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Criteria for purity.,determination of purity,effects of impurity on melting point and boiling point

Answer Text:
Criteria for purity.
Pure substance:
- Is a substance that contains only one type of compound or element.
Determination of purity
(a). Solids:
- Purity of solids is determined by measuring the melting point.
- Pure solids melt sharply over a narrow temperature range.
Examples:
- Naphthalene melts at# 80^oC
– 81^oC#.
- Water melts at #0^oC#.
Effects of impurity on melting point.
- Impurities lower themelting point of a substance making it melt over a wide range of temperatures i.e. the melting point is not sharp.
Applications:
1. Extraction of metals.
- Impurities are added to purified metal ores to lower their melting points, hence save energy and extraction costs.
2. To prevent knocking of engines due to freezing of water in car radiators during cold seasons, impurities like ethylene glycol or salt (NaCl) is added.
3. Defrosting of frozen roads and sidewalks in temperate countries, by sprinkling a salt such as sodium chloride.
(b). Liquids:
- Purity of a liquid is determined by measuring its boiling point.
- A pure liquid has a sharp boiling point.
Examples:
Pure water boils at #100^oC# at 1 atmospheric pressure. Pure ethanol boils at #78^oC#.
Effect of impurity on boiling point.
- Impurities raise the boiling point of a liquid.
Example:
- Sea water boils at a higher temperature than pure water due to the presence of dissolved salts.