The chemical properties of each substance in the mixture are unchanged.

The idea

When substances are mixed without chemical reaction, each substance keeps its own chemical properties. For example, the oxygen in air still supports combustion and the nitrogen still behaves as a relatively unreactive gas; the salt in seawater still gives the characteristic chloride test. This is because no new chemical bonds form between the components, so the particles remain the same and their reactions are unchanged. Physical properties such as melting point and boiling point may change because the components affect each other physically; for instance, seawater boils at a higher temperature than pure water. This principle allows mixtures to be separated and the components recovered with their original chemical identities.

What earns the marks

• States that each substance in a mixture keeps its own chemical properties because no chemical bonds form between the components.
• Explains that the particles of each substance remain unchanged, so their reactions and characteristic tests are the same as when the substance is pure.
• Contrasts this with a compound, where the elements have reacted and the compound has different chemical properties from its constituent elements.
• Uses examples such as air, where oxygen still supports combustion, or a salt solution, where the salt still gives a positive flame test or precipitation reaction.
• Distinguishes chemical properties, which are unchanged, from physical properties such as boiling point, which may change in a mixture.
• Links the unchanged chemical properties to the possibility of separating a mixture and recovering the original substances.

Common mistakes and how to put them right

• Claiming that a mixture has the average properties of its components.

  Put it right: State that each component keeps its own chemical properties, although measured physical properties may vary with composition.

• Confusing a mixture with a compound and saying new chemical properties appear.

  Put it right: Explain that new properties arise only when elements react chemically to form a compound.

• Assuming that because a mixture looks different from its components, its chemical behaviour has changed.

  Put it right: Test or recall that the components still react in their characteristic ways.

Exam tips

• Use a named example and state the unchanged chemical property, such as oxygen in air supporting combustion.
• Contrast the mixture with a compound formed from the same elements to show the difference in chemical properties.
• Avoid saying properties are always identical; specify that chemical properties are unchanged while physical properties may vary.