UPSC Prelims 2002 · General Studies

Question 145: Science and Technology

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

aBoth A and R are individually true and R is the correct explanation of A

bBoth A and R are individually true but R is not a correct explanation of A

cA is true but R is false

dA is false but R is true

AnswerThe answer is (a).

Assertion A is true: synthetic detergents lather well even in hard water. Reason R is true and explains it: their calcium and magnesium salts are soluble, whereas ordinary soaps form an insoluble scum with these ions and stop lathering. This is why detergents, not soaps, are used for washing in hard-water areas.

A. Synthetic detergents lather well in hard water.
R. Their calcium and magnesium salts are soluble, so no scum forms.

How you could have got this

If you were sure statement 1 is correct, (d) goes. Then judge the second statement; if it is correct too, decide whether it explains the first.

This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.

Which options go is worked out from UPSC’s key and the options, not by a model.

Why each option is right or wrong

(a) Right: both true, and R explains A.
(b) R is the direct explanation of A.
(c) R is true.
(d) A is true.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2002, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.