Distillation, crystallisation and chromatography

Chemists choose a separation method by asking: what is mixed with what, and how are they different?

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Filtration and crystallisation

Filtration separates an insoluble solid from a liquid. The solid left in the filter paper is the residue; the liquid that passes through is the filtrate.
Crystallisation gets a soluble solid out of a solution. Heat the solution to evaporate some of the water, then leave it to cool. Crystals form as the solution becomes saturated.
Simple evaporation to dryness also leaves the solid, but the crystals are smaller and some solids break down if heated too strongly.

Simple distillation

Distillation gets a pure liquid (the solvent) from a solution, such as pure water from sea water.
The solution is heated and the water boils. The steam passes into a condenser, where cold water flowing around the outside cools it, so it condenses back to liquid.
The pure liquid that is collected is the distillate. The salt stays behind in the flask.
A thermometer at the top shows the boiling point: 100 °C for pure water.

Chromatography

Chromatography separates the dyes in an ink.
Draw a start line in pencil (ink would run). Put spots of ink on it and stand the paper in a solvent, keeping the spots above the solvent.
The solvent soaks up the paper and carries each dye a different distance, depending on how soluble it is.
A pure substance gives one spot. If a spot matches the height of a known dye, it is probably that dye.
Worked example

Choose a method to get each of these: (a) sand from muddy water, (b) pure water from ink, (c) copper sulfate crystals from copper sulfate solution.

  1. Sand is insoluble, so filter it.
  2. Water is the solvent we want, so distil it.
  3. Copper sulfate is a dissolved solid, so crystallise it.

Answer: (a) filtration, (b) distillation, (c) crystallisation

Key idea

Filtration: insoluble solid from liquid. Crystallisation: dissolved solid from solution. Distillation: pure solvent from solution, using boiling then condensing. Chromatography: separates dyes; a pure substance gives one spot.

Key words
Distillation
Separating a liquid from a solution by boiling it and then condensing the vapour.
Chromatography
A way of separating dissolved substances, such as the dyes in ink, as a solvent moves up paper.
Filtration
Separating an insoluble solid from a liquid using filter paper.
Solvent
The liquid that a solute dissolves in.
Condensing
The change of state from a gas to a liquid.
Pure substance
A single element or compound with nothing else mixed in. It melts and boils at fixed temperatures.

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  1. In filtration, what is the solid left on the filter paper called?

    1. The distillate
    2. The filtrate
    3. The residue
    4. The solvent
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    Answer: The residue

    The residue stays behind.

  2. In distillation, what is the liquid collected called?

    1. The solute
    2. The residue
    3. The filtrate
    4. The distillate
    Show the answer

    Answer: The distillate

    The distillate is the condensed liquid.

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Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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