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

• A. Because pen inks can dissolve in the solvent, causing colors to spread and distort results.
• B. Because pencils are easier to see.
• C. Because pencil marks are permanent.
• D. Because black ink dries slower.

Answer: (A)

In chromatography, the origin line must stay fixed and not be carried along by the developing solvent. Pen inks contain dyes that can dissolve in the solvent, so drawing the line with ink would allow the solvent to move the ink up the plate, smearing the baseline and distorting the pattern of spots. Pencil marks, made with graphite, are largely insoluble in the common solvents used for chromatography, so the origin line remains in place and does not contaminate the developing front. This gives a clean, reliable reference point for measuring how far substances travel.