Separation of Solutions and Mixtures

Forensic chemists separate the dyes in a pen's ink to match a ransom note to a pen. Refineries split crude oil into gasoline, kerosene and diesel. Water plants remove grit before treating water. Each method works because the parts of a mixture differ in some physical property, and each property traces back to the forces between particles.

Filtration: by particle size

Filtration separates a solid that has not dissolved from a liquid. The liquid and everything dissolved in it pass through the tiny pores of filter paper (the liquid that comes through is the filtrate); the solid particles are too large and stay behind. Filtration cannot separate a solution: dissolved particles are individual ions or molecules, far smaller than the pores, so salt water goes straight through.

Chromatography: by attraction to two phases

Chromatography separates the parts of a mixture by how strongly each is attracted to two things: a stationary phase that stays put (paper, a coated plate, or a packed column) and a mobile phase that moves past it (a liquid solvent, or a gas). Every component is pulled both ways. One that interacts more strongly with the stationary phase spends more time stuck to it and moves more slowly. One that interacts more strongly with the mobile phase is carried along faster.

In paper chromatography, paper is made of cellulose, which is covered in –OH groups, so the stationary phase is very polar. With a nonpolar or weakly polar solvent, polar components hydrogen-bond to the paper and lag behind, while nonpolar components travel with the solvent.

Each spot is described by its retention factor:

R_f = distance traveled by the spot / distance traveled by the solvent front

Both distances are measured from the start line where the sample was placed. R_f is always between 0 and 1. A large R_f means the component moved nearly as far as the solvent: it is attracted more to the mobile phase. A small R_f means it is held by the stationary phase. With the same paper and solvent, a substance always gives the same R_f, so comparing an unknown's spots with known substances identifies them.

Changing the solvent changes the result. A more polar solvent competes better for polar components and carries them farther, so their R_f values rise. In column chromatography the same idea runs in a tube: the components come out of the bottom one after another, the most weakly held first.

Distillation: by volatility

Distillation separates liquids, or a liquid from dissolved solids, by heating the mixture so that the most volatile component boils off first, then cooling its vapor so it condenses in a separate flask. It works because the components differ in vapor pressure, and vapor pressure depends on intermolecular forces: the component with weaker forces between its molecules has the higher vapor pressure and the lower boiling point, so its vapor is richer in it.

Salt water can be distilled into pure water: water boils off and condenses, and the ions stay behind, because their attractions are far too strong for them to enter the vapor. When two liquids have close boiling points, one distillation gives only partial separation; a tall column packed with glass beads (fractional distillation) repeats the boil-and-condense cycle many times and does better.