Chemistry · General Principles Of Organic Chemistry · NEET
Yes, Rf is always less than 1 for any real spot. The substance is carried up by the solvent, so it can move at most as far as the solvent front, never farther. That makes the top number (x) always smaller than the bottom number (y), so the ratio stays below 1. Rf = 1 would mean the spot did not stick to the stationary phase at all and moved exactly with the solvent, which does not happen in practice. Rf = 0 would mean the spot never moved from the base line.
No. Rf is a ratio of two distances (both measured in cm or mm), so the units cancel out. It is a pure number with no unit. In NEET, if an option gives Rf 'in cm', that option is wrong.
A high Rf means the compound travelled a long way with the solvent, so it was weakly adsorbed on the stationary phase and is more soluble in the moving solvent (mobile phase). A low Rf means the compound stuck strongly to the stationary phase and barely moved. So Rf is inversely related to how strongly a substance is adsorbed.
Both distances are measured from the base line (the starting spot line where you loaded the sample), not from the very bottom edge of the plate. x is base line to the centre of the spot; y is base line to the solvent front. Measuring from the wrong reference is the most common calculation mistake.
Each compound has its own balance of adsorption (sticking to stationary phase) and solubility (moving with the mobile phase). A compound that dissolves better in the solvent and sticks less to the plate moves higher and gets a larger Rf. Because this balance is characteristic of each compound under fixed conditions, Rf helps identify substances.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Rf stands for Retardation Factor. It measures how much the movement of a substance is retarded (held back) relative to the solvent front in chromatography.
Rf = distance moved by the substance from the base line (x) / distance moved by the solvent from the base line (y). Both distances are taken from the same starting line.
No. The spot cannot travel farther than the solvent that carries it, so x is always less than y and Rf stays between 0 and 1.
It depends on the compound's adsorption on the stationary phase and its solubility in the mobile phase, plus fixed conditions like the solvent, adsorbent, and temperature. Under identical conditions, Rf is characteristic of a compound and helps identify it.
Rf is used mainly in thin layer chromatography (TLC) and paper chromatography, where the spot and the solvent front travel measurable distances on a plate or paper.
