Paper Chromatography
Separates coloured substances using solvent and solubility differences
Filtration
Separates insoluble solids from liquids using filter paper
Crystallisation
Isolates dissolved solids by cooling a saturated solution
Distillation
Separates liquids using differences in boiling points
Rf Values
Calculate and use to identify unknown substances (extended tier)
Purity Testing
Use melting and boiling points to assess sample purity
What is it and why does it work?
Chromatography is a separation technique that splits a mixture into individual substances. The clever bit? It works because different substances have different solubilities in a given solvent.
Think of it like this: imagine you've mixed several different coloured inks to make black ink. Each colour has a different "preference" for water — some dissolve and travel easily, others stick around. By using a paper and solvent, you can make each colour travel at a different speed, separating them as the solvent moves up.
The solvent travels up the paper by capillary action — the same force that pulls water up into a paper towel. As it moves, it carries the dissolved substances with it. Those with higher solubility in the solvent travel further, and those with lower solubility travel less far.
Key Principle: Higher solubility in the solvent = substance travels further up the paper
How to carry out paper chromatography
- Draw a pencil line near the bottom of your chromatography paper (called the baseline). Use pencil because ink would run with your samples and mess up the results.
- Spot your samples on the pencil line. Place the black ink spot on the line, and also place samples of the known pure colours (red, blue, yellow) for comparison.
- Set up the apparatus. Pour solvent (usually water) into a beaker. Lower the paper into the beaker so that the pencil line sits just above the solvent surface. The paper dips into the solvent, but your sample spots don't wash straight into the solvent.
- Wait and watch. The solvent travels up the paper by capillary action. Different substances travel at different rates. Leave it running until the solvent front reaches near the top.
- Remove and mark. Take out the paper and quickly mark where the solvent reached (the solvent front) with a pencil. Allow it to dry.
Examiner's Top Tip: Always use a pencil for the baseline, never ink. If a question asks why, the answer is: "Because ink would dissolve and travel with the samples, making the results unclear."
Interpreting chromatograms
Pure substances produce only one spot on a chromatogram. They're made of a single component, so only one substance travels.
Impure substances (mixtures) produce multiple spots — one for each component. The brown ink separates into red, yellow, and blue because it's a mixture of these three dyes.
To identify an unknown: