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Why does soap scum form in hard water that contains #Mg^(2+)# ions when a sodium stearate soap is used?

Answer 1

Because magnesium and calcium stearates are insoluble beasts.

The common result of sodium stearates is the formation of insoluble calcium and magnesium salts that need to be scrubbed away in the bath or toilet. The next time you clean the bathroom, consider this and the cleaning solution you are using (vinegar is a cheap alternative; why would it work?).

#2C_17H_35C(=O)O^−Na^+ + Ca^(2+) → (C_17H_35COO)_2Cadarr + 2Na^+#

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Answer 2

Jose Ayala

When sodium stearate soap is used in hard water with Mg^(2+) ions, soap scum forms because the magnesium ions react with the soap molecules to form insoluble magnesium stearate, which is a white precipitate that is difficult to remove from surfaces.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.