Pure Substances & Mixtures
Distinguish between pure substances and mixtures, identify formulations as useful mixtures with precise compositions, and use melting and boiling points to test for purity
What a learner can do afterwards
- Explains why a pure substance has a sharp, fixed melting point but a mixture melts over a range
- Identifies common formulations (medicines, alloys, paints, fuels) as deliberate mixtures
- Explains what impurities do to melting and boiling points
- Distinguishes mixtures from compounds at the particle level
The lesson
You already know that atoms can bond into compounds, like water, or stay mixed without bonding. A pure substance is just one substance: one element or one compound, with nothing else in it. A mixture has two or more substances mixed together, but they aren't bonded, so you can still separate them apart.
Melting point is one of the best clues for spotting a pure substance. A pure substance melts at one exact, fixed temperature: pure ice always melts right at 0°C. A mixture is messier. It starts softening at a lower temperature and doesn't finish melting until several degrees higher, because the extra particles get in the way of the neat pattern a pure substance needs.
Formulations are mixtures made on purpose, with an exact recipe. A paracetamol tablet mixes the drug with fillers in a precise amount, so every tablet works the same way. Bronze mixes copper and tin in a set ratio to make a metal harder than either one alone. Paints and fuels are formulations too, each ingredient measured for the job it does.
To test if a sample is pure, measure its melting or boiling point. Impurities usually lower the melting point and raise the boiling point, spreading both into a range instead of one sharp value. If your sample melts or boils exactly at the known value, all at once, it's pure.
A pure substance melts and boils at one sharp temperature; a mixture, including formulations, melts and boils across a range.
Watch it
Where it sits
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Where this leads
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