A mixture consists of two or more elements or compounds not chemically…

The idea

A mixture contains two or more substances that are together in the same space but are not joined by chemical bonds. The substances keep their own identities, so a mixture can usually be separated by physical methods such as filtration, crystallisation, simple distillation, fractional distillation or chromatography. Air is a mixture of nitrogen, oxygen, argon, carbon dioxide and other gases; seawater is a mixture of water and dissolved salts. In a mixture the components can be present in any proportion, unlike a compound which has a fixed composition by mass. No new chemical bonds form between the components, so the mixture does not have a single fixed formula.

What earns the marks

• Defines a mixture as two or more elements or compounds that are not chemically combined together.
• Distinguishes mixtures from compounds: mixtures have variable composition, no chemical bonds between components and can be separated by physical means.
• Gives examples of mixtures such as air, seawater, alloys and crude oil, and identifies the elements or compounds present.
• Explains that the components of a mixture retain their individual identities and can be separated using techniques matched to their physical properties.
• Recognises that no new substance is formed when a mixture is made, so the process is not a chemical reaction.
• Uses particle diagrams or descriptions to show different types of particles mixed but not bonded together.

Common mistakes and how to put them right

• Describing a mixture as a compound because the substances are together.

  Put it right: State that a compound involves chemical bonding and fixed composition, whereas a mixture does not.

• Assuming a mixture always has a fixed ratio of components.

  Put it right: Note that mixtures can be made in any proportion.

• Thinking that a mixture cannot be separated because it looks uniform.

  Put it right: Recall that physical separation methods can separate even apparently uniform mixtures such as salt solution.

Exam tips

• When asked to classify a substance, look for a fixed formula and chemical bonding to identify a compound, and variable composition with no bonding to identify a mixture.
• Name the components of a named mixture and state the physical property that allows separation, such as boiling point or solubility.
• Use particle diagrams to support your answer; show different particles close together but not joined.