Why is a pencil used to draw the origin line in chromatography?

• A. Pen would dissolve and give colours, making the experiment inaccurate.
• B. Pencils are cheaper than pens.
• C. Pen marks are water-soluble and would blur.
• D. Pen would attract the solvent.

Answer: (A)

In chromatography, the starting line must stay fixed and not interact with the solvent. Pencil graphite is largely insoluble in the common solvents used, so drawing the origin with a pencil keeps that mark at the bottom without it moving or dissolving. If a pen were used, its ink could dissolve in the mobile phase or be carried up the paper, adding colour from the line itself and distorting the true pattern of the sample components. That interference would make the results unreliable. The other factors mentioned aren’t the main issue; the key idea is to avoid any drawing material that could dissolve or travel with the solvent and tamper with the chromatogram.