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Purity (IGCSE)
2.2 Purity
2.2.1 Criteria of purity

• Identify substances and assess their purity from melting point and boiling point information.
A pure substance has a definite, sharp melting point and boiling point.

When a substance contains an impurity:
– its melting point falls and its boiling point rises
– it melts and boils over a range of temperatures, not sharply.
Example: melting point of seawater is about 2.5oC. boiling point of seawater is about 102oC
The more impurity there is:
– the bigger the change in melting and boiling points
– the wider the temperature range over which melting and boiling occur.
 Example, pure ice melts at 0Oc and pure water boils at 100Oc. If we dissolve 58 grams of salt in 1000cm3 of water, the boiling point will increase by 0.5 degrees.
Understand the importance of purity in substances in everyday life, e.g. foodstuffs and drugs.

In real life, very few substances are pure. For example, tap water contains small amounts of calcium ions and chloride ions. You can bathe using tapwater water without being harmed.
But sometimes purity is very important. If you are making a new medical drug, or a flavouring for food, there must be no impurity. Impurities in foodstuffs or drugs can produce undesirable and harmful effects.