Purity and Formulations — AQA GCSE Chemistry
In chemistry, "pure" has a precise meaning, and many useful products are carefully designed mixtures called formulations.
Pure substances
In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
- A pure substance melts and boils at a specific, sharp temperature.
- An impure substance melts/boils over a range of temperatures. Impurities usually lower the melting point and raise the boiling point.
Measuring the melting point is therefore a way to test purity: a sharp melting point at the expected value shows a pure substance.
Formulations
A formulation is a mixture that has been designed as a useful product, with each component present in a measured quantity to give the product its required properties.
Examples of formulations:
- medicines (active drug + fillers, binders, coatings),
- fuels, cleaning products, paints, fertilisers, alloys, cosmetics and foods.
By carefully controlling the proportions, manufacturers ensure the product performs exactly as intended.
Exam tips
- In chemistry, pure = a single element or compound (not "clean").
- A pure substance has a sharp melting/boiling point; impurities broaden and shift it.
- A formulation is a mixture made in measured quantities for a purpose — give an example.
- Use melting point as a test of purity.