80. At the critical point of a pure substance, what happens to saturated liquid and saturated vapour?
[SSC JE Mechanical, December 2025]
(A) Latent heat becomes infinite
(B) Latent heat remains finite and nonzero
(C) Enthalpy becomes zero
(D) Their properties become identical
Answer: (D) Their properties become identical
Explanation: At the critical point, the distinction between saturated liquid and its vapour disappears. Their densities and other properties converge, and the latent heat of vaporisation approaches zero. Beyond the critical point there is no ordinary liquid-vapour phase boundary.
79. For adiabatic compression, how much heat crosses the system boundary?
[SSC JE Mechanical, December 2025]
(A) Maximum heat transfer
(B) Minimum positive heat transfer
(C) Zero heat transfer
(D) Very high heat transfer
Answer: (C) Zero heat transfer
Explanation: An adiabatic process is defined by zero heat exchange with the surroundings, Q = 0. Temperature may still rise during compression because work done on the gas increases its internal energy. Adiabatic does not mean constant temperature or zero work.
78. The energy needed to change one kilogram of a solid into liquid at its melting point is called the ____ heat of fusion.
[SSC CHSL Tier-I, November 2025]
(A) Latent
(B) Boiling
(C) Melting
(D) Absolute
Answer: (A) Latent
Explanation: Specific latent heat of fusion measures the energy needed to melt unit mass at constant temperature. During melting, heat changes molecular arrangement rather than producing a temperature rise. For an arbitrary mass, the required energy equals mass multiplied by specific latent heat.
77. What is the direct change from gas to solid without forming an intermediate liquid called?
[SSC CHSL Tier-I, November 2025]
(A) Solidification
(B) Deposition
(C) Freezing
(D) Evaporation
Answer: (B) Deposition
Explanation: Deposition converts a vapour straight into a solid without passing through the liquid phase. Frost forming from atmospheric water vapour is an example. The reverse process, solid changing directly to gas, is sublimation. Both processes exchange latent heat.
76. Which examples demonstrate cooling by evaporation:
I. Acetone applied to the palm;
II. Water sprinkled on a hot roof?
[SSC CHSL Tier-I, November 2025]
(A) Only I
(B) Only II
(C) Both I and II
(D) Neither I nor II
Answer: (C) Both I and II
Explanation: Acetone rapidly evaporates, extracting latent heat from the skin. Water evaporating from a sun-heated roof similarly removes thermal energy from that surface. In both examples a liquid changes into vapour while drawing heat from its surroundings.
75. Why can steam at 100°C cause worse burns than liquid water at the same temperature?
[SSC CGL Tier-I, September 2025]
(A) Steam is hotter than boiling water
(B) Steam contains more latent heat of vaporisation
(C) Steam has a higher specific heat capacity
(D) Steam molecules are moving faster
Answer: (B) Steam contains more latent heat of vaporisation
Explanation: Steam contains latent heat acquired during vaporisation. When it touches cooler skin, some steam condenses and releases this extra energy in addition to sensible heat. Consequently, steam at the same measured temperature can transfer substantially more heat than boiling water.
74. Why are the bases of many cooking utensils made from copper?
[SSC CGL Tier-I, September 2025]
(A) Copper is a strong thermal insulator
(B) Copper melts at very low temperatures
(C) Copper conducts heat well, helping distribute it evenly
(D) Copper must chemically react with food
Answer: (C) Copper conducts heat well, helping distribute it evenly
Explanation: Copper has high thermal conductivity because electrons transport energy efficiently through the metal. A copper layer spreads burner heat across the base and reduces local hotspots. The good conductor therefore promotes rapid, more uniform heating of the cooking vessel.
73. Which statement best describes a usual urban heat-island cause?
[SSC CGL Tier-I, September 2025]
(A) High humidity alone
(B) Extensive tree cover
(C) Dense concrete surfaces, limited vegetation and waste heat
(D) High elevation above sea level
Answer: (C) Dense concrete surfaces, limited vegetation and waste heat
Explanation: Buildings and paved surfaces absorb and store solar energy, while limited vegetation reduces cooling by transpiration. Traffic, air conditioners and other activities release additional heat. Together, these effects can make heavily built-up urban areas warmer than nearby rural surroundings.
72. Regarding pressure cookers, which statements are correct?
I. They lower pressure and lower boiling point;
II. Increased pressure raises water’s boiling point;
III. Higher-temperature steam helps food cook faster.
[SSC Stenographer, August 2025]
(A) Only II and III
(B) All three
(C) Only I and II
(D) Only I and III
Answer: (A) Only II and III
Explanation: A pressure cooker traps steam, raising the internal pressure. Water can then remain liquid above its ordinary 100°C boiling point, so cooking occurs at a higher temperature. Statement I reverses the pressure effect; statements II and III express the operating principle.
71. Boiling and evaporation are respectively which types of phenomena?
[SSC GD Constable, February 2025]
(A) Bulk and bulk
(B) Surface and bulk
(C) Surface and surface
(D) Bulk and surface
Answer: (D) Bulk and surface
Explanation: Boiling produces vapour bubbles throughout a liquid once its vapour pressure matches the surrounding pressure. Evaporation instead occurs at the exposed surface and can happen below boiling point. The distinction also explains why boiling has a characteristic temperature at fixed pressure.
