Q20. At approximately which temperature in kelvin does pure water have its maximum density?
[UPPCS Mains – 2015]
(A) 373 K
(B) 277 K
(C) 273 K
(D) 269 K
Answer: (B) 277 K
Explanation: Water reaches maximum density near 4°C at normal atmospheric pressure. Converting Celsius to kelvin gives approximately 4 + 273 = 277 K. The phenomenon reflects anomalous expansion: water becomes less dense when cooled further toward its freezing point.
Q19. How does the pressure inside a soap bubble compare with atmospheric pressure outside it?
[UPPCS Preliminary – 2014]
(A) It is greater
(B) It is smaller
(C) It is equal
(D) It is half as large
Answer: (A) It is greater
Explanation: The curved soap film produces an excess internal pressure because surface tension pulls the film inward. For a spherical soap bubble with two surfaces, the ideal pressure difference is 4T/r. Consequently, the air pressure inside exceeds surrounding atmospheric pressure.
Q18. Without capillarity, which would be affected:
(1) using a kerosene lamp,
(2) drinking through a straw,
(3) blotting paper,
(4) growth of tall trees?
[UPSC CSE Preliminary – 2012]
(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: A wick draws kerosene by capillarity, and blotting paper absorbs fluid through narrow pores. Capillarity also aids water movement in plants, alongside transpiration. Drinking with a straw chiefly depends on a pressure difference created by suction, not capillarity.
Q17. Why can a carefully placed iron needle remain supported on the surface of water?
[UPPCS Mains – 2012]
(A) Water upthrust alone
(B) Surface tension
(C) Viscosity
(D) Gravitational attraction
Answer: (B) Surface tension
Explanation: A light needle placed gently can deform the water surface without piercing it. The surface-tension force along the contact line supplies an upward component that balances its weight. Once the surface film is disturbed sufficiently, the needle may sink.
Q16. Which of these metals has the highest electrical conductivity?
[Chhattisgarh PSC Preliminary – 2011]
(A) Silver
(B) Copper
(C) Aluminium
(D) Iron
Answer: (A) Silver
Explanation: Among these pure metals at ordinary conditions, silver has the greatest electrical conductivity. Copper is widely used for wiring because it is much cheaper while remaining an excellent conductor. Conductivity depends on electron mobility and scattering within the metallic structure.
Q15. Why does kerosene normally form a floating layer on water?
[Uttarakhand Lower Subordinate Preliminary – 2010]
(A) Its density is greater than water
(B) Its density is lower than water
(C) Its density equals water
(D) None of these
Answer: (B) Its density is lower than water
Explanation: Kerosene has a lower density than water and the two liquids are largely immiscible, so kerosene generally remains above water. This arrangement minimises gravitational potential energy for the two layers and does not mean that kerosene has no mass.
Q14. Which among alcohol, water, honey and gasoline is usually the most viscous?
[Uttarakhand Lower Subordinate Preliminary – 2010]
(A) Alcohol
(B) Water
(C) Honey
(D) Gasoline
Answer: (C) Honey
Explanation: Honey generally resists flowing more strongly than alcohol, water or gasoline at similar room temperatures. Its concentrated sugars produce substantial internal resistance. Viscosity describes flow resistance and changes with temperature, so comparing substances assumes approximately the same conditions.
Q13. Why does a bucket filled with water feel heavier after it rises above the water surface in a well?
[UPPCS Preliminary – 2009]
(A) Its apparent weight increases outside water
(B) Its apparent weight decreases outside water
(C) Its stability disappears
(D) Its mass increases
Answer: (A) Its apparent weight increases outside water
Explanation: A submerged bucket receives upward buoyant force from the surrounding well water, making its apparent weight smaller. As it emerges, that support diminishes, so more lifting force is required. Its actual mass has not increased merely because of emerging.
Q12. An ice cube floats in a glass filled with water. What happens to the water level after the ice has completely melted?
[MPPSC Preliminary – 2008]
(A) It rises
(B) It falls
(C) It stays the same
(D) It rises and then falls
Answer: (C) It stays the same
Explanation: Floating ice displaces a mass of water equal to its own mass. After melting, the resulting water occupies exactly the same volume as that previously displaced, assuming pure water and ordinary conditions. Therefore, the level stays essentially unchanged.
Q11. Two soap bubbles of unequal diameter are connected by a narrow tube. What is the likely outcome?
[RAS/RTS Preliminary – 2007]
(A) Both remain unchanged
(B) The smaller shrinks and the larger grows
(C) Both become equal-sized
(D) Both burst immediately
Answer: (B) The smaller shrinks and the larger grows
Explanation: Excess pressure inside a soap bubble is inversely proportional to its radius. The smaller bubble therefore has higher internal pressure, driving air through the connecting tube into the larger bubble. It shrinks while the larger one expands.
