U3.9 · Separation of Solutions and Mixtures (Chromatography, Distillation)Practice · 混合物的分离（色谱、蒸馏）

Rf = distance moved by the spot ÷ distance moved by the solvent front, so the spot’s distance = Rf × the front’s distance = 0.36 × 7.5 = 2.7 cm. Rf is always between 0 and 1, since a spot can’t pass the solvent front.

The silica surface carries –OH groups that hydrogen-bond or form dipole–dipole attractions with polar molecules. The more polar a compound, the more strongly it sticks to the stationary phase, the more slowly it moves, and the smaller its Rf. Hydrocarbon X is least polar and stays mostly in the nonpolar mobile phase, so its Rf is largest; Z, with two –OH groups, hydrogen-bonds to the silica and has the smallest Rf; Y is in between.

Distillation relies on a difference in boiling points: the bigger the difference, the purer the first fraction. Boiling-point differences: $\ce{C5H12}$ / $\ce{C6H14}$ 32.6 °C, $\ce{C6H14}$ / $\ce{C7H16}$ 29.7 °C, $\ce{C7H16}$ / $\ce{C8H18}$ 27.2 °C, $\ce{C5H12}$ / $\ce{C8H18}$ 89.5 °C. Pentane and octane differ the most, so they separate most easily.

Matching Rf values under the same conditions mean that component distributes between the two phases the same way X does, so it is probably X. But different compounds can share an Rf by chance, so it isn’t proof; chemists often repeat the test with another mobile phase. On a polar stationary phase, the component at Rf 0.40 moves more slowly, so it is more polar than X, not less.

Pen and marker inks are themselves mixtures of dyes that dissolve in the mobile phase, travel up with it, and form their own spots among the sample’s spots. Graphite doesn’t dissolve in the usual solvents, so it stays put and serves only as a mark.