Why Can Soap Carry Away Grease That Water Cannot?

When plain water runs over a greasy plate, it often beads around the oily film. This is not because water lacks force. Water is a polar liquid whose molecules interact readily with one another, while cooking oils consist mostly of non-polar molecules. Mixing the two is unfavourable, so they tend to separate.

The key ingredients in soap and many detergents are surfactants. A typical surfactant molecule has two contrasting parts: a charged or polar head that interacts with water, and a long non-polar tail that prefers oil. During rubbing or scrubbing, the tails enter the grease while the heads remain in the water, breaking the oily film into smaller droplets. Enough surfactant molecules can surround a droplet to form a dispersed aggregate, often simplified as a micelle. Its water-friendly exterior allows the oil-containing particle to remain suspended and be carried away in the rinse.

Soap therefore does not make grease vanish, nor does cleaning require the grease to be chemically destroyed. It changes the interfaces among oil, water and the solid surface, turning a stubborn film into small droplets that can move with water. Warm water may soften some fats and help them flow, but it is the surfactant's amphiphilic structure that connects the oil to the water phase.

Sources: https://openstax.org/books/organic-chemistry/pages/27-2-soap
https://edu.rsc.org/resources/chemistry-in-your-bathroom-resource/1050.article