Chromatography splits a mixture into its parts by making them race. Every component travels at its own speed, and the speed depends on something you already understand — how strongly it is attracted to each of the two phases.
📚 What you need to know
Chromatography separates mixtures using a mobile phase (a moving solvent) and a stationary phase (something that stays put).
Components move at different rates depending on their solubility in the mobile phase and their attraction to the stationary phase.
Those attractions are just intermolecular forces — the ones from the last set of notes.
Paper chromatography: cellulose paper (polar –OH groups) is the stationary phase.
Thin-layer chromatography (TLC): a thin layer of silica or alumina on glass or plastic instead of paper.
Rf = distance moved by the component ÷ distance moved by the solvent front. Always less than 1, and it has no units.
The two phases
Everything in chromatography comes down to a tug of war between two things:
The mobile phase — a liquid solvent that soaks up the paper by capillary action, carrying the mixture with it.
The stationary phase — the paper itself, which stays where it is and grabs at the components as they pass.
A component that is very soluble in the solvent and only weakly attracted to the paper gets carried a long way. One that clings to the paper and dissolves poorly barely moves. Because every substance has its own balance, they separate.
Chromatography paper is made of cellulose, which is covered in polar –OH groups. Those can hydrogen bond. So polar substances stick to the paper and travel slowly, while nonpolar substances are carried along by the solvent and travel further.
Doing paper chromatography
The solvent climbs the paper by capillary action and carries the components with it — each one only as far as its own balance of attractions allows.
🧩 The method, step by step
Draw a baseline in pencil near the bottom of the paper. Pencil, not pen — ink would dissolve and run up the paper with everything else.
Put small spots of the mixture on the baseline, along with any reference substances you want to compare against.
Stand the paper in the solvent so that the baseline sits above the solvent level. If it dips below, the spots simply wash off into the solvent.
Put the lid on and let the solvent rise by capillary action, carrying the components at different rates.
Take the paper out before the solvent reaches the top, and immediately mark the solvent front in pencil — it disappears as the paper dries.
Let it dry. The finished paper is called a chromatogram.
💡 The three marks people throw away
Pencil baseline, because ink is itself a mixture of dyes and would separate too.
Baseline above the solvent, or the sample dissolves away before it can travel.
Mark the solvent front straight away, or you have no way to calculate Rf.
Thin-layer chromatography (TLC)
TLC works on exactly the same principle. The only change is the stationary phase: instead of paper, a thin layer of an inert solid — usually silica or alumina — is coated onto a rigid plate of glass or plastic.
That surface also carries –OH groups, so it forms the same kinds of intermolecular forces with the sample. Components are adsorbed onto the surface to different extents, and again that decides how far each one travels. TLC is faster than paper and gives sharper spots.
Rf values
A spot’s position on its own is not much use — it depends on how long you left the experiment running. What is reproducible is how far the spot travelled compared with the solvent. That ratio is the retardation factor, Rf.
Retardation factor
Rf = distance travelled by the component ÷ distance travelled by the solvent front
Both distances are measured from the baseline, and the spot distance is taken to the centre of the spot.
Two things follow straight from that definition:
Rf is always less than 1, because a component is carried by the solvent and so can never overtake it.
Rf has no units, because you are dividing a length by a length — the units cancel.
A high Rf means the substance is very soluble in the mobile phase and weakly held by the stationary phase. A low Rf means the opposite.
WORKED EXAMPLE
On a chromatogram the solvent front moved 8.0 cm from the baseline. Spot A moved 6.0 cm and spot B moved 2.4 cm. Calculate both Rf values and say which substance is more polar.
Spot AR f = 6.0 ÷ 8.0= 0.75Spot BR f = 2.4 ÷ 8.0= 0.30B moved less, so it was held more strongly by the paperB is the more polar substance
WORKED EXAMPLE
A student runs a chromatogram and one component does not move off the baseline at all. Suggest what went wrong and what they should do.
The component is insoluble in that solventIf it will not dissolve in the mobile phase, nothing can carry it up the paper.It may also be very strongly attracted to the stationary phaserepeat the experiment with a different solventChemists often have to try several solvents before every component separates clearly.
Why Rf is useful. Under the same conditions — same solvent, same paper, same temperature — a given substance always gives the same Rf. So you can compare an unknown spot with a known reference value, or run a reference substance alongside your sample and see whether the spots line up.
⚠️ Common mix-up
Measure from the baseline, not from the bottom of the paper, for both distances.
Measure to the centre of the spot, not to its top or bottom edge.
Rf is never greater than 1. If you get a number above 1, you have divided the wrong way round.
Don’t write a unit after an Rf value. It is a plain number.
Mobile vs stationary: the solvent moves, the paper stays still. Easy to swap under pressure.
That completes Topic 2, Models of Bonding & Structure — ionic, covalent, and everything that follows from them. Next you’ll move into Topic 3, where these structures start reacting.
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