A farmer buys a liquid weedkiller product from a garden centre. The label states that it contains several different active ingredients mixed together in carefully measured quantities to give the product its required properties. Explain what type of mixture this weedkiller product is, and describe how paper chromatography could be used to show that the product contains more than one active ingredient.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The product is a formulation – a mixture made up of exact quantities of pure active ingredients designed for a particular use.
In paper chromatography the paper is the stationary phase and a solvent is the mobile phase. The sample is applied as a spot near the bottom of the paper. As the solvent rises, each component moves a different distance because of its differing solubility in the solvent.
If the chromatogram shows more than one distinct spot, this proves that the weedkiller contains more than one active ingredient.
In paper chromatography the paper is the stationary phase and a solvent is the mobile phase. The sample is applied as a spot near the bottom of the paper. As the solvent rises, each component moves a different distance because of its differing solubility in the solvent.
If the chromatogram shows more than one distinct spot, this proves that the weedkiller contains more than one active ingredient.
Examiner tips
- Use the term ‘formulation’ and explain it is a mixture of pure substances in measured amounts.
- Describe the roles of stationary and mobile phases and why components separate.
- State that multiple spots indicate multiple substances.
Common mistakes
- Calling the product a ‘solution’ instead of a formulation.
- Failing to mention the solvent as the mobile phase.
- Assuming a single spot proves only one ingredient without justification.
Mark scheme (5 marks)
- The weedkiller is a formulation
- A formulation is a mixture designed for a particular use / made up of exact quantities of pure substances
- In paper chromatography, paper acts as the stationary phase and a solvent (mobile phase) carries the mixture up the paper
- Different substances travel different distances up the paper because they have different solubilities in the solvent
- More than one spot on the chromatogram shows the product contains more than one substance / active ingredient
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain and Describe" question →
- Decode the mark scheme abbreviations →
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