silica

Meaning: Silica is silicon dioxide, SiO₂. In the organic lab it is used as silica gel, the polar stationary phase in thin-layer chromatography (TLC) and column chromatography. Its surface carries polar Si–OH groups that hydrogen bond to solutes. Polar compounds such as alcohols, carboxylic acids and amines bind more strongly and move slowly. Nonpolar compounds such as alkanes interact weakly and travel faster with the mobile phase (eluent).

Equation: Rf = dcompound / dfront, where dcompound is the distance the spot moved from the origin line (m), dfront is the distance the solvent front moved from the same line (m), and Rf is dimensionless, between 0 and 1.

On the MCAT: A passage describes a TLC plate or column packed with silica and an eluent such as hexane or ethyl acetate. You predict which compound has the higher Rf or elutes first, or what happens when the eluent becomes more polar: compounds are carried farther and Rf values rise. You may also compute Rf from measured distances.

Common confusion: Students assume the polar compound travels farthest because the plate is polar. The opposite holds: silica holds the polar compound more tightly, so it has the lower Rf with a typical eluent. Reversed-phase chromatography uses a nonpolar stationary phase and flips the elution order.