The tutor remembers, in a lab, receiving this basic explanation from a grad student.  While I’ve never fielded this question during tutoring, it might be of general interest.

 
A primary fact of chemistry is that “like dissolves like”. Yet, soap doesn’t dissolve dirt; it emuslfies it. So we’ll alter the principle to “like attracts like”.

A reader might protest; don’t opposites attract? Opposite charges do, yes, but opposite bond types do not.

The bond type of water is essentially polar. The oxygen hoards the electrons, leaving the hydrogen nuclei relatively bare. Therefore, the oxygen end of a water molecule is relatively negative, while the hydrogen end is correspondingly positive.

In contrast to water, the bond type in oils is non-polar. The electrons remain more or less even throughout, preventing localized charges from developing. Any cook knows that “oil and water don’t mix.” Now you know why: water is polar, while oil is non-polar. From a molecular point of view, like attracts like; opposites repel.

You know water alone won’t clean dirty hands, dirty clothes, or dirty dishes. That’s because the fundamental component of “dirt” is oil (grease being an example). Soap is a chemical “middle man” that is attracted both to water and to oil; hence, soapy water will clean off the dirt.

How can soap attract both water and oil, which are basically opposites? Soap begins as an oil, but is chemically treated (often with sodium hydroxide) so that one of its “ends” receives a polar bond. The end with the polar bond attracts water.

At the same time, the other end of the soap molecule maintains its non-polar character. This non-polar end attracts “dirt” (oil, grease, earth, or what have you).

Soap has to be wet to work. In such a setting, the non-polar ends of the soap molecules latch onto the grease molecules. Then, the polar ends of the soap molecules, which are “attached” to water molecules, get pulled in different directions by the motion of the water. As the soap molecules are pulled away, they carry the grease molecules with them; the lumps of grease are thus pulled apart into smaller units. This process is called emulsification; as it repeats, the grease droplets become tiny enough to be completely removed by the water.

The classic “bubble” you get from soap is, essentially, a “water-free” zone surrounded by the nonpolar ends of soap molecules. Water, in turn, surrounds the bubble, since it’s attracted to the polar ends of those same soap molecules.

With potentially so many dishes to wash tonight (the Canadian Thanksgiving), this article might provide some “food for thought” for anyone peering into the bubbles:)

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.