Why does soap work?
Because soap molecules have two ends that want incompatible things, and that split personality is what lets water carry away substances water cannot normally touch.
The problem soap solves. Water is polar — its molecules have partial charges and attract each other strongly. Oils and greases are non-polar. Polar and non-polar substances do not mix, which is why rinsing greasy hands with water alone achieves almost nothing.
The molecule. A soap or detergent molecule is amphiphilic: a long hydrophobic hydrocarbon tail that avoids water and mixes readily with oil, and a hydrophilic head — charged or strongly polar — that dissolves happily in water.
What happens on the surface:
Micelle formation. Above a certain concentration, soap molecules cluster into spheres with tails pointing inward and heads outward. Grease and oily dirt are captured inside the tail-lined core, while the water-friendly exterior lets the whole assembly be rinsed away suspended in water.
Surface tension reduction. Soap molecules sit at the air–water interface and disrupt the hydrogen bonding that makes water bead up. Lower surface tension means water spreads and penetrates into fabric and skin crevices instead of sitting on top — which is why the word surfactant means surface-active agent.
Why soap destroys enveloped viruses, which became widely discussed during the pandemic. Viruses such as coronaviruses and influenza are wrapped in a lipid envelope — essentially a fat membrane. Soap's hydrophobic tails insert into that membrane and prise it apart, destroying the virus rather than merely washing it off. This is why twenty seconds of soap and water is genuinely effective, and why hand sanitiser needs sufficient alcohol content to do something comparable.
Why hot water helps — it makes greases more fluid and speeds the process — though warm is sufficient, and water hot enough to matter thermally would scald.
Hard water and soap scum occur when calcium and magnesium ions react with traditional soap; synthetic detergents were developed partly to avoid this.