Chemistry · General Principles Of Organic Chemistry · NEET
It is partition chromatography, not adsorption. This is a very common NEET trap. In adsorption chromatography (like column and thin layer chromatography, TLC) the components stick to the surface of a solid adsorbent such as silica gel or alumina. In paper chromatography the components do not stick to a solid surface; they share themselves between two liquid phases (water in the paper and the moving solvent). So paper chromatography is always classified as partition chromatography.
The stationary phase is the water trapped inside the chromatography paper, not the paper itself. Chromatography paper is a special quality paper that holds water in its fibres. The paper is only the support that carries this water. Students often wrongly write 'paper' or 'cellulose' as the stationary phase; NCERT clearly says the trapped water acts as the stationary phase.
The mobile phase is the solvent (or a mixture of solvents) that rises up the paper strip by capillary action. As this solvent moves up and flows over the spot, each component travels a different distance depending on how it partitions between the moving solvent and the fixed water. This is why the spots end up at different heights on the developed paper, which is called a chromatogram.
They separate because each component divides itself differently between the two phases. A component that prefers the mobile solvent moves up faster and travels farther; a component that prefers the stationary water stays back and moves less. This continuous, repeated partitioning (sharing) all along the paper is what pulls the mixture apart into separate spots.
Coloured compounds are seen directly by their own colour. Colourless compounds are made visible either by viewing the paper under ultraviolet (UV) light or by spraying a suitable reagent on it. For example, amino acids are detected by spraying ninhydrin solution, which gives coloured spots. This detection step is the same idea used in TLC.
Paper chromatography is an example of
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Partition chromatography. The separation happens by partitioning between water (stationary) and the moving solvent (mobile), not by sticking to a solid surface.
The water trapped inside the chromatography paper acts as the stationary phase. The paper is only the support that holds this water.
A solvent or a mixture of solvents that rises up the paper by capillary action is the mobile phase.
The developed paper strip, on which the separated components appear as spots at different heights, is called a chromatogram.
After the spots separate, each spot's position is measured as an Rf value = distance moved by the substance divided by distance moved by the solvent. Rf is a fixed number for a compound under set conditions.
