UPSC Prelims 2002 Q143: Science & Technology – Chemistry
Assertion (A): With the increase of temperature, the viscosity of glycerine increases.
Reason (R): Rise of temperature increases kinetic energy of molecules.
- (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: (d) A is false but R is true
Explanation
Assertion-Reason: viscosity of glycerine and temperature
| Statement | Verdict |
|---|---|
| Assertion: viscosity of glycerine increases with temperature | False — viscosity of a liquid decreases as temperature rises |
| Reason: temperature rise increases kinetic energy of molecules | True |
| Link | Greater kinetic energy overcomes intermolecular forces, so the liquid flows more easily |
| Result | A is false but R is true |
With the rise of temperature, the molecules of a liquid gain kinetic energy and overcome the attractive forces between them, so the viscosity of glycerine falls. The assertion is false, but the reason is a true statement.
Answer: (d) A is false but R is true.
Why each option is right or wrong
- (a) Incorrect. A is false, so R cannot explain it.
- (b) Incorrect. A is false, so both are not true.
- (c) Incorrect. R is true, so the statement 'R is false' fails.
- (d) CORRECT. A is false (viscosity falls) and R is true.
Key point
Liquids: viscosity falls with temperature; gases: viscosity rises with temperature.
Remember
Liquids get less viscous when heated.
Common mistakes
- Applying the gas rule to glycerine.
- Believing that thicker means hotter.
- Choosing option (a) without checking the assertion.
Exam tip
Test A with honey or oil: heating makes them runny, not thick.
Sources
- NCERT Class XI Physics — Mechanical Properties of Fluids
- NCERT Class XI Chemistry — States of Matter (viscosity of liquids)