UPSC Prelims 2006 · General Studies
Question 5: Science and Technology
Consider the following chemicals:
- 1.Benzene
- 2.Carbon tetrachloride
- 3.Sodium carbonate
- 4.Trichloroethylene
Which of the above is/are used as dry-cleaning chemicals?
AnswerThe answer is (c).
Dry-cleaning uses organic solvents that dissolve grease without water. Benzene, carbon tetrachloride and trichloroethylene have all been used for it, though benzene and carbon tetrachloride are toxic and have been phased out in favour of safer solvents. Sodium carbonate is washing soda, an ordinary water-based laundry chemical, and is not a dry-cleaning solvent. So 1, 2 and 4 are right.
- 1. Benzene has been used as a dry-cleaning solvent.
- 2. Carbon tetrachloride was a common dry-cleaning solvent.
- 3. Sodium carbonate (washing soda) works in water, not as a dry-cleaning solvent.
- 4. Trichloroethylene is a chlorinated solvent used in dry cleaning.
How you could have got this
- If you were sure statement 3 is not correct, (d) goes: it includes 3.
What you needed to know: Sodium carbonate (washing soda) works in water, not as a dry-cleaning solvent.
- Then, if you were sure statement 4 is correct, (a) and (b) go: each of them leaves 4 out.
What you needed to know: Trichloroethylene is a chlorinated solvent used in dry cleaning.
Only (c) is left. You didn’t need statements 1 or 2.
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) Benzene is used, but so are carbon tetrachloride and trichloroethylene.
- (b) Carbon tetrachloride is used, but so are benzene and trichloroethylene.
- (c) Correct: benzene, carbon tetrachloride and trichloroethylene; sodium carbonate is left out.
- (d) Wrongly includes sodium carbonate, a water-based washing chemical.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2006, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.