Pure Substances (AQA GCSE Chemistry: Combined Science)

• In everyday language, we use the word pure to describe when something is natural or clean and to which nothing else has been added
• In chemistry, a pure substance may consist of a single element or compound which contains no other substances
• For example, pure water contains only H₂O molecules and nothing else
• Drinking water would not be classed as a pure substance because it contains H₂O molecules and additional substances like dissolved ions and chlorine

Pure v Impure Water 

Pure water will consist of only H₂O molecules, whereas tap water has additional substances so is not chemically pure

How can purity be distinguished?

• Pure substances melt and boil at specific and sharp temperatures
  • E.g. pure water has a boiling point of 100 °C and a melting point of 0 °C
• Impure substances have a range of melting and boiling points as they consist of different substances 
• Generally, impure substances have lower melting points and higher boiling points than the pure substance 
• Melting and boiling point data can therefore be used to distinguish pure substances from mixtures
• Melting point analysis is routinely used to assess the purity of drugs
• This is done using a melting point apparatus which allows you to slowly heat a small amount of the sample, making it easier to observe the exact melting point
• This is then compared to data tables
• The closer the measured value is to the actual melting or boiling point then the purer the sample is
• Measuring purity is also important in foodstuffs