Colours that climb at different speeds

The idea

Paper chromatography separates the components of a mixture by their different interactions with the solvent and the paper: the liquid carries them up the paper at different speeds; the solvent may be water, alcohol or a mixture of solvents.

A drop on a sketch pen

A drop of water falls on something written with a black sketch pen. The colour spreads out.

What do you expect to see?

• Several different colours spreading to different distances
• Only black spreading evenly
• Nothing spreads at all

Let the water climb

The pen mark starts on the pencil line. Move the minutes slider. Then try spinach extract.

Paper chromatography

Paper chromatography separates the components of a mixture using the differences in their interactions with the solvent and the paper. The liquid carries the substances up the paper, separating them by how fast they move.

The solvent may be water, alcohol or a mixture of solvents: water will not work in every case. The same method separates the pigments of spinach leaves or flower petals.

Curiosity. *Chromatography* comes from the Greek *chroma* (colour) and *graphein* (to write): “writing with colour”. It was first used to separate dyes and inks.

Notes

Paper chromatography separates mixtures because the solvent carries different substances up the paper at different speeds.

Check yourself

In paper chromatography the substances separate because they…

What carries the substances up the paper?

Is water a good solvent for every mixture in chromatography?

The word *chromatography* means…

• all move at the same speed. Then they would not separate.
• dissolve in the paper completely. The liquid carries them along the paper.
• move up the paper at different speeds — correct. Yes.
• The liquid solvent — correct. Yes.
• A magnet. Chromatography does not use a magnet.
• Light from a lamp. Not a part of the method.
• Yes, always. In some cases you need a different solvent.
• No. Sometimes alcohol or a mixture of solvents is needed — correct. Yes.
• No, chromatography needs no solvent at all. The solvent carries the substances.
• cutting with heat. Not the meaning.
• boiling a liquid. Not the meaning.
• writing with colour — correct. Yes. Chroma is colour; graphein is to write.