CAPF AC 2020 Q20: Science & Technology – Chemistry
- (a) bulk modulus
- (b) shear strain
- (c) volume effect
- (d) surface tension
Answer: (d) surface tension
Explanation
Why oil and water do not mix
How to think: three of the options are about squeezing or twisting a material.
Only one is about the surface between two liquids.
Water molecules pull strongly on each other.
Oil molecules are not attracted to water in the same way.
So the two liquids keep a boundary between them, and that boundary behaves like a stretched skin.
This is surface tension (between two liquids it is called interfacial tension).
| Option | What it means | Verdict |
|---|---|---|
| (a) Bulk modulus | Resistance to being compressed | Incorrect |
| (b) Shear strain | Change of shape under a sideways force | Incorrect |
| (c) Volume effect | Not a property that explains mixing | Incorrect |
| (d) Surface tension | Force that makes a liquid surface shrink | Correct |
Why soap helps: soap lowers the surface tension, so oil breaks into tiny drops and washes away.
Deeper reason: water is polar and oil is non-polar.
Answer: (d) surface tension.
Why each option is right or wrong
- (a) Incorrect. Bulk modulus tells how hard it is to compress a material.
- (b) Incorrect. Shear strain is a change of shape under a sideways force.
- (c) Incorrect. Volume effect is not a physical property that explains mixing.
- (d) CORRECT. Surface tension at the oil-water boundary keeps the two liquids apart.
Key point
Oil and water stay apart because of surface (interfacial) tension; water is polar, oil is non-polar. Soap lowers surface tension and lets them mix.
Remember
Oil and water do not mix: surface tension; soap lowers it.
Common mistakes
- Looking for the word polarity in the options and then guessing at random.
- Confusing surface tension with viscosity, which is resistance to flow.
- Forgetting that detergents work by lowering surface tension.
Exam tip
Educated guess: bulk modulus and shear strain belong to solids under stress, so drop them first.
Sources
- NCERT Class XI Physics — Mechanical Properties of Fluids (surface tension, detergents)
- Encyclopaedia Britannica — surface tension