A paint manufacturer produces a decorative wall paint by mixing several pure substances together in carefully measured quantities. Explain why this wall paint is described as a formulation and not a pure substance, and describe how paper chromatography could be used to show that the coloured pigment in the paint is a mixture rather than a pure substance.

Eduqas A-Level Chemistry — 1.1 Atoms, molecules and stoichiometry · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A paint manufacturer produces a decorative wall paint by mixing several pure substances together in carefully measured quantities to give the product specific properties.

Model answer (5 marks)

A formulation is a mixture designed for a particular use, made from exact or carefully measured quantities of pure substances.
The paint is not a pure substance because it contains more than one compound or element that are not chemically bonded together.
To show that the coloured pigment is a mixture, a small spot of the pigment is applied to a strip of chromatography paper (the stationary phase). The paper is then placed in a solvent (the mobile phase). The solvent rises up the paper by capillary action, carrying the components of the pigment with it. Because the different components have different solubilities in the solvent, they move at different speeds and travel different distances.
If the pigment were a pure substance, only one spot would appear on the paper. The appearance of more than one spot indicates that the pigment is a mixture.

Examiner tips

  • Use the term ‘formulation’ and explain it as a mixture of pure substances. Mention that the paint contains multiple uncombined compounds. Describe the chromatography set‑up: spot, stationary phase, mobile phase. State that multiple spots mean a mixture, one spot means a pure substance.

Common mistakes

  • Calling the paint a pure substance. Forgetting to explain that the components are not chemically bonded. Describing chromatography but not linking multiple spots to a mixture.

Mark scheme (5 marks)

  1. A formulation is a mixture designed for a particular use, made from exact/carefully measured quantities of pure substances
  2. The paint is not a pure substance because it contains more than one compound/element that are not chemically bonded/combined together
  3. A spot of the pigment is placed on paper (the stationary phase) above a solvent (the mobile phase)
  4. The solvent (mobile phase) travels up the paper, carrying the components of the pigment at different speeds/distances because they have different solubilities in the solvent
  5. A mixture will produce more than one spot on the paper, whereas a pure substance produces only a single spot, so multiple spots confirm the pigment is a mixture

Key terms in this question

formulation · pure substance · mixture · paper chromatography

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