Pure Substance vs Mixture (Edexcel IGCSE Chemistry)

Spec point — 1.9

Pure substance vs mixture

• 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 mixture and not a pure substance because it contains H₂O molecules and additional substances like dissolved ions and chlorine

Pure substance v mixture

Pure water consists of only H₂O molecules whereas tap water is a mixture

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

Worked Example Sulfur has a melting point of 114 ^oC.

A student tests the melting point of a sample of sulfur. It begins to melt at 100 ^oC and finishes melting at 113 ^oC.

Explain whether the substance is pure or impure.

Final answer: Answer:

• The substance is impure because its melts over a range of temperatures.