Explain the formation of scum when hard water is treated with soap?


The word ‘hard water’ refers to water with a high mineral content. Hard water is formed when water percolates into calcareous, gypsum, or chalk deposits that are mainly composed of carbonates of magnesium or calcium, bicarbonates, and sulphate.

  • Calcium and magnesium salts are commonly found in hard water.
  • A precipitate is formed when soap molecules react with calcium and magnesium salts.
  • This precipitate starts to float on the surface of the water as an off-white layer. Scum is the name for this layer.
  • Because of the formation of scum, soaps lose their cleansing ability in hard water.

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