Why does soap leave a film even after rinsing? © Getty Images

Why does soap leave a film even after rinsing?

Scum occurs when the ions found in hard water react with the ingredients of soap.

Asked by: Rudiger Jonser, by email

Advertisement

Hard water contains calcium and magnesium ions. These react with the sodium stearate molecules in soap to give calcium or magnesium stearate, which forms an insoluble scum but at least rinses away. Artificially softened water swaps calcium and magnesium ions for sodium and potassium ions, which doesn’t precipitate out so easily and instead clings to the slightly charged surface of your skin. Naturally soft water has no ions and rinses away soap the most effectively.


Advertisement

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.