Soap scum
Not scale and not dirt: a metal soap formed when hard water meets true soap. It is the reason a shower screen films over in one house and not another.
In detail
Soap scum is a chemical product of the water rather than a soiling of the surface. Soap is a sodium salt of a fatty acid; hard water carries dissolved calcium and magnesium; and when the two meet, those metals displace the sodium and produce calcium and magnesium salts of the same fatty acids. Those salts are not soluble in water, so they come out of solution as a curd which then dries onto glass, tiles, taps and the bath. That single reaction explains almost everything readers find puzzling about it. It appears in hard-water areas and not in soft ones. It appears with bar soap far more than with body wash, because synthetic detergents were developed precisely so their heads would not precipitate with hardness ions. And it resists both of the obvious treatments used alone: a descaler attacks the mineral half and leaves a greasy film, while a detergent attacks the fatty half and leaves a mineral haze. The deposit is a hybrid and usually needs an acidic cleaner and a surfactant together, or one then the other with a rinse between. Prevention is more effective than any product, because the film needs the water to evaporate in place in order to deposit at all.
Why
- Soap scum is explained by Insoluble metal soaps
- Soap scum is explained by Water hardness
- Soap scum is explained by Evaporative deposition
- Soap scum is explained by Adhesion and cohesion
- Soap scum is explained by Why different surfaces get different dirt
Aftercare and prevention
- Squeegee helps prevent Soap scum
- Moisture control helps prevent Soap scum
Hacks and claims involving this
- One spray for the whole bathroom Does not work
Sources for this page
Sources
Handbook of Detergents (Surfactant Science Series)
CRC Press / Taylor & Francis · Reference work
Relied on for: That calcium and magnesium ions in hard water interact with surfactants, reducing detergency and forming insoluble deposits. (background and mechanism only)
Spot Solver: spot and spill removal procedures
The Carpet and Rug Institute (CRI) · Trade or professional body
Relied on for: That soap residue is treated by opening with warm water and then a vinegar solution rather than with detergent, because what is being removed is alkaline residue. (states this directly)