UPSC CSE Prelims 2012

Correct Option (b):

Capillarity is the phenomenon where a liquid spontaneously rises or falls in a narrow tube or porous material due to the interplay of cohesive forces within the liquid and adhesive forces between the liquid and the solid surface.

• Statement 1: Correct. Kerosene lamps function by drawing fuel upwards through the wick. This process relies on the capillary action within the fine pores of the wick, which allows the kerosene to reach the flame. Without capillarity, the lamp would not operate effectively.
• Statement 3: Correct. Blotting paper absorbs liquids like ink due to its porous structure. The numerous tiny channels within the paper act as capillaries, drawing the liquid into the paper through capillary action. If capillarity were absent, blotting paper would lose its absorbent property.
• Statement 4: Correct. The ascent of water from the roots to the leaves in tall trees occurs primarily through xylem vessels. This transport mechanism, known as the cohesion-tension theory, involves a significant contribution from capillary action within the narrow xylem conduits, alongside transpiration pull and cohesive forces of water molecules. Without capillarity, the efficient transport of water against gravity to significant heights would be severely impeded, thus limiting the growth of tall trees.

Incorrect Options:

• Statement 2: Incorrect. The use of a straw to consume a soft drink relies on the principle of atmospheric pressure and suction, not capillarity. When air is removed from the straw by suction, the pressure inside the straw decreases, allowing the higher atmospheric pressure on the surface of the liquid to push the liquid up the straw.
• Option (a) is incorrect because it includes statement 2, which is not a consequence of the absence of capillarity.
• Option (c) is incorrect because it omits statement 1, which correctly describes a difficulty arising from the absence of capillarity.
• Option (d) is incorrect because it includes statement 2, which is based on atmospheric pressure and suction, not capillarity.