Which of the following separation techniques would show that black ink is a mixture of chemical compounds?

1. Analyze the question We need the technique that can separate the different coloured compounds (dyes/pigments) dissolved in black ink, and so show that the ink is a mixture.

2. Explain the concept Chromatography separates substances in a mixture based on how differently they dissolve in a solvent (the mobile phase) and how strongly they stick to a surface such as filter paper (the stationary phase). Components that are more soluble and less strongly attracted to the paper travel farther, so they separate into distinct spots or bands.

3. Solution, step by step

• Put a spot of black ink near the bottom of a strip of filter paper.
• Dip the bottom edge of the paper in a solvent (water or alcohol), keeping the ink spot above the solvent level.
• The solvent rises up the paper and carries the ink components with it at different rates.
• Black ink separates into several colours (for example blue, yellow, and red or pink bands). This proves it is a mixture of different dyes.

Checking the other options:

• A. Crystallization: separates a dissolved solid from its solution. It does not separate the individual dyes in ink.
• C. Filtration: separates an insoluble solid from a liquid. The dyes in ink are dissolved, so they pass straight through the filter paper.
• D. Sublimation: separates a solid that sublimes (such as ammonium chloride or iodine) from one that does not. Ink dyes are not separated this way.

4. Answer The correct option is B. Chromatography.

5. Exam tip Remember: "Chromatography = Colours." When a WASSCE, JAMB or NECO question mentions inks, dyes, pigments or plant colours (such as chlorophyll), think chromatography first.