Distillation and Filtration

Filtration is a size sort. Sand grains in water are still solid chunks of matter, far bigger than the pores in filter paper, so they get trapped while water molecules slide through. Dissolved salt is a different story: once NaCl dissolves, it breaks apart into Na+ and Cl- ions that are no bigger than the water molecules around them. No filter paper has pores fine enough to stop an ion but let a water molecule pass, so filtration simply cannot touch a true solution.

Distillation separates by volatility instead of size, which is why it can pull apart two liquids that look identical to a filter. Heat a mixture and the liquid with the higher vapor pressure (lower boiling point, usually weaker intermolecular forces) evaporates more readily, so the vapor above the mixture is richer in that component than the liquid was. That vapor rises through a column, cools, condenses back to liquid, and drips into a separate collection flask, chemically unchanged but now enriched in the more volatile substance. In a tall fractionating column, this evaporate-condense cycle repeats many times on the way up, each step boosting the purity a little more, like doing several distillations stacked on top of each other.

A single pass struggles when boiling points are close, because the vapor pressures of the two liquids are nearly equal and neither one dominates the vapor. The enrichment per cycle is tiny, so separating liquids boiling only a couple degrees apart demands many repeated cycles, fractional distillation, rather than one simple distillation. This ties distillation directly back to intermolecular forces: the method you choose, filtration, distillation, or something else entirely, depends on whether you are separating by size (filtration) or by the strength of attractions holding molecules in the liquid versus vapor phase (distillation).