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Cleansing Action of Soap

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Question

what is the reaction between soap molecule (steraic acid soap) with magnesium and calcium salts present in hard water?

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Solution

The hard water contains Ca⁺²and Mg⁺²ions which remain in the form of carbonates and bicarbonates which are insoluble in soaps and gives the insoluble product with the soap molecules like sodium stearate etc. which is known as scum.
This scum is formed because the calcium and magnesium ions are not soluble in water.
For ex:
2C₁₇H₃₅COONa + CaCO₃ --> (C₁₇H₃₅COO)₂Ca (insoluble scum) + Na₂CO₃

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Q. Assertion :Scum is formed when hard water is treated with soap. Reason: Hard water contains calcium and magnesium ions. When soap comes in contact with hard water, these ions form calcium and magnesium salts of fatty acids which are insoluble in water. These calcium or magnesium salts precipitate out in the form of scum.

Q. Assertion (A): Hard water gives easy lather with soap.
Reason (R): Salts of calcium and magnesium react with soap to form insoluble salts.

Soap forms insoluble precipitates with calcium and magnesium ions present in hard water.

Q. Which of the following is true regarding these statements?

Statement 1: Hard water contains dissolved salts of calcium and magnesium.
Statement 2: Hard water lathers easily with soap.

Q. Soaps do not work with hard water containing calcium and magnesium ions because:

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Cleansing Action of Soap

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