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8.1 Purity, formulations and chromatography

8.1 Purity, formulations and chromatography

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Question 8

A forensic scientist is analyzing a black ink sample from a suspected forged signature using thin-layer chromatography (TLC).

At the end of the experiment, the following measurements and observations were recorded:

  • The solvent front traveled a distance of 12.5 cm12.5\text{ cm}12.5 cm from the pencil baseline (start line).
  • The black ink separated into two distinct component spots: a purple spot and a yellow spot.
  • The purple spot traveled a distance of 10.0 cm10.0\text{ cm}10.0 cm from the baseline.
  • The yellow spot traveled a distance of 5.5 cm5.5\text{ cm}5.5 cm from the baseline.
a.

Explain how this chromatogram indicates that the black ink is a mixture rather than a pure substance.

[2]
b.

Calculate the RfR_fRf​ value of the purple spot using the equation:

Rf=distance moved by spotdistance moved by solvent R_f = \frac{\text{distance moved by spot}}{\text{distance moved by solvent}} Rf​=distance moved by solventdistance moved by spot​
[2]
c.

Identify which of the two spots (purple or yellow) has the greater RfR_fRf​ value.

[2]
Markscheme

8.1 Purity, formulations and chromatography Questions

  1. GCSE
  2. /Chemistry
  3. /8.1 Purity, formulations and chromatography

28 exam-style questions on AQA GCSE Chemistry 8.1 Purity, formulations and chromatography, covering 8.1.1 Pure substances, 8.1.2 Formulations, and 8.1.3 Chromatography. Each one has a worked solution and a mark scheme showing where the marks go.

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