Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C12.2 — Parts (a)(i), (a)(ii), (b)(i), (b)(ii), (c)
(a)(i) Pencil does not dissolve in the solvent
Ink would dissolve in the solvent used during chromatography.
This would cause the start line itself to run and interfere with the separation.
Pencil (graphite) is insoluble in the solvent, so the start line stays fixed.
(a)(ii) \( R_f = 0.928 \)
\( R_f = \dfrac{\text{distance moved by substance}}{\text{distance moved by solvent}} \).
\( R_f = \dfrac{14.2}{15.3} \).
\( R_f = 0.928 \) (no unit).
(b)(i) Mixture X
An insoluble colouring cannot travel with the solvent up the paper.
It remains on the start line instead of forming a spot higher up.
Mixture X shows a spot still on the start line, indicating an insoluble component.
(b)(ii) Mixture Z
The number of spots above the start line shows the number of soluble colourings present.
Mixture Z produces the greatest number of separated spots above the line.
This indicates it contains the most soluble food colourings.
(c) The sample is impure
A pure substance melts sharply at one fixed temperature.
This sample melts over a range, from 96°C to 113°C, rather than sharply.
The melting range is also lower than the data-book value of 116°C, both of which indicate impurity.
