Pure Substances and Formulations
Build confidence in Pure Substances and Formulations for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A pure substance melts and boils at one fixed temperature, while a formulation is a mixture made to an exact recipe, like paint or fuel.
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Topic overview
In chemistry, a pure substance is one made of only a single element or compound. This differs from the everyday meaning, where pure often just means natural or unadulterated.
Pure substances melt and boil at specific, sharp temperatures. Mixtures melt and boil over a range of temperatures, so measuring the melting or boiling point is a way of testing purity.
A formulation is a mixture that has been designed as a useful product. The components are present in carefully measured quantities so that the product has the required properties. Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods are all formulations.
Revision notes
Pure substances
In chemistry a pure substance consists of a single element or compound only.
This is stricter than the everyday meaning. Pure orange juice is a mixture chemically, even though it is described as pure in the shops.
Testing purity
Pure substances melt and boil at specific sharp temperatures.
Mixtures melt and boil over a range of temperatures. So a sharp melting point indicates a pure substance, and a broad range indicates a mixture — this is the standard purity test.
Formulations
A formulation is a mixture designed as a useful product, with components in carefully measured quantities.
Each component is present in a specific amount so the product has the required properties. Examples: fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
Key points
- A pure substance is a single element or compound.
- Pure substances melt at a sharp temperature.
- Mixtures melt over a range of temperatures.
- Melting point is used to test purity.
- A formulation is a mixture designed as a useful product.
- Formulation components are in measured quantities.
Worked examples
Example 1
Explain how melting point can be used to test whether a substance is pure. [2 marks]
Model answer
A pure substance melts at a specific sharp temperature (1 mark), whereas a mixture melts over a range of temperatures, so a broad melting range indicates impurity (1 mark).
Example 2
Define a formulation. [2 marks]
Model answer
A formulation is a mixture that has been designed as a useful product (1 mark), in which the components are present in carefully measured quantities so the product has the required properties (1 mark).
Example 3
Explain why orange juice labelled pure is not chemically pure. [2 marks]
Model answer
Chemically, pure means a single element or compound only (1 mark), but orange juice is a mixture of water, sugars and many other compounds (1 mark).
Common mistakes
Using the everyday meaning of pure.
In chemistry it means a single element or compound, nothing else.
Saying mixtures have sharp melting points.
They melt over a range; only pure substances are sharp.
Calling any mixture a formulation.
A formulation is deliberately designed with measured quantities for a purpose.
Not giving an example of a formulation.
Paint, medicine and alloys are all valid.
Exam tips
- Use the chemical definition of pure in your answers.
- Contrast sharp melting points with melting ranges.
- Say designed and measured quantities when defining a formulation.
- Learn several examples of formulations.
Key terms
- Pure substance
- A single element or compound.
- Formulation
- A mixture designed as a useful product.
- Melting point
- The temperature at which a solid melts, sharp if pure.
- Mixture
- Two or more substances not chemically combined.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.