GCSE Chemistry revision

Pure substances

Purity, formulations and chromatography

AQA 4.8.1.1 Foundation & Higher

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A pure substance contains only one type of element or compound and is not mixed with anything else.
Examples of pure substances include pure water (containing only water molecules) and pure sodium chloride.
A mixture such as salt water is not considered pure because it contains two different compounds combined together.

1. Pure Substances in Chemistry AQA

A pure substance contains only one type of element or compound and is not mixed with anything else.
Examples of pure substances include pure water (containing only water molecules) and pure sodium chloride.
A mixture such as salt water is not considered pure because it contains two different compounds combined together.

Only pure substances melt and boil at specific, fixed temperatures, which can be used to identify unknown substances.
For example, pure water boils at exactly 100°C and sodium chloride melts at exactly 801°C.
Testing the melting or boiling point of a substance is a physical test, as it examines the physical properties of that substance.
Unknown substances can be identified by comparing their measured melting or boiling points against values in a data book.

2. Melting and Boiling Points of Pure Substances AQA

Only pure substances melt and boil at specific, fixed temperatures, which can be used to identify unknown substances.
For example, pure water boils at exactly 100°C and sodium chloride melts at exactly 801°C.
Testing the melting or boiling point of a substance is a physical test, as it examines the physical properties of that substance.
Unknown substances can be identified by comparing their measured melting or boiling points against values in a data book.

Impure substances do not have fixed melting or boiling points; instead, they melt and boil over a range of temperatures.
In general, the presence of impurities lowers the melting point and raises the boiling point of a substance.
For example, salt water has a melting point of approximately −2°C (compared to 0°C for pure water) and a boiling point of around 100.5°C (compared to 100°C).

3. Effect of Impurities on Melting and Boiling Points AQA

Impure substances do not have fixed melting or boiling points; instead, they melt and boil over a range of temperatures.
In general, the presence of impurities lowers the melting point and raises the boiling point of a substance.
For example, salt water has a melting point of approximately −2°C (compared to 0°C for pure water) and a boiling point of around 100.5°C (compared to 100°C).