Soap molecules are composed of a long hydrocarbon chain with a negatively charged head group. In aqueous solution, soap molecules form micelles, which are spherical aggregates with the hydrocarbon chains pointing inward and the head groups pointing outward. These micelles can trap dirt and oil molecules, allowing them to be rinsed away with water.

In an acid solution, the pH of the solution is below 7, which means that there are more hydrogen ions (H+) in the solution than hydroxide ions (OH-). The H+ ions can react with the soap molecules, neutralizing the charge on the head groups. This causes the micelles to disintegrate, releasing the dirt and oil molecules back into the solution. As a result, soap is not as effective at cleaning in an acid solution as it is in a neutral or basic solution.