What Is Chromatography?
A separation technique used to separate mixtures of substances based on how they travel through a stationary phase when carried by a moving phase (the mobile phase).
Chromatography is one of the most powerful and widely used techniques in chemistry for separating and identifying the components of a mixture. The word itself comes from the Greek chroma (colour) and graphe (writing) — it was first used to separate coloured plant pigments.
Today, chromatography is used in fields ranging from forensic science and food testing to pharmaceutical research and environmental monitoring.
Think of chromatography like a race. Imagine several runners (the different substances in a mixture) all starting at the same line. Some runners are faster and get carried further; others are slower and fall behind. At the end of the race, each runner is in a different position — and that's exactly how chromatography separates substances!
All types of chromatography share two key components:
- Stationary phase — the material that stays still (e.g. paper, silica gel)
- Mobile phase — the solvent or gas that moves through the stationary phase, carrying the substances with it
Substances separate because different molecules have different affinities (attractions) for the stationary phase versus the mobile phase.
Paper Chromatography — How It Works
A type of chromatography that uses absorbent paper as the stationary phase and a liquid solvent as the mobile phase to separate mixtures.
Paper chromatography is the simplest and most commonly performed type in MYP labs. Here is how the process works step by step:
- A small spot of the mixture (e.g. ink, food colouring, plant extract) is placed near the bottom of a strip of chromatography paper.
- The paper is placed in a container with a small amount of solvent (the mobile phase) — the solvent level must be below the spot.
- The solvent slowly travels up the paper by capillary action, carrying the substances in the mixture with it.
- Different substances move at different speeds depending on:
- How soluble they are in the solvent (more soluble = travels further)
- How strongly they stick to (adsorb onto) the paper (sticks more = travels less)
- After the solvent nearly reaches the top, the paper is removed and dried. The final solvent position is marked as the solvent front.
The pencil line where the spots are placed is called the baseline or origin. Always use pencil (not pen) to mark this line, because pen ink would dissolve and travel with the solvent, ruining your results.
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