JEE Mains Chemistry · Organic Chemistry - Some Basic Principles and Techniques

Chromatography

Chromatography separates a mixture as a mobile phase carries it over a stationary phase: a component that clings more to the stationary phase travels less, so it has a lower Rf on a plate and leaves a column later.

All Organic Chemistry - Some Basic Principles and Techniques notes JEE Mains Chemistry strategy

Why this matters

Twenty PYQs, five of them asking for a number, and one from 2026. Ten test the types: adsorption in column and thin-layer chromatography, partition in paper chromatography, the adsorbents used and how colourless spots are made visible. Ten are about Rf: working it out from a plate, reading polarity from it, and turning it into an order of elution.

Concept 1 of 2: Adsorption and partition chromatography

In every kind of chromatography a moving solvent (the mobile phase) carries the mixture over something that stays put (the stationary phase). Each component divides its time between the two. In adsorption chromatography the stationary phase is a solid that holds the components on its surface; in partition chromatography it is a liquid film, and the components share themselves between two liquids.

Definition

Adsorption chromatography: the components are adsorbed to different extents on a solid adsorbent, silica gel or alumina. Column chromatography and thin-layer chromatography (TLC) work this way.
Partition chromatography: the components are distributed continuously between a liquid held as a thin film on an inert support (the stationary phase) and a moving liquid.
Paper chromatography is partition chromatography: the paper is only the support, and the stationary phase is the WATER held in its pores.
Colourless spots on a TLC plate are found with ultraviolet light (for fluorescent compounds), with iodine vapour (brown spots) or by spraying a reagent (ninhydrin for amino acids). A visualising agent is never added to the mobile phase.
Preparative TLC separates small amounts, around 100 mg; column chromatography handles larger amounts.
The separation depends on the adsorbent, the solvent, the solubility of each component and the length of the column or plate.

Technique	Principle	Stationary phase	Mobile phase
Column chromatography	Adsorption	Silica gel or alumina packed in a glass column	A solvent (the eluant) run down the column
Thin-layer chromatography (TLC)	Adsorption	A thin layer of silica gel or alumina on a glass plate	A solvent rising up the plate by capillary action
Paper chromatography	Partition	Water held in the pores of the paper	A solvent rising up the paper The paper itself is only the support, not the stationary phase.

Column and TLC share a principle; paper chromatography is the odd one out.

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The same idea in a real exam question:

JEE Mains · 2025 · 29 Jan 2025 · Q103Moderate

Example 1 · Organic Chemistry - Some Basic Principles and Techniques · Chromatography

Given below are two statements : Statement (I) : In partition chromatography, stationary phase is thin film of liquid present in the inert support. Statement (II) : In paper chromatography, the material of paper acts as a stationary phase. In the light of the above statements, choose the correct answer from the options given below :

The paper is not the stationary phase

In paper chromatography the stationary phase is the water held in the paper's pores. A statement that the paper material itself is the stationary phase is false.

TLC works by adsorption, not partition

Thin-layer chromatography uses a solid adsorbent on a plate, just like column chromatography. Only paper chromatography, among the three, works by partition.

No visualising agent in the mobile phase

Spots are revealed after the plate is run: by UV light, iodine vapour or a spray. Adding a visualising agent to the solvent is not a method of locating spots.

Drill 9 more on adsorption and partition chromatography

Concept 2 of 2: Retardation factor and order of elution

Rf compares how far a spot travelled with how far the solvent travelled. Both distances are measured from the base line, where the spot started, so Rf is a fraction between 0 and 1 with no unit. A compound held tightly by the stationary phase lags behind the solvent and has a small Rf.

Definition

Measure both distances from the base line (the pencil line where the sample was spotted), never from the bottom edge of the plate.
Rf has no unit and is always less than 1. For a given compound it changes with the solvent and the stationary phase.
On polar silica gel or alumina, a more polar compound is held more strongly: it has a LOWER Rf and, on a column, it comes out LATER.
The order of elution from a column is the order of decreasing Rf on a TLC plate run in the same system. The compound that elutes first has the higher Rf and the weaker adsorption.
A rough polarity guide on silica: carboxylic acid > alcohol > aldehyde or ketone > ether > hydrocarbon.

Retardation factor

R_f = \dfrac{\text{distance moved by the compound from the base line}}{\text{distance moved by the solvent front from the base line}}

Worked example

On a silica gel TLC plate the solvent front travels 8.0 cm from the base line. Spot A travels 3.2 cm and spot B 6.0 cm. Find each Rf, say which compound is more polar, and which comes out first from a silica gel column.

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The same idea in a real exam question:

JEE Mains · 2026 · 6 Apr 2026 Shift 1 · Q38Moderate

Example 2 · Organic Chemistry - Some Basic Principles and Techniques · Chromatography

R_{f}

value for 2-methylpropene in a solvent system (Ethyl acetate + ether) is 0.42 . 2-methylpropene is treated with dilute

H_{2}SO_{4}

to give major organic product (X).

R_{f}

value for (X) in the same solvent system under identical condition will be :

Measure from the base line, not the plate edge

If a distance is given from the bottom of the plate, subtract the height of the base line first, for both the spot and the solvent front.

More polar means lower Rf on silica

A polar compound, such as an alcohol or an acid, sticks to polar silica gel and travels less. Its Rf is SMALLER than that of a non-polar compound in the same solvent.

The first compound out has the higher Rf

On a column, the compound that is adsorbed least runs fastest and elutes first. The same compound travels furthest on a TLC plate, so it has the higher Rf.

Drill 9 more on retardation factor and order of elution

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Retardation factor and order of elution
Retardation factor
$R_f = \dfrac{\text{distance moved by the compound from the base line}}{\text{distance moved by the solvent front from the base line}}$

Reference tables (1)

Adsorption and partition chromatography3 rows

Technique	Principle	Stationary phase	Mobile phase
Column chromatography	Adsorption	Silica gel or alumina packed in a glass column	A solvent (the eluant) run down the column
Thin-layer chromatography (TLC)	Adsorption	A thin layer of silica gel or alumina on a glass plate	A solvent rising up the plate by capillary action
Paper chromatography	Partition	Water held in the pores of the paper	A solvent rising up the paper The paper itself is only the support, not the stationary phase.

Column and TLC share a principle; paper chromatography is the odd one out.

Watch out for (6)

The paper is not the stationary phase
→ Adsorption and partition chromatography
TLC works by adsorption, not partition
→ Adsorption and partition chromatography
No visualising agent in the mobile phase
→ Adsorption and partition chromatography
Measure from the base line, not the plate edge
→ Retardation factor and order of elution
More polar means lower Rf on silica
→ Retardation factor and order of elution
The first compound out has the higher Rf
→ Retardation factor and order of elution

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