UPSC Prelims 2002 Q28: Science & Technology – Chemistry
- (a) The boiling point of an aqueous solution is higher than that of pure water
- (b) Addition of solutes to a solution causes an increase in its water potential
- (c) The vapour pressure of the water in a solution is lower than that of pure water
- (d) When a solution is separated from water by a semi permeable membrane, water movement can be prevented by applying pressure to the solution
Answer: (b) Addition of solutes to a solution causes an increase in its water potential
Explanation
Colligative properties: the statement that is NOT correct
| Statement | Verdict |
|---|---|
| Boiling point of an aqueous solution is higher than pure water | True — boiling point elevation |
| Adding solutes increases the water potential | False — solutes lower (make more negative) the water potential |
| Vapour pressure of water in a solution is lower than pure water | True — Raoult's law |
| Pressure on the solution can stop water moving across a semi-permeable membrane | True — this is the osmotic pressure idea |
The question asks for the statement that is NOT correct. Adding a solute lowers the water potential, so statement (b) is false. The other three statements are standard facts on boiling point elevation, vapour pressure lowering and the pressure that stops osmosis. The bank had dropped the word NOT, and the stored key (b) fits only the NOT reading.
Answer: (b) Addition of solutes to a solution causes an increase in its water potential.
Why each option is right or wrong
- (a) Incorrect. This statement is true, since solutes raise the boiling point.
- (b) CORRECT. This is the false statement; solutes decrease the water potential.
- (c) Incorrect. This statement is true, since the vapour pressure of a solution is lower.
- (d) Incorrect. This statement is true, since applied pressure can stop osmosis.
Key point
Pure water has the highest water potential (zero); every solute makes it more negative.
Remember
Solute added: boiling point up, vapour pressure down, water potential down.
Common mistakes
- Reading the stem as 'correct' and choosing a true statement such as (a).
- Thinking more solute means more water potential.
- Confusing boiling point elevation with freezing point elevation.
Exam tip
Three statements are textbook truths; the odd one out is the false statement.
Sources
- NCERT Class XII Chemistry — Solutions
- NCERT Class XI Biology — Transport in Plants (water potential)