Why Soap Can Grab What Water Cannot
Grease repels water because its molecules are mostly nonpolar: they do not form the kinds of attractions that let water molecules mingle easily. Soap molecules have two contrasting parts. One end interacts readily with water; the other is attracted to oily substances. At a greasy surface, soap molecules gather with their oil-loving ends tucked into the grease and their water-friendly ends exposed to the surrounding water.
As rubbing and rinsing break the grease into smaller bits, soap helps keep those bits dispersed instead of letting them merge back into a slick. When enough soap is present, many molecules can cluster around oily material in tiny structures called micelles. Their outer surfaces interact with water, so the grease can travel through the rinse rather than cling to a dish or sink.
The Rinse Completes the Job
Soap does not make grease disappear or turn it into water. It makes oily residue easier to lift and carry away. Friction loosens it from a surface; soap helps surround and disperse it; flowing water removes the resulting mixture. Without the rinse, the grease and soap would still be there, just redistributed.
That is why a quick splash may leave a slippery film, while scrubbing followed by a thorough rinse works better: the water has to carry the loosened, soap-coated grease out of the sink.
