Investigation 5: Chromatography

Two inks can look like the same color and still be made of different dyes mixed together to produce that color. Each dye has its own set of intermolecular forces, so each one interacts differently with the paper (stationary phase, polar, full of -OH groups from cellulose) and the solvent (mobile phase). Components that cling more to the paper barely move. Components that dissolve better in the solvent and interact weakly with the paper travel far up the strip. The result is a trail of separated spots instead of one clean dot.

To run the chromatogram: spot a small, concentrated drop of the sample near the bottom of the paper, let it dry, then stand the paper in a shallow layer of solvent in a covered container, keeping the spot above the liquid level. The solvent wicks upward by capillary action, carrying each component at its own rate. Before the solvent reaches the top, pull the paper out and immediately mark the solvent front in pencil, since it fades once the paper dries. Measuring each spot's distance against the front gives a retention factor, Rf, equal to distance traveled by the spot divided by distance traveled by the solvent.

Swap a polar solvent like water for a nonpolar one like hexane and the order of separation can flip: polar components travel farther in the polar solvent, and nonpolar components travel farther in the nonpolar solvent, following the like-dissolves-like rule from intermolecular forces. This links chromatography to the unit's broader theme of designing experiments that reveal invisible structural differences, and to error analysis, since Rf values are only as reliable as the precision of marking the front and spot centers.