AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Compounds can only be separated into elements by chemical reactions.
Full specification wording
Atoms, elements and compounds: Compounds can only be separated into elements by chemical reactions.
Because the elements in a compound are chemically combined, separating them requires a chemical reaction that breaks the bonds holding the atoms together.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 7 of 10
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Because the elements in a compound are chemically combined, separating them requires a chemical reaction that breaks the bonds holding the atoms together. For example, passing electricity through molten sodium chloride decomposes it into sodium and chlorine, and heating mercury oxide decomposes it into mercury and oxygen. These are decomposition reactions. Physical methods such as filtration, distillation, crystallisation and chromatography separate mixtures, not compounds, because they rely on differences in physical properties and do not break chemical bonds. The statement means that no physical separation technique can split a compound into its elements; only a chemical change can do so.
What earns the marks
- Elements in a compound are held together by chemical bonds, so separating them requires breaking those bonds.
- Chemical reactions such as thermal decomposition or electrolysis can break a compound into its elements.
- Physical separation methods separate mixtures by differences in physical properties and do not break chemical bonds.
- Examples include electrolysis of molten sodium chloride to give sodium and chlorine, and heating mercury oxide to give mercury and oxygen.
- The products of decomposition are elements or simpler compounds, showing that a chemical change has occurred.
Common mistakes and how to put them right
Suggesting filtration or distillation can split a compound into elements.
Put it right: These methods separate mixtures only.
Confusing decomposition with physical change.
Put it right: Decomposition is a chemical reaction because new substances form.
Assuming all compounds decompose easily on heating.
Put it right: Some require electrolysis or other conditions, and some are very stable.
Exam tips
- Name the separation method and state whether it separates a mixture or breaks a compound, linking this to chemical bonds.
- Give a specific example of a decomposition reaction and name the elements produced.
- Use the phrase chemical reaction when explaining how a compound is separated into elements.
Try the lesson
The start of the Atoms, elements and compounds: Compounds can only be separated into elements by chemical reactions. lesson, made from checked teaching notes for this specification point.
Have a go first
Which process is needed to separate the compound copper oxide into copper and oxygen?
Listen
Because the elements in a compound are chemically combined, separating them requires a chemical reaction that breaks the bonds holding the atoms together. For example, passing electricity through molten sodium chloride decomposes it into sodium and chlorine, and heating mercury oxide decomposes it into mercury and oxygen. These are decomposition reactions. Physical methods such as filtration, distillation, crystallisation and chromatography separate mixtures, not compounds, because they rely on differences in physical properties and do not break chemical bonds. The statement means that no physical separation technique can split a compound into its elements; only a chemical change can do so. A strong answer earns marks for these points. Elements in a compound are held together by chemical bonds, so separating them requires breaking those bonds. Chemical reactions such as thermal decomposition or electrolysis can break a compound into its elements. Physical separation methods separate mixtures by differences in physical properties and do not break chemical bonds. Examples include electrolysis of molten sodium chloride to give sodium and chlorine, and heating mercury oxide to give mercury and oxygen. A common slip: Suggesting filtration or distillation can split a compound into elements. Instead, these methods separate mixtures only. Exam tip: Name the separation method and state whether it separates a mixture or breaks a compound, linking this to chemical bonds.
Key words
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Decomposition reactions
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Chemical bonding and bond breaking
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Physical versus chemical separation methods
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Mixtures versus compounds
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See every definition
- Decomposition reactions
- Chemical reactions that break a compound into its elements or simpler compounds, for example passing electricity through molten sodium chloride or heating mercury oxide.
- Chemical bonding and bond breaking
- The elements in a compound are held together by chemical bonds, so separating them requires breaking those bonds.
- Physical versus chemical separation methods
- Physical methods such as filtration, distillation, crystallisation and chromatography separate mixtures by differences in physical properties and do not break chemical bonds, whereas only a chemical change can split a compound into its elements.
- Mixtures versus compounds
- Physical methods separate mixtures, not compounds, because the elements in a compound are chemically combined.
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