UPSC Prelims 2012 Q93: Science & Technology – Physics
Consider the following statements: If there were no phenomenon of capillarity
1. It would be difficult to use a kerosene lamp
2. One would not be able to use a straw to consume a soft drink
3. The blotting paper would fail to function
4. The big trees that we see around would not have grown on the Earth
Which of the statements given above are correct?
- (a) 1, 2 and 3 only
- (b) 1, 3 and 4 only
- (c) 2 and 4 only
- (d) 1, 2, 3 and 4
Answer: (b) 1, 3 and 4 only
Explanation
If there were no capillarity
Capillarity is the rise of a liquid in a very narrow tube or fine pore, caused by surface tension.
| Statement | Verdict | Reason |
|---|---|---|
| 1. It would be difficult to use a kerosene lamp | Correct | Oil climbs the wick by capillary action |
| 2. One would not be able to use a straw to consume a soft drink | Incorrect | A straw works by atmospheric pressure: sucking lowers the pressure inside, and the air outside pushes the drink up |
| 3. The blotting paper would fail to function | Correct | Ink is drawn into the fine pores of the paper |
| 4. The big trees that we see around would not have grown on the Earth | Correct | Water rises in the narrow xylem vessels partly by capillarity |
More examples of capillarity:
| Example | How |
|---|---|
| Towel or sponge | Soaks up water |
| Soil | Water rises from below to the roots |
| Brick wall | Dampness climbs up |
Answer: (b) 1, 3 and 4 only.
Why each option is right or wrong
- (a) Incorrect. A straw does not depend on capillarity, and Statement 4 is left out.
- (b) CORRECT. The kerosene lamp, blotting paper and big trees depend on capillarity.
- (c) Incorrect. A straw works by pressure, and Statements 1 and 3 are left out.
- (d) Incorrect. Statement 2 is false; using a straw needs atmospheric pressure, not capillarity.
Key point
Capillarity (surface tension in narrow tubes): lamp wicks, blotting paper, towels, rise of water in soil and xylem. A drinking straw works by atmospheric pressure.
Remember
Straw = air pressure. Wick and blotting paper = capillarity.
Common mistakes
- Counting the straw because liquid rises in a tube; the tube is far too wide.
- Forgetting that a wick is a bundle of fine capillaries.
- Thinking trees lift water by capillarity alone; transpiration pull does most of it.
Exam tip
Capillarity needs a very narrow tube. A straw is wide, so something else, air pressure, lifts the drink.
Sources
- NCERT Class XI — Physics Part II, Ch on Mechanical Properties of Fluids (surface tension and capillary rise)
- NCERT Class XI — Biology, Ch on Transport in Plants