KS3 · Chemistry

Chromatography

A note written in black ink, three black pens on the table. Could one tiny dot of ink tell you which pen wrote it?

Chemistry · The mystery ink

Which inks are hiding in ink U?
solventpencil lineABCU

View: 2 · After the run. Showing 2 layers: Paper and start line, Spots after the run

View

Explore

Step through the four views ↑ · tap an ink to focus it

The solvent has soaked up the paper and carried the inks with it. Some dots stayed as one spot; others split into several. This pattern is a chromatogram.

A tiny dot of each ink was put on the pencil line, then the paper was stood in a little solvent. Step through the views, then tap each ink.

Why the spots end up at different heights

1Solvent soaks up2345

Stage 1 of 5: Solvent soaks up. paused

Solvent soaks up · 1/5Drag the scrubber to run the chromatogram.

The absorbent paper soaks up the solvent towards the pencil line.

Scrub from left to right and watch one dot become several spots.

Setting it up

Get the set-up in the right order

The order you would do these steps in, to set up paper chromatography

1 · First step5 · Last step

  1. Lower the paper gently into the solvent

  2. Add the sample to the X, a drop at a time

  3. Mark an X at the centre of the line

  4. Fasten the paper to a support so it hangs vertically

  5. Draw a straight line with a pencil and ruler, 1–2 cm from one end

Watch out: Every step is there to keep the chromatogram readable. Skip one and the answer you read off the paper can't be trusted.

Pencil, not pen

Why must the start line be in pencil?

The method for paper chromatography says to draw the start line in pencil, not pen.

Which is closest to what you think right now?
How sure are you?

Your turn

Read a new chromatogram

Sort each sample, first by counting its spots, then by comparing it with ink K.

How many spots are on this sample's own line?

Still to sort

Pure substance (0)

One spot on its line.

Where the line is: Count only the spots on this sample's own line. One spot: pure. Two or more: a mixture.

Mixture (0)

Two or more spots on its line.

5 of 5 still to sort.

Known ink K gives one spot, 5 cm above the start line. Here are five more samples, sample by sample.

Watch out: When a sample was run on a separate paper, line its start line up with this one first. Only then can you compare heights.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One tiny dot of ink, split into a pattern that tells you exactly what's in it.

What you need to know

  • Chromatography separates mixtures and analyses them, and it works on really small samples.
  • The solvent dissolves the sample, and the different substances are carried different distances up the paper.
  • More soluble in the solvent = travels further. Less soluble stays near the X. Insoluble doesn't move.
  • The start line is pencil (insoluble), the spot is small and dried, and the solvent stays below the line and spot.
  • Read up a sample's line: one spot = pure, several = mixture. Read across: a spot at the same height = the same substance.

The big picture

Chromatography separates and analyses mixtures, even really small samples such as the inks in pens. A tiny spot sits on a pencil line on absorbent paper, and a solvent soaks up the paper, dissolving the sample and carrying each substance a different distance: the more soluble it is in the solvent, the further it travels, and an insoluble substance stays on the X. The pattern left behind is a chromatogram. Read up a sample's line to see if it is pure (one spot) or a mixture (several), and read across at the same height to find which known substances it contains.

Key points

1Chromatography is one of the simple techniques for separating mixtures. It can be used both to separate really small samples and to analyse them, for example the inks in pens, drugs, or debris.
2A sample is placed on absorbent paper and a solvent is applied. The solvent dissolves the sample to make a solution, and as that solution is absorbed along the paper, the different parts of the sample become separated and easier to see.
3The pattern that forms is called a chromatogram.
4The more soluble a substance is in the solvent, the further from the X it travels. Less soluble substances stay close to the X, and an insoluble substance does not move from the X.
5Set-up: a pencil-and-ruler line about 1–2 cm from one end, an X at its centre, a small spot (under 1 cm wide) dried drop by drop, then the paper hung vertically and lowered gently so the solvent stays below the line and the spot.
6Pencil is used for the line because it is insoluble in water and most solvents. Many inks are soluble, so a pen line could separate out and muddle the chromatogram.
7Read vertically (up from each X): one spot means a pure substance, several spots mean a mixture. Read horizontally (across at the same height): spots at the same height as a known sample's spots are the same substances.
8Use a ruler to read a chromatogram, and when comparing separate chromatograms, line up their pencil start lines first.

Worked example

Problem

Ink from a note was run on one strip of paper, and inks from two pens, Pen 1 and Pen 2, were run on another. The note's ink gave spots 2 cm and 6 cm above its start line. Pen 1 gave spots at 2 cm and 6 cm. Pen 2 gave spots at 2 cm and 4 cm. Which pen's ink matches the note?

⚠ Watch out

Saying the start line is drawn in pencil 'so you can rub it out'. The real reason is solubility: pencil doesn't dissolve in the solvent, but many inks do, and a pen line would separate out into spots of its own and muddle the chromatogram.

🧠

Memory hook

Soluble travels. The better a substance dissolves in the solvent, the further it rides up the paper, and an insoluble one never leaves the X. Then read it like a map: UP for how many, ACROSS for which ones.

✓

Check yourself

Cover the page. Explain why one dot of ink can end up as three spots at three different heights. Then say what a single spot that never left its X tells you about that substance.

Flashcards

(13)
What is chromatography used for?
Separating mixtures and analysing them, even when you only have a really small sample.
Name three things chromatography can be used to test.
The different inks in pens, different drugs, and different debris.
What is a chromatogram?
The pattern of spots that forms as a result of chromatography.
What does the solvent do when it reaches the sample?
It dissolves the sample, making a solution that is carried along the paper as the paper soaks it up.
Which travels further: a more soluble substance or a less soluble one?
The more soluble one. Less soluble substances stay close to the X.
What happens to a substance that is insoluble in the solvent?
It does not move from the X.
Where is the start line drawn, and with what?
About 1–2 cm from one end of the paper, with a pencil and a ruler, with an X at its centre.
Why is the start line drawn in pencil, not pen?
Pencil is insoluble in water and most solvents. Many inks are soluble and could separate out and muddle the chromatogram.
Why must the sample spot be small?
A spot that is too large makes the chromatogram muddled and difficult to interpret. Keep it under 1 cm wide.
Why let each drop of sample dry before adding the next?
It concentrates the sample while keeping the spot small.
Why must the solvent stay below the pencil line and the spot?
If the spot goes into the solvent, the sample could dissolve into the solvent instead of being separated up the paper.
Reading up a sample's line: what do one spot and several spots mean?
One spot: a pure substance. Several spots: a mixture.
Reading across a chromatogram: what does a spot at the same height as a known sample's spot tell you?
That spot is the same substance as the known sample.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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