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š¬ Think about it
Drop water on something written with a black sketch pen and the colour spreads into different shades. That spreading is a clue that black ink is a mixture of colours. We can use it to separate them. Let us investigate.
What is paper chromatography?
- It separates the components of a mixture on paper.
- It uses differences in how components interact with the solvent and the paper.
- The liquid carries the substances up the paper.
- They separate based on how fast each one moves.
In this Activity, we will separate the colours in black ink by letting water rise up a strip of paper.
Procedure
1. Take a 3 cm wide strip of chromatographic (or filter) paper and draw a pencil line 2 cm from the bottom.
2. Mark a spot with a black sketch pen at the centre of the line.
3. Put a thin layer of water at the bottom of a gas jar or beaker.
4. Dip the strip so the lower end is in the water, with the water level below the spot.
5. Watch as the water rises through the paper.
6. As the water rises, the ink separates into different colour spots.
2. Mark a spot with a black sketch pen at the centre of the line.
3. Put a thin layer of water at the bottom of a gas jar or beaker.
4. Dip the strip so the lower end is in the water, with the water level below the spot.
5. Watch as the water rises through the paper.
6. As the water rises, the ink separates into different colour spots.
Explanation
The water carries the ink up the paper. Different colours travel at different speeds, so they separate into spots. This shows that black ink is a mixture of several colours. You can also try green food colour with a 2% m/v salt solution, or separate spinach pigments.
ā Summary
- Put an ink spot on paper.
- Dip the strip in water.
- Water rises and carries colours.
- Colours separate into spots.
Important Points
- Paper chromatography separates a mixture on paper using a solvent.
- Components separate by how fast they move up the paper.
- It is great for separating coloured substances like inks and pigments.
š§© Threads of Curiosity
- The word comes from the Greek chroma (colour) and graphein (to write).
- It means 'writing with colour'.
- It was first used to separate coloured substances like dyes and inks.
š§¬ Pause and Ponder
-
Salt can be separated from a salt solution by evaporation or distillation.
True. -
Distillation can separate two liquids even when they have the same boiling point.
False — distillation needs a difference in boiling points; liquids with the same boiling point cannot be separated this way. -
In paper chromatography, the solvent level should be above the sample spot at the start.
False — the solvent level must be below the spot, or the spot would dissolve into the solvent instead of rising up the paper. -
Evaporation and crystallization are the same processes.
False — evaporation removes the solvent to leave the solute, while crystallization forms pure crystals by cooling a saturated solution.
ā Test Yourself
-
What does paper chromatography separate?
View Answer
The components of a mixture, especially coloured ones like inks and pigments. -
What carries the substances up the paper?
View Answer
The solvent (the liquid). -
Why must the water level be below the ink spot?
View Answer
So the spot does not dissolve into the water instead of rising up the paper.
Important Definitions
- Paper chromatography — separating a mixture on paper based on how fast components move in a solvent.
