UPSC Prelims 2002 Q145: Science & Technology – Chemistry
Assertion (A): Synthetic detergents can lather well in hard water.
Reason (R): Synthetic detergents form soluble calcium and magnesium salts with hard water.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not a correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (a) Both A and R are individually true and R is the correct explanation of A
Explanation
Assertion-Reason: synthetic detergents and hard water
| Statement | Verdict |
|---|---|
| Assertion: synthetic detergents lather well in hard water | True |
| Reason: they form soluble calcium and magnesium salts | True — unlike soap, which forms insoluble scum |
| Link | Soluble salts do not precipitate, so lather is not lost |
| Result | Both true and R explains A |
Soap forms insoluble calcium and magnesium salts (scum) with hard water, which stops it from lathering. Synthetic detergents form soluble salts, so they lather well even in hard water. The reason correctly explains the assertion.
Answer: (a) Both A and R are individually true and R is the correct explanation of A.
Why each option is right or wrong
- (a) CORRECT. Soluble salts explain why detergents lather in hard water.
- (b) Incorrect. R does explain A, so option (b) fails.
- (c) Incorrect. R is true; detergents form soluble salts.
- (d) Incorrect. A is true; detergents lather in hard water.
Key point
Soaps: sodium salts of fatty acids, precipitate in hard water; detergents: sulphonates or sulphates, with soluble Ca and Mg salts.
Remember
Detergents work in hard water because their Ca and Mg salts are soluble.
Common mistakes
- Thinking detergents form scum like soaps.
- Treating hardness as irrelevant to lathering.
- Confusing temporary and permanent hardness.
Exam tip
Lather lost means insoluble salts; here the salts are soluble, so lather stays.
Sources
- NCERT Class XII Chemistry — Chemistry in Everyday Life
- Encyclopaedia Britannica — detergent