Purity, formulations and chromatography · GCSE Chemistry

Pure substances

In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
Pure substances melt and boil at specific temperatures. Mixtures melt and boil over a range of temperatures.
Melting point and boiling point data can tell pure substances apart from mixtures. Impurities usually lower the melting point.
In everyday language, 'pure' can mean a substance that has had nothing added to it, such as 'pure' orange juice, which is really a mixture.

A formulation is a mixture that has been designed as a useful product.
The components are mixed in carefully measured quantities so the product has the properties needed.
Examples: fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

Chromatography separates mixtures and helps identify substances.
It uses a stationary phase (the paper) and a mobile phase (the solvent). Substances move different distances depending on how they are attracted to each phase.
Rf = distance moved by the substance ÷ distance moved by the solvent. Rf values are always less than 1.
Different compounds have different Rf values in a given solvent, so a substance can be identified by comparing it with known substances.
A pure compound produces a single spot in all solvents. A mixture may separate into several spots.

Draw a start line in pencil (it won't run like ink) about 1 cm above the bottom of the paper.
Put small spots of the samples on the line, and place the paper in a beaker of solvent with the solvent level below the line.
Cover with a lid so the solvent doesn't evaporate. Remove the paper when the solvent is near the top, and mark the solvent front.
Measure the distances and calculate the Rf value of each spot.

Pure substance: A single element or compound, not mixed with anything else.
Impurity: An unwanted substance mixed with a pure substance.
Formulation: A mixture designed as a useful product, with components in measured quantities.
Chromatography: A method of separating and identifying substances in a mixture.
Stationary phase: The part of chromatography that doesn't move, such as the paper.
Mobile phase: The solvent that moves through the stationary phase.
Rf value: Distance moved by a substance ÷ distance moved by the solvent.
Solvent front: The furthest point the solvent reaches up the paper.