After separating substances from mixtures, it is often necessary to determine their degree of purity. Several methods are employed to check whether a substance is pure or contains impurities.
Methods of Testing Purity
1. Melting Point (Solids)
- A pure solid melts at a fixed temperature.
- The presence of impurities causes the solid to:
- Melt over a range of temperatures, or
- Melt at a lower temperature than the pure solid.
Example: Ice melts more quickly when salt is sprinkled on it.
2. Boiling Point (Liquids)
- A pure liquid boils at a fixed temperature.
- Impurities cause the liquid to:
- Boil over a range of temperatures, or
- Boil at a higher temperature than the pure liquid.
3. Paper Chromatography (Coloured Substances)
- Used for testing the purity of dyes and pigments.
- A pure substance produces a single spot on the chromatogram.
- An impure substance produces multiple spots.
General Effect of Impurities
- Impurities lower the melting point of solids.
- Impurities raise the boiling point of liquids.
Real‑World Applications
- Salting icy roads: Salt lowers the melting point of ice, helping it melt quickly in winter.
- Antifreeze in car radiators: Added to prevent water from freezing in regions where temperatures fall below 0°C.
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