100. An insulated vessel initially has 250 g water at 20°C. Steam at 100°C heats it to 25°C and the water mass becomes 252 g. If the specific heat of water is 1 cal/g°C, what is the steam’s latent heat?
[UPSC NDA-II GAT, September 2026]
(A) 530 cal/g
(B) 540 cal/g
(C) 550 cal/g
(D) 560 cal/g
Answer: (C) 550 cal/g
Explanation: The original water gains 250 × 5 = 1,250 calories. Two grams of steam condense and their resulting water cools from 100°C to 25°C. Therefore 2(L + 75) = 1,250, giving L = 550 cal/g.
99. Choose the correct order of processes in the natural water cycle.
[RRB Group D, August 2026]
(A) Precipitation → runoff → evaporation
(B) Evaporation → precipitation → condensation
(C) Evaporation → condensation → precipitation
(D) Condensation → precipitation → evaporation
Answer: (C) Evaporation → condensation → precipitation
Explanation: Sunlight causes liquid water to evaporate. Rising moist air cools until water vapour condenses into droplets, forming clouds. Droplets later fall as precipitation. These phase transitions exchange latent heat and play a central role in Earth’s hydrological and energy cycles.
98. Arrange these stages of the water cycle: 1. Condensation; 2. Evaporation; 3. Precipitation; 4. Collection in water bodies.
[RRB Group D, August 2026]
(A) 3 → 2 → 1 → 4
(B) 1 → 2 → 3 → 4
(C) 2 → 3 → 1 → 4
(D) 2 → 1 → 3 → 4
Answer: (D) 2 → 1 → 3 → 4
Explanation: Water absorbs solar energy and evaporates first. Cooling converts vapour into condensed droplets; precipitation follows when droplets grow heavy enough. Water subsequently collects in rivers, lakes and oceans. The repeated evaporation-condensation sequence involves absorption and release of latent heat.
97. Which substance has the lowest coefficient of thermal conductivity?
[UPSC CDS-I General Knowledge, April 2026]
(A) Hydrogen gas (H₂)
(B) Liquid water (H₂O)
(C) Copper (Cu)
(D) Aluminium (Al)
Answer: (A) Hydrogen gas (H₂)
Explanation: Hydrogen is a gas, with thermal conductivity much lower than liquid water or the listed metals under ordinary conditions. Copper and aluminium contain mobile conduction electrons and transfer heat efficiently. Thermal conductivity quantifies energy transferred through a material per temperature gradient.
96. According to Joule’s heating law, for fixed resistance and time, heat produced in a resistor is proportional to what?
[RRB NTPC Graduate CBT-I, March 2026]
(A) Square of electric current
(B) Square of resistance
(C) Inverse square of current
(D) Inverse square of resistance
Answer: (A) Square of electric current
Explanation: Joule’s law gives H = I²Rt, where I is current, R is resistance and t is time. Holding resistance and duration constant, doubling current produces four times the heat. This principle underlies electric heaters, irons and protective fuses.
95. Why are cotton garments preferred during hot summer weather?
[SSC MTS 2025 paper, February 2026]
(A) They stop sweat from evaporating
(B) They increase body temperature by trapping heat
(C) They absorb sweat and facilitate evaporation
(D) They cannot absorb moisture
Answer: (C) They absorb sweat and facilitate evaporation
Explanation: Cotton fibres readily absorb moisture and spread sweat over a larger surface. Evaporation then requires latent heat, which is taken partly from the skin. The associated energy loss produces a cooling effect, making cotton clothing comfortable in hot weather.
94. Why does a paper cup containing water usually not ignite when heated carefully?
[SSC MTS 2025 paper, February 2026]
(A) Water evaporation immediately extinguishes every flame
(B) Water absorbs heat and holds paper below its ignition temperature
(C) Wet paper becomes permanently fireproof
(D) Steam automatically puts out the fire
Answer: (B) Water absorbs heat and holds paper below its ignition temperature
Explanation: Water has substantial heat capacity and absorbs the supplied energy. While water remains in contact with the paper, its temperature stays relatively low, often near its boiling point. Thus, the paper may not reach the temperature required for ignition.
93. At normal atmospheric pressure, what is the temperature at which a solid begins to turn into a liquid called?
[SSC MTS 2025 paper, February 2026]
(A) Boiling point
(B) Melting point
(C) Absolute point
(D) Vaporisation point
Answer: (B) Melting point
Explanation: The melting point is the temperature at which solid and liquid phases can coexist in equilibrium at a specified pressure. Added heat during melting mainly overcomes particle attractions rather than raising temperature. Different substances possess different characteristic melting points.
92. People sprinkle water on a sun-heated roof mainly to take advantage of which effect?
[SSC MTS 2025 paper, February 2026]
(A) Heating of the roof by incoming water
(B) Increase in the surface temperature
(C) Large latent heat of vaporisation of water
(D) Increased slipperiness of the surface
Answer: (C) Large latent heat of vaporisation of water
Explanation: Water absorbs substantial energy while evaporating from a hot surface. Its latent heat of vaporisation is drawn from the roof and nearby air, lowering their temperature. This cooling mechanism also operates when sweat evaporates and when water is stored in porous earthen pots.
91. What produces water droplets on the outside of a glass containing ice-cold water?
[SSC MTS 2025 paper, February 2026]
(A) Evaporation of the water inside the glass
(B) Condensation of water vapour present in surrounding air
(C) Absorption of moisture through solid glass
(D) Heating of the air next to the glass
Answer: (B) Condensation of water vapour present in surrounding air
Explanation: Warm ambient air contains water vapour. At the cold glass surface, nearby air cools, possibly below its dew point, causing vapour to condense into droplets. The moisture therefore originates outside the glass, not from water penetrating the vessel.