Chemistry · Chemical analysis. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
In chemistry a pure substance is a single element or a single compound, with nothing else mixed in. A pure substance melts and boils at one fixed temperature. An impure substance melts over a range of temperatures, and below the melting point of the pure substance.
A formulation is a mixture designed as a useful product. Each part is added in a measured quantity and has a purpose. Fuels, paints, medicines, alloys, fertilisers, cleaning agents and foods are formulations.
Paper chromatography separates the substances in a mixture. A solvent moves up the paper and carries the substances with it. Each substance moves a different distance, so the mixture spreads into separate spots.
Students divide the wrong way round and get an Rf bigger than 1. The solvent always travels further than the spot, so the solvent distance goes on the bottom.
Answer: Rf = 0.60 (no unit).
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