90. Why do droplets appear on the outside of a cold drinking glass on a warm day?
[RRB Group D, February 2026]
(A) Freezing
(B) Sublimation
(C) Condensation
(D) Evaporation
Answer: (C) Condensation
Explanation: The glass cools adjacent humid air. When this air reaches its dew point, gaseous water changes into liquid droplets on the outer surface. Condensation releases latent heat. The glass does not leak; the deposited moisture comes from atmospheric water vapour.
89. Evaporation of a liquid can occur at which temperature?
[RRB Group D, February 2026]
(A) Only below its freezing point
(B) Only at 100°C
(C) Only at the boiling point
(D) At any temperature where the liquid exists
Answer: (D) At any temperature where the liquid exists
Explanation: Evaporation occurs at a liquid’s exposed surface whenever some molecules possess enough energy to escape. Unlike boiling, it does not require the entire liquid to reach its boiling point. Higher temperature generally accelerates the process by increasing molecular kinetic energy.
88. Which change does NOT increase the rate of evaporation?
[RRB Group D, February 2026]
(A) Increasing exposed surface area
(B) Increasing wind speed
(C) Decreasing temperature
(D) Decreasing humidity
Answer: (C) Decreasing temperature
Explanation: Lowering temperature reduces the average kinetic energy of liquid molecules and generally slows their escape from the surface. A larger surface, stronger airflow and drier air normally increase evaporation. The precise rate depends on the liquid and surrounding conditions.
87. Which change normally increases evaporation by exposing more liquid molecules to air?
[RRB Group D, February 2026]
(A) Reducing temperature
(B) Increasing atmospheric humidity
(C) Reducing wind speed
(D) Increasing the liquid’s surface area
Answer: (D) Increasing the liquid’s surface area
Explanation: Evaporation is a surface process. A wider exposed area allows more molecules to escape into the atmosphere during a given interval. This explains why wet clothes spread on a line dry faster than when they remain bundled together.
86. What should be done to evaporate water more quickly?
[RRB Group D, January 2026]
(A) Cover the container
(B) Use a deep, narrow vessel
(C) Keep it in a cool, humid room
(D) Raise its temperature and use a fan
Answer: (D) Raise its temperature and use a fan
Explanation: Warmer water has more energetic molecules, so more can overcome surface attraction. A fan replaces moisture-rich air with drier moving air and prevents vapour accumulation above the surface. Both changes promote more rapid evaporation compared with a covered container.
85. Why does skin feel cooler when sweat evaporates?
[RRB Group D, January 2026]
(A) Sweat crystallises on skin
(B) Sweat freezes before vaporising
(C) Evaporating sweat absorbs heat from the skin
(D) Evaporating sweat releases heat to the skin
Answer: (C) Evaporating sweat absorbs heat from the skin
Explanation: Vaporisation requires latent heat. Sweat obtains this energy from its surroundings, including the body surface, so the skin loses heat and feels cooler. In humid weather evaporation slows, which is why sweating becomes a less effective cooling mechanism.
84. What is the likely sequence when a typical solid is continuously heated?
[RRB Group D, January 2026]
(A) It expands and immediately becomes gas
(B) It always sublimates directly
(C) It continually contracts
(D) It may melt and later vaporise
Answer: (D) It may melt and later vaporise
Explanation: Heating generally raises particle kinetic energy until the solid reaches its melting point. Added energy then changes the phase into liquid. Continued heating can bring the liquid to its boiling point, where it changes into vapour; exceptions include subliming substances.
83. Ice has latent heat of fusion 3.34 × 10⁵ J/kg. How much heat melts 0.20 kg of ice at 0°C without raising its temperature?
[Delhi Police Head Constable AWO/TPO 2025 paper, January 2026]
(A) 6.68 × 10³ J
(B) 1.67 × 10⁴ J
(C) 3.34 × 10⁴ J
(D) 6.68 × 10⁴ J
Answer: (D) 6.68 × 10⁴ J
Explanation: Use Q = mL because the ice changes phase at constant temperature. Multiplying 0.20 kg by 3.34 × 10⁵ J/kg gives 6.68 × 10⁴ J. This energy disrupts the ice crystal structure rather than heating the meltwater.
82. Which heat-transfer mechanism works without any material medium, including through a vacuum?
[Delhi Police Head Constable AWO/TPO 2025 paper, January 2026]
(A) Only conduction
(B) Only convection
(C) Both conduction and convection
(D) Radiation
Answer: (D) Radiation
Explanation: Thermal radiation consists of electromagnetic waves and can travel through empty space. Conduction involves microscopic energy transfer through matter, while convection needs movement of a liquid or gas. Consequently, solar heat reaches Earth across the near-vacuum of space through radiation.
81. One kilojoule of heat energy is equal to how many joules?
[SSC MTS 2025 paper, February 2026]
(A) 10
(B) 100
(C) 1,000
(D) 10,000
Answer: (C) 1,000
Explanation: The SI prefix kilo means one thousand. A kilojoule is therefore 10³ joules. The joule is the SI unit for heat, work and energy, whereas the watt measures energy transfer per second and is a unit of power.
