40. Calorimetry is primarily concerned with measuring what?
[SSC MTS, September 2023]
(A) Compressive strength
(B) Mechanical force
(C) Electrical current
(D) Heat exchanged in a process
Answer: (D) Heat exchanged in a process
Explanation: Calorimetry determines the amount of heat absorbed or released during physical or chemical processes. By recording temperature changes and knowing heat capacities, one can evaluate energy transfer. Carefully insulated calorimeters reduce unwanted exchange with the environment.
39. Chloroform boils near 334 K and methane near 111 K. Which stated boiling temperatures are correct?
[SSC CGL Tier-I, July 2023]
(A) Chloroform only
(B) Neither
(C) Methane only
(D) Both
Answer: (D) Both
Explanation: Chloroform has a normal boiling point near 61°C, which equals approximately 334 K. Methane boils near −162°C, approximately 111 K, at atmospheric pressure. The disparity reflects major differences in molecular interactions and volatility between the two compounds.
38. Who investigated carbon dioxide near its critical temperature and pressure in the nineteenth century?
[SSC CHSL Tier-I, August 2023]
(A) Thomas Andrews
(B) Giulio Natta
(C) John Dalton
(D) Marguerite Perey
Answer: (A) Thomas Andrews
Explanation: Thomas Andrews performed influential nineteenth-century experiments on carbon dioxide. He showed how the distinction between liquid and gas disappears near a critical point. His work established foundational ideas about phase behaviour, critical temperature and pressure in thermodynamics.
37. A substance absorbs 20 kJ of heat and warms by 10°C. Its specific heat is 400 J/kg°C. What is its mass?
[UPSC NDA-I GAT, April 2022]
(A) 0.1 kg
(B) 1 kg
(C) 10 kg
(D) 5 kg
Answer: (D) 5 kg
Explanation: The heat equation is Q = mcΔT. Converting 20 kJ to 20,000 J and substituting c = 400 J/kg°C and ΔT = 10°C gives m = 20,000/(400 × 10) = 5 kg.
36. Which conditions generally cause a liquid to evaporate faster?
[UPSC NDA-I GAT, April 2022]
(A) Higher temperature and greater exposed surface area
(B) Lower temperature and smaller exposed area
(C) Higher humidity and lower wind speed
(D) Higher humidity and smaller surface area
Answer: (A) Higher temperature and greater exposed surface area
Explanation: Heating increases the fraction of molecules energetic enough to escape a liquid surface. A broader surface permits more escaping molecules at once. Low humidity and moving air can further enhance evaporation by removing vapour from the region near the surface.
35. What is the latent heat of vaporisation associated with?
[UPSC NDA-I GAT, April 2022]
(A) Liquid becoming vapour with increasing temperature
(B) Liquid becoming vapour at constant temperature
(C) Vapour becoming liquid with temperature increase
(D) Vapour becoming liquid with temperature decrease only
Answer: (B) Liquid becoming vapour at constant temperature
Explanation: At the boiling temperature under fixed pressure, heat can convert a liquid into vapour without changing temperature. This energy is latent heat of vaporisation. It increases molecular separation and potential energy rather than raising mean molecular kinetic energy.
34. Which claim about sound speed in air is incorrect?
[UPSC NDA-II GAT, September 2022]
(A) Sound normally travels faster in steel than water
(B) Sound speed in air decreases as temperature increases
(C) Sound speed in air increases as temperature increases
(D) Sound travels faster in water than air
Answer: (B) Sound speed in air decreases as temperature increases
Explanation: For an ideal gas, sound speed is approximately proportional to the square root of absolute temperature. As air warms, the speed of sound therefore rises. Typical solids transmit sound faster than liquids, and liquids faster than gases, though exceptions exist.
33. Which pair consists of especially good conductors of heat?
[UPSC CDS-II General Knowledge, September 2022]
(A) Silver and copper
(B) Silver and lead
(C) Copper and mercury
(D) Lead and mercury
Answer: (A) Silver and copper
Explanation: Silver and copper have exceptionally high thermal conductivity among familiar metals. Their abundant mobile electrons rapidly carry thermal energy. Lead and mercury conduct less effectively, so the pair containing silver and copper best satisfies the question.
32. Earth’s atmosphere is heated most directly from below by which radiation?
[UPPSC State Services Prelims, June 2022]
(A) Long-wave radiation emitted by Earth
(B) Scattered solar radiation
(C) Reflected solar radiation
(D) Incoming short-wave solar radiation
Answer: (A) Long-wave radiation emitted by Earth
Explanation: Solar radiation warms Earth’s surface, which subsequently emits long-wave infrared energy. Greenhouse gases absorb and re-emit part of this outgoing radiation, heating the lower atmosphere. The atmosphere is relatively transparent to much incoming short-wave sunlight.
31. The term thermal transmittance is also known as what?
[MPPSC Assistant Engineer Mechanical, July 2022]
(A) Energy
(B) Mass transfer
(C) Radiation
(D) Overall heat-transfer coefficient
Answer: (D) Overall heat-transfer coefficient
Explanation: Thermal transmittance, commonly denoted U, measures steady heat flow through an assembly per unit area per unit temperature difference. Its SI unit is W/m²K. It combines effects of conduction and boundary convection, making it useful in insulation and heat-exchanger analysis.
