MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Soap and hard water. Hard water is water that contains magnesium and calcium ions, with bicarbonate. These minerals form limescale when water is heated, and soap scum when detergents are used. The 'scum' (also called lime soap) is a white precipitate which forms by the reaction of fatty acids in the soap with calcium or magnesium ions.
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