Mixtures and separation · Process
Chromatography
Everything in the mixture is dissolved, and all of it is the same colour. How do you separate substances that will not sit still to be picked apart?
Start here
Black ink is not black. It is three or four colours pretending.
A note has been written in black ink and four black pens have been collected. All four inks look identical on paper, and no amount of staring at them will tell you which pen wrote the note.
Draw a dot of each ink near the bottom of a strip of paper and let water creep up through it. What happens?
Each dot climbs and splits into separate coloured spots — and the pattern is different for each pen, because each manufacturer mixes its black from a different set of dyes. The inks are indistinguishable until you separate them. Then they are as distinctive as handwriting.
Chromatography separates substances that are all dissolved in the same solvent. The solvent soaks up through the paper and carries the dyes with it — but each dye is pulled two ways: it sticks to the paper, and it dissolves in the moving solvent. A dye that holds the solvent's side travels a long way. A dye that clings to the paper barely moves.
Nothing here is about size, and nothing is about how much you put on. It is a tug of war, and each substance loses it by a different amount.
Your turn · the case of the four pens
Set the paper up, run it, and see whether it can be read
Three decisions before you start. Each one can ruin the whole run — and each one ruins it in its own particular way.
The baseline is drawn in
The solvent in the tank starts
You take the paper out
The note
Pen A
Pen B
Pen C
Pen D
A chromatogram you can read.
Five lanes, run side by side on one piece of paper, in one solvent, for the same length of time. That is the only way the heights can be compared — and now they can be.
The baseline has run, and it has run through everything.
You drew the baseline in pen. Pen ink is a mixture of dyes dissolved in a solvent — exactly the thing this experiment separates — so the line itself climbed the paper and smeared its own colours across all five lanes. Nothing can be read. Pencil is graphite: insoluble, so it stays where you drew it.
The spots have gone. All of them.
The solvent level was above the baseline, so the ink dots were sitting in the liquid rather than above it. They dissolved straight into the solvent in the tank and were carried nowhere. The solvent has to start below the spots and climb up to them.
Every spot is jammed against the top edge.
You left the paper in until the solvent reached the top. Once the solvent front runs off the end, the fast spots pile up against the edge and stop being separable — and there is no solvent front left to measure distances against. Take the paper out while the front is still short of the top.
Which pen wrote the note?
A match means every spot at the same height, and no spot that the other one does not have.
Pen A. Three spots, three matching heights, nothing extra.
The note and Pen A separate into the same three dyes at the same three heights. Pen B has no yellow at all. Pen C has yellow and cyan but both sit slightly low, and it has no magenta. Pen D was never in the running: one blue dye, and the note has three.
Pen A is the match — look again at the heights.
Pen B has magenta and cyan, and its magenta sits lower than the note. It is also missing the yellow entirely — a missing spot rules a sample out as firmly as a wrong one.
Pen A is the match — look again at the heights.
Pen C looks close because it shares yellow and cyan, but both spots sit lower than the note and there is no magenta. Close is not a match.
Pen A is the match — look again at the heights.
Pen D gives one blue spot. The note gives three, in three colours, so they cannot be the same ink.
Key fact
Chromatography separates dissolved substances by how strongly each one sticks to the paper compared with how well it dissolves in the moving solvent. The further a substance travels, the more it favoured the solvent.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“The spot that travelled furthest must be the dye there is most of in the ink.”
It went the furthest, so surely there was the most of it pushing. Commit before you read on.
How far a spot travels says nothing about how much of it there is. It is decided entirely by the tug of war between the paper and the solvent — and one particle of a dye is pulled exactly as hard as a million of them. Put a tiny dot of a fast dye on the paper and it still goes to the top; put a heavy blob of a slow dye on and it still crawls.
What the amount changes is the size and darkness of the spot, not its height. So a chromatogram is read two ways: height tells you which substance, and darkness hints at how much. Confusing the two is how you end up accusing the wrong pen.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What kind of mixture can chromatography separate?
Rung 2 · The one that catches people
Why must the baseline be drawn in pencil rather than pen?
Rung 3 · Explain
On one chromatogram, a yellow dye travels a third of the way up and a blue dye travels most of the way. Explain what makes the difference — and explain why the height of a spot says nothing about how much of that dye was in the ink.
Rung 4 · Take it somewhere new
A food company is accused of using a banned red colouring in its sweets. You have the sweets, pure samples of the banned dye and of three legal red dyes, and ordinary lab equipment. Describe the investigation, and say exactly what result would convict them.
Key note
Chromatography separates substances dissolved in the same solvent. The solvent rises through the paper and each substance is pulled two ways — sticking to the paper, or dissolving in the moving solvent. Whichever pull wins decides how far it travels. The baseline is pencil, the solvent starts below the spots, and everything being compared is run on the same paper at the same time.
Going further
Height is only comparable within one run — a warmer room, a different paper or a different solvent moves every spot. So chemists quote a ratio instead: the distance the spot travelled divided by the distance the solvent travelled. That number, the Rf value, is between 0 and 1, has no units, and can be compared between one lab and another. It is the reason a chromatogram can be evidence rather than an anecdote.
Most substances worth separating are colourless, which makes the school version of this look easy and the real version look like a blank sheet of paper. Amino acids from a protein, sugars from a plant extract and drugs in an athlete's urine all separate perfectly well and all arrive invisible; the paper is then sprayed with a chemical that reacts to show them, or looked at under ultraviolet light. The separation is the same. Only the seeing is harder.
Before this lesson
Next in this unit
At GCSE this becomes
- Rf values calculated and compared, and chromatography as a required practical for identifying substances.
Where to next
Ask Mr Badmus AI
Still not sure why the baseline has to be pencil?
Lesson content © MrBadmusAI.