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π¬ Think about it
- Write something with a black sketch pen and let a drop of water fall on it.
- The colour spreads — and different colours come out of it.
- So black ink was never just one colour. How do we separate them properly?
What is paper chromatography?
Chromatography
chroma
colour
graphein
to write
↓
"Writing with colour"
because it was first used to separate coloured substances like dyes and inks
- The word comes from the Greek words chroma and graphein .
- Paper chromatography separates the different soluble components of a complex liquid mixture — inks, dyes and natural plant pigments.
- It uses the differences in how each component interacts with the solvent and with the paper .
- The liquid carries the substances up the paper and separates them by how fast each one moves .
Definition
Paper chromatography is a method of separating the components of a mixture based on the differences in their interactions with the solvent and with the paper.
ποΈ
Activity 5.5 — Let us investigate
In this Activity, we will separate the colours hidden inside black sketch pen ink using nothing more than paper and water.
Materials needed
A 3 cm wide strip of chromatographic paper (or filter paper), a pencil and scale, a black sketch pen, water, and a gas jar, measuring cylinder or beaker.
Procedure
1. Take a 3 cm wide strip of chromatographic paper and draw a straight, horizontal line with a
pencil
, 2 cm from the bottom.
2. Mark a spot with the black sketch pen at the centre of that line and let it dry a little.
3. Take enough water to make a thin layer at the bottom of a gas jar, measuring cylinder or beaker.
4. Place the paper strip vertically in the container so that its lower end dips into the water, but the water level stays below the ink spot .
5. Leave the set-up undisturbed and watch the water rise through the paper.
6. Observe as the ink starts to separate into different colour spots.
2. Mark a spot with the black sketch pen at the centre of that line and let it dry a little.
3. Take enough water to make a thin layer at the bottom of a gas jar, measuring cylinder or beaker.
4. Place the paper strip vertically in the container so that its lower end dips into the water, but the water level stays below the ink spot .
5. Leave the set-up undisturbed and watch the water rise through the paper.
6. Observe as the ink starts to separate into different colour spots.
Observation
As the water rises, the ink separates into spots of different colours at different heights on the paper.
Explanation
Black ink is not a single colour — it is a mixture of different soluble dyes. The colour that is most soluble in the water travels fastest and goes highest, while the less soluble colours stay lower down on the paper.
You may also try
Try it with green food colour instead of black ink, using a 2 % m/v salt solution as the solvent. You can also separate the pigments in the green extract of spinach leaves, or the coloured pigments of flower petals. A
pigment
is a substance that gives colour.
Will water always work as the solvent?
No. In some cases a different solvent is needed, such as alcohol, or a mixture of solvents.
This activity proves that black ink is not one single colour but a mixture of different soluble dyes, which chromatography can pull apart.
β Summary
- An ink spot is placed above the water level on a paper strip
- Rising water carries the dyes up at different speeds
- The dyes separate into spots at different heights
Pause and Ponder
π€ Pause and Ponder — Question 6
-
6. State whether the following statements are True or False. Also, correct the False statements.
(i) Salt can be separated from a salt solution by evaporation or distillation.
(ii) Distillation can be used for separation of two liquids even when these have the same boiling point.
(iii) In paper chromatography, the solvent level should be above the sample spot at the beginning of the experiment.
(iv) Evaporation and crystallization are the same processes.View answer
Answer(i) True • (ii) False • (iii) False • (iv) FalseExplanation
Statement Verdict Correction (i) Salt can be separated from a salt solution by evaporation or distillation True Evaporation gives us the salt; distillation gives us both the salt and the water. (ii) Distillation works even when the two liquids have the same boiling point False Distillation needs a difference of at least about 25 °C in boiling points. At the same boiling point both would vaporise together. (iii) The solvent level should be above the sample spot at the beginning False The solvent level must be below the spot, otherwise the spot would simply dissolve into the solvent. (iv) Evaporation and crystallization are the same process False They are different. In evaporation the solvent evaporates and the solid left behind often still carries impurities. In crystallization a hot saturated solution cools slowly, pure crystals form, and the impurities stay dissolved in the solution. Only the first statement is true; the other three each miss one essential condition.
Let us summarise the three methods for homogeneous mixtures
| Method | How it works | Example |
|---|---|---|
| Crystallization | Cool a saturated solution, or evaporate the solvent, so the solid comes out as pure crystals while impurities stay in solution | Salt from sea water; purifying impure copper sulfate |
| Distillation | The liquid with the lower boiling point vaporises first, condenses in the condenser and is collected separately | Acetone and water |
| Paper chromatography | The more soluble component climbs the paper faster, so the components separate at different heights | Dyes in black ink; plant pigments |
π Summary - What we have learnt
- Paper chromatography separates soluble components of a mixture using how fast each one moves up the paper.
- The word means "writing with colour", from chroma and graphein .
- The most soluble component travels highest; the least soluble stays lowest.
- The solvent level must always start below the sample spot.
Recap — Section 5.3 — Methods of Separation of Homogeneous Mixtures
β
Test Yourself
-
A saturated solution of a salt is heated further so more salt dissolves, then cooled slowly. What comes out?
View Answer
Crystals — the extra solute that the cooler solution can no longer hold separates out as crystals. That is crystallization. -
Water and acetone are miscible, and their boiling points differ by 44 °C. Would you use fractional distillation here?
View Answer
No, plain distillation is enough. Fractional distillation is needed only when boiling points differ by less than about 25 °C. -
In paper chromatography, what two things decide how far up the paper a component travels?
View Answer
How that component interacts with the solvent and with the paper — components that interact differently move at different speeds and end up at different heights.
Important Definitions
- Crystal
- A solid whose particles are arranged in a regular geometric pattern.
- Crystallization
- The process of forming crystals from a saturated solution.
- Mother liquor
- The solution left behind after crystals have separated out, holding the impurities.
- Distillation
- Separating miscible liquids by boiling off the one with the lower boiling point and condensing it.
- Fractional distillation
- Separating components whose boiling points differ by less than 25 °C, as with crude oil.
- Paper chromatography
- Separating soluble components of a mixture by how fast each moves up a paper strip.
Questions at the End of the Chapter — Related to This Section
π NCERT Question 8 — Two miscible liquids A and B
Two miscible liquids A and B have boiling points 30 °C apart — name the method to separate them and draw the set-up.
View Answer →
π NCERT Question 9 — Evaporation, crystallization and distillation compared
Compare evaporation, crystallization and distillation, and say when each one should be chosen.
View Answer →
π NCERT Question 12 — Why distillation suits water and acetone
Explain why distillation is such a good method for separating a mixture of water and acetone.
View Answer →
π NCERT Question 15 — Identifying and labelling the apparatus
Identify a labelled apparatus from a figure and say which mixtures the technique can separate.
View Answer →
