compound is sparingly soluble at room temperature but appreciably soluble when hot. Impurities stay in the mother liquor. Coloured impurities can be removed with activated charcoal.
Simple distillation: separates liquids with a large difference in boiling points, or a volatile liquid from non-volatile impurities. NCERT's example is chloroform (334 K) from aniline (457 K).
Fractional distillation: for liquids with close boiling points. The fractionating column gives repeated condensation and vaporisation. It is used to separate fractions of crude oil.
Distillation under reduced pressure: for liquids that decompose at or below their normal boiling point. Lowering the pressure lowers the boiling point. NCERT's example is glycerol from spent lye in the soap industry.
Steam distillation: for compounds that are steam volatile and immiscible with water. The mixture boils when the two vapour pressures add up to atmospheric pressure, p₁ + p₂ = p, so it boils below 373 K. Aniline is purified this way.
Differential extraction: shaking an aqueous solution with an immiscible organic solvent in which the compound is more soluble, then separating the layers in a separating funnel.
Chromatography: separates components by how strongly they are held by a stationary phase as a mobile phase moves past.
Worked example 1: choose the method
o-nitrophenol and p-nitrophenol: steam distillation. The NCERT alcohols, phenols and ethers chapter explains that the ortho isomer is steam volatile because its hydrogen bond is intramolecular, while the para isomer is associated by intermolecular hydrogen bonds.
Two liquids boiling at 334 K and 457 K: simple distillation.
Two liquids boiling a few kelvin apart: fractional distillation.
Impure benzoic acid: crystallisation from hot water, as in the NCERT Class 11 chemistry laboratory manual.
Glycerol, which decomposes near its boiling point: distillation under reduced pressure.
How does chromatography separate a mixture?
NCERT describes two main types. In adsorption chromatography, a solid such as silica gel or alumina is the stationary phase; components that are adsorbed more strongly move more slowly. Column chromatography and thin layer chromatography (TLC) work this way. In partition chromatography, the stationary phase is a liquid. The NCERT Class 12 laboratory manual explains that in paper chromatography, water held in the pores of the paper is the stationary phase.
The retardation factor is:
Rf = (distance moved by the substance from the base line) / (distance moved by the solvent front from the base line)
The lab manual notes that Rf has no unit and is always less than one, because the solvent front moves fastest. Colourless spots can be seen under UV light, with iodine vapour, or with a spray reagent such as ninhydrin for amino acids, as NCERT describes.
Worked example 2: calculating Rf
A paper strip has its base line 3.0 cm from the bottom edge. After developing, the solvent front is 12.0 cm above the base line. A leaf extract gives two spots, with centres 3.0 cm and 8.4 cm above the base line.
Spot A: Rf = 3.0/12.0 = 0.25
Spot B: Rf = 8.4/12.0 = 0.70
Spot B is held less strongly by the stationary phase. If you measured from the paper's edge instead, spot A would give 6.0/15.0 = 0.40, a wrong answer. Always measure from the base line, to the centre of the spot.
How do you know the compound is pure?
The lab manual states that pure solids and liquids have sharp melting and boiling points, so these are used as criteria of purity. An impure solid melts over a range. Our post on colligative properties explains why impurities change these temperatures in solutions.
Which mistakes cost marks in the practical?
Drawing the base line in ink, which dissolves and runs. Use pencil.
Letting the solvent touch the spots at the start.
Disturbing the jar or leaving the lid off while the chromatogram develops.
Evaporating all the solvent during crystallisation, or cooling it fast, which gives small or disfigured crystals.
Washing benzoic acid crystals with alcohol, in which they dissolve. The lab manual says to use cold water.
A practice routine
Make a table in your notebook with three columns: method, physical difference, NCERT example. Fill it from memory.
Write ten "which method?" questions for a friend, and answer theirs.
Calculate Rf for three spots from a real or textbook chromatogram, measuring from the base line.
Before the practical, rehearse the steps of crystallisation and paper chromatography aloud. Our guide to functional group tests covers the next practical skill.
When you are unsure why a technique works, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise vapour pressure or adsorption on its own.
Tonight, explain to someone in three sentences why aniline can be steam distilled below 373 K.
