Question map
Not attempted Correct Incorrect ★ Bookmarked
Loading…
Q51 (CDS-I/2013) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Applied everyday chemistry Answer Verified

Why hard water does not give lather with soap ?

Result
Your answer: —  ·  Correct: A
Explanation

Hard water is characterized by the presence of dissolved salts, primarily bicarbonates, sulphates, and chlorides of calcium (Ca²⁺) and magnesium (Mg²⁺) [1]. Soap consists of sodium or potassium salts of long-chain fatty acids, such as sodium stearate [2]. When soap is added to hard water, the calcium and magnesium ions react with the soap molecules to form insoluble precipitates known as 'scum' [1]. For example, sodium stearate reacts with calcium chloride to produce insoluble calcium stearate. This chemical reaction consumes the soap molecules, preventing them from lowering surface tension and forming the micelles necessary to produce lather. Consequently, lather only forms after all the calcium and magnesium ions have been precipitated by an excess of soap. Detergents avoid this issue because their charged ends do not form insoluble precipitates with these ions [1].

Sources

  1. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Activity 4.12 > p. 76
  2. [2] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Activity 4.10 > p. 75
How others answered
Each bar shows the % of students who chose that option. Green bar = correct answer, blue outline = your choice.
Community Performance
Out of everyone who attempted this question.
50%
got it right
✓ Thank you! We'll review this.

SIMILAR QUESTIONS

CDS-I · 2009 · Q90 Relevance score: 4.01

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.

NDA-I · 2021 · Q71 Relevance score: 0.46

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

NDA-I · 2025 · Q71 Relevance score: 0.05

Which one of the following statements is NOT correct?

IAS · 2002 · Q145 Relevance score: -0.33

Assertion (A) : Synthetic detergents can lather well in hard water. Reason (R) : Synthetic detergents form soluble calcium and magnesium salts with hard water.