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Q90 (CDS-II/2012) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Applied everyday chemistry Answer Verified

Soaps cannot be used in acidic condition because they lose their cleansing effect due to formation of insoluble

Result
Your answer: —  ·  Correct: D
Explanation

Soaps are sodium or potassium salts of long-chain carboxylic acids, also known as fatty acids [2]. In acidic conditions, the high concentration of hydrogen ions (H+) reacts with the soap molecules (RCOOⁱNa⁰). This chemical reaction displaces the sodium or potassium ions, converting the soluble soap back into its parent long-chain fatty acids. Unlike the soap salts, these free fatty acids are insoluble in water and precipitate out as a waxy solid. Because these insoluble fatty acids lack the ionic hydrophilic head necessary to form micelles, they cannot emulsify oily dirt, leading to a complete loss of cleansing effect [4]. This is distinct from the reaction in hard water, where soap forms insoluble calcium or magnesium salts (scum) [3]. Therefore, soaps are ineffective in acidic media due to the formation of insoluble long-chain fatty acids.

Sources

  1. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Activity 4.10 > p. 75
  2. [2] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > CH COOH CH CH OH CH C C CH CH H O 3 3 2 3 2 3 2 Acid O (E - + - - - - - + thanoic acid) (Ethanol) (Ester) O > p. 73
  3. [4] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > What you have learnt > p. 77
  4. [3] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Activity 4.12 > p. 76
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SIMILAR QUESTIONS

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Consider the following statements : 1. Soap cannot be used in acidic water. 2. Ionic part of a soap is —COO~.Na+. 3. Soap dissolves in water faster than detergent. Which of the statements given above is/are correct?

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Which one of the following statements about the cleansing action of soap is not true?

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CDS-I · 2009 · Q90 Relevance score: -2.62

Assertion (A) : Soaps do not form lather with water containing salts of calcium and magnesium. Reason (R) : Calcium and magnesium salts of long-chain fatty acids are insoluble in water.