Pure substances and separation techniques

Pure vs mixture

In chemistry pure means a single element or compound, with no other substance present. Pure ice is just H₂O. "Pure" orange juice is a mixture (water + sugars + citric acid).

Test for purity: a pure substance has a sharp, fixed melting/boiling point. A mixture melts/boils over a range, usually lower than the pure component.

Filtration

Separates an insoluble solid from a liquid (residue / filtrate). Use a filter funnel and filter paper. Example: removing sand from salty water.

Crystallisation

Recovers a soluble solid from its solution. Heat the solution gently to evaporate most of the water, then leave to cool — pure crystals form as the solvent evaporates. Used to obtain copper sulfate from solution.

Simple distillation

Separates a liquid from a dissolved solid (e.g. pure water from sea water). The liquid evaporates, vapour passes into a condenser, cools and is collected. Salt is left in the flask.

Fractional distillation

Separates a mixture of liquids with different boiling points (e.g. ethanol + water; crude oil into fractions). A fractionating column has a temperature gradient — lower-bp liquids reach the top first; higher-bp liquids condense lower down.

Paper chromatography

Separates and identifies coloured substances in a mixture (e.g. inks, food dyes).

1. Draw a baseline in pencil (ink would dissolve).
2. Spot samples of mixture and references.
3. Place in solvent below baseline.
4. Solvent rises; substances rise different distances depending on attraction between stationary phase (paper) and mobile phase (solvent).
5. Calculate Rf:

Rf = distance moved by spot / distance moved by solvent

Rf is always between 0 and 1; identical Rf in same solvent suggests the same substance.

OCR PAG C5 — chromatography

Tests inks; separates components and compares Rf with knowns. Solvent must not reach the baseline; lid covers tank to keep solvent saturated.

OCR exam tip

Two-mark questions on "why pencil for the baseline" want both: pencil is insoluble in the solvent; ink would dissolve and confuse the result.