Meanings Of Purity (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note
Exam code: 8465
Pure substances
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
Compared to pure substances, impure substances have:
Lower melting points
Higher boiling points
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.
Answer:
The substance is impure because:
It melts over a range of temperatures (100–113 °C)
The melting point is also lower than the expected value of 114 °C
Examiner Tips and Tricks
When explaining that a substance is pure, use the word specific or constant temperature — not "fixed", "same" or "consistent", as these can lose marks
Remember that impurities:
Lower melting points
Raise boiling points
When analysing melting point data, look for two pieces of evidence of impurity:
The substance melting over a range of temperatures
Whether the measured melting point is lower than the known value
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