GCSE Chemistry AQA 8462

Pure substances

In chemistry, a pure substance is a single element or compound, not mixed with any other substance.

Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.

In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.

Students should be able to use melting point and boiling point data to distinguish pure from impure substances.

Formulations

A formulation is a mixture that has been designed as a useful product. Many products are complex mixtures in which each chemical has a particular purpose. Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties. Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

Students should be able to identify formulations given appropriate information.

Students do not need to know the names of components in proprietary products.

Chromatography

Chromatography can be used to separate mixtures and can give information to help identify substances. Chromatography involves a stationary phase and a mobile phase. Separation depends on the distribution of substances between the phases.

The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent can be expressed as its R_f value:

$R_f = \dfrac{\text{distance moved by substance}}{\text{distance moved by solvent}}$

Different compounds have different R_f values in different solvents, which can be used to help identify the compounds. The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.

Students should be able to:

• explain how paper chromatography separates mixtures
• suggest how chromatographic methods can be used for distinguishing pure substances from impure substances
• interpret chromatograms and determine R_f values from chromatograms
• provide answers to an appropriate number of significant figures.