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

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Chapter 4

NCERT

Class 10

Science

Solutions

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

Question:

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

Answer:

Calcium and magnesium ions are present in the hard water. These ions react with soap to form scum. The sum is an insoluble substance which appears as white, off-white, or sticky layer on water surface. The formation of the scum reduces the soap's effectiveness.

Chemical Reaction

Sodium and potassium ion present in the soap molecules are displaced by the magnesium and calcium ions present in the hard water resulting in formation of scum. It reduces the soap's cleansing properties.

Reaction

RCOO^-Na⁺ + Ca²⁺ → (RCOO^-)₂Ca + 2Na⁺

RCOO^-Na⁺ + Mg²⁺ → (RCOO^-)₂Mg + 2Na⁺

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(All Q's of this Chapter solved)

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