UPSC Prelims 2006 Q5: Science & Technology – Chemistry
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?
- (a) 1 only
- (b) 2 only
- (c) 1, 2 and 4 only
- (d) 1, 2, 3 and 4
Answer: (c) 1, 2 and 4 only
Explanation
Dry-cleaning chemicals
Dry cleaning uses non-aqueous organic solvents that dissolve grease and oil without water. Sodium carbonate works only in water, so it belongs to ordinary laundering.
| Chemical | Use in dry cleaning? | Reason |
|---|---|---|
| 1. Benzene | Yes | Organic solvent that dissolves grease |
| 2. Carbon tetrachloride | Yes | Chlorinated non-flammable solvent |
| 3. Sodium carbonate | No | Washing soda, an aqueous laundry builder |
| 4. Trichloroethylene | Yes | Chlorinated solvent used for degreasing and dry cleaning |
So chemicals 1, 2 and 4 are used in dry cleaning.
Answer: (c) 1, 2 and 4 only.
Why each option is right or wrong
- (a) Incorrect. Benzene alone is incomplete; carbon tetrachloride and trichloroethylene are also dry-cleaning solvents.
- (b) Incorrect. Carbon tetrachloride alone is incomplete; benzene and trichloroethylene are also used.
- (c) CORRECT. Benzene, carbon tetrachloride and trichloroethylene are organic solvents used for dry cleaning.
- (d) Incorrect. Sodium carbonate is washing soda used with water, not a dry-cleaning solvent.
Key point
Dry cleaning relies on organic solvents such as benzene, carbon tetrachloride and trichloroethylene, while sodium carbonate is only a water-based laundry builder.
Remember
Dry cleaning solvents: benzene, carbon tetrachloride, trichloroethylene; sodium carbonate is not one.
Common mistakes
- Including sodium carbonate because it is a common cleaning chemical, ignoring the 'dry' (no water) condition.
- Forgetting that trichloroethylene is a dry-cleaning and degreasing solvent.
- Assuming only one solvent is used instead of the group of organic solvents.
Exam tip
'Dry' cleaning means no water, so pick organic solvents and reject water-soluble salts like washing soda.
Sources
- NCERT Class XII Chemistry — Haloalkanes and Haloarenes (uses and environmental effects of polyhalogen compounds)
- Encyclopaedia Britannica — Dry cleaning