Question

Why can the fluids used to dry-clean clothing remove grease even when water cannot?

Answer 1

Can oil and water mix; what about water and hexanes?

Explanation:

A strongly polar solvent such as water is incapable of dissolving non-polar solutes and solvents. A grease stain is certainly composed of long chain hydrocarbons, with which water will not interact.

On the other hand, the use of dry cleaning fluids, which is typically `"perc"`, `"perchloroethylene"`, `"Cl"_2"C"="CCl"_2`, will lift such a stain, as it is non-polar and capable of dissolving grease.

Note other solvents than `"perc"` are used for dry cleaning. And typically, a dry cleaner (so I am told) will spot obvious stains with some other solvent or stain remover before it is put thru the regular cycle.