Chemistry · General Principles Of Organic Chemistry · NEET
Column chromatography is a type of ADSORPTION chromatography. The stationary phase is a solid adsorbent (silica gel or alumina) packed in a glass column, and separation happens because compounds are adsorbed to different degrees. This is a common NEET trap: paper chromatography is partition, but column and TLC are adsorption.
Both are adsorption chromatography with the same principle. In column chromatography the adsorbent is packed as a column in a glass tube and the eluant flows DOWN; components come out at the bottom at different times. In TLC the adsorbent is a thin ~0.2 mm layer coated on a glass plate (chromaplate) and the eluant rises UP; components move up to different heights. TLC is faster and mainly used to check separation and purity.
The stationary phase is the solid adsorbent — commonly silica gel or alumina. The mobile phase is the liquid eluant (a single solvent or mixture of solvents) that moves over the adsorbent and carries the components along. Separation depends on how strongly each component sticks to the stationary phase versus how well it dissolves and moves in the mobile phase.
The eluant is the liquid or mixture of liquids allowed to flow slowly down the column. It is the mobile phase. As it moves, it washes the components down the column at different speeds depending on how strongly each is adsorbed. Choosing the right eluant polarity controls how well the mixture separates.
The LEAST strongly adsorbed compound comes out first. Since it sticks weakly to the adsorbent, the eluant carries it down fastest, so it comes down to the bottom sooner. The most strongly adsorbed compound stays near the top and comes out last. Remember: sticks more = moves less.
Rf (retardation factor) = distance moved by the substance from the base line (x) ÷ distance moved by the solvent from the base line (y). It is a fraction (always less than 1) that is characteristic of a compound under fixed conditions, so it helps identify components on a TLC plate. Rf has its own detailed page linked below.
The most suitable method of separation of a 1:1 mixture of ortho- and para-nitrophenols is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Different compounds are adsorbed onto a solid adsorbent (silica gel or alumina) to different degrees. When a mobile phase moves over this stationary adsorbent, components travel different distances and separate.
The name comes from the Greek word chroma meaning colour, because the method was first used to separate coloured substances from plants.
Silica gel and alumina are the commonly used adsorbents (the solid stationary phase) in both column chromatography and TLC.
A chromaplate is the TLC plate — a glass plate coated with a thin (~0.2 mm) layer of adsorbent such as silica gel or alumina on which the mixture is separated.
Yes. NEET regularly tests classification of chromatography (adsorption vs partition), which technique separates a given mixture, and the meaning of stationary phase, mobile phase, eluant and Rf. These are direct, high-scoring one-mark questions.
