Q50. Why are liquids far less compressible than gases?
[RRB Group D – December 2025]
(A) They always contain dissolved gases
(B) They are always less dense than solids
(C) Their mass cannot change
(D) Their particles have relatively little empty space between them
Answer: (D) Their particles have relatively little empty space between them
Explanation: Liquid particles are packed much closer than gas particles, leaving comparatively little space available for compression. As a result, applying pressure causes a relatively small volume change. Most liquids still have finite compressibility, though it is low.
Q49. Which typical property explains why many metals make a ringing sound when struck?
[RRB Group D – December 2025]
(A) Brittleness
(B) Plasticity
(C) Sonority
(D) Low conductivity
Answer: (C) Sonority
Explanation: Sonority means that a material can produce a ringing or resonant sound when struck. Metallic solids can vibrate and sustain such audible oscillations. This is one reason metals and alloys are useful in bells, cymbals and similar instruments.
Q48. For most materials, what happens to density as temperature increases, if mass remains unchanged?
[RRB Group D – December 2025]
(A) It never changes
(B) It always increases
(C) It usually decreases
(D) It must double
Answer: (C) It usually decreases
Explanation: Heating normally increases a substance’s volume through thermal expansion. Because density equals mass divided by volume, the same mass occupying a larger volume has lower density. Water between 0°C and 4°C is a well-known exception to this general pattern.
Q47. Which statement correctly identifies the SI unit of pressure?
[UPSC NDA-II – 2025]
(A) Pressure is a directed vector
(B) Pressure decreases with depth in water
(C) Action and reaction act on one body
(D) One pascal equals one newton per square metre
Answer: (D) One pascal equals one newton per square metre
Explanation: Pressure is normal force divided by area and is measured in pascals. Thus one pascal equals one newton per square metre. In a stationary liquid, hydrostatic pressure usually rises with depth rather than falling, making the other statements incorrect.
Q46. How does the surface tension of an ordinary liquid generally change as its temperature increases?
[UPSC NDA-II – 2025]
(A) It increases
(B) It decreases
(C) It always stays constant
(D) It decreases for either warming or cooling
Answer: (B) It decreases
Explanation: Raising temperature generally weakens the effective cohesive influence responsible for surface tension. As molecular motion increases, less work is needed to increase the liquid surface. Surface tension usually approaches zero near the liquid’s critical temperature.
Q45. A metal sphere has volume 2 × 10⁻⁴ m³ and is completely immersed in water. What buoyant force acts on it if water density is 1000 kg/m³ and g = 10 m/s²?
[UPSC CDS-I – 2025]
(A) 0.5 N
(B) 1.5 N
(C) 2 N
(D) 2.5 N
Answer: (C) 2 N
Explanation: The upward force is the weight of displaced water: F = ρVg. Substituting 1000 × 2 × 10⁻⁴ × 10 gives 2 newtons. The sphere’s own mass does not enter this buoyancy calculation when its immersed volume is known.
Q44. Which property of a liquid is related to its internal resistance to the relative motion of adjacent layers?
[SSC JE Mechanical – November 2024]
(A) Adaptability
(B) Transferability
(C) Permeability
(D) Viscosity
Answer: (D) Viscosity
Explanation: Viscosity expresses internal friction in a fluid and determines how strongly one moving layer resists sliding past another. Molecular interactions contribute to it. Honey and glycerine generally flow less readily than water because their viscosities are considerably higher.
Q43. Which pair of metal properties is particularly useful when producing thin, shiny sheets of silver for sweets?
[SSC MTS – November 2024]
(A) Ductility and lustre
(B) Malleability and sonority
(C) Lustre and sonority
(D) Malleability and lustre
Answer: (D) Malleability and lustre
Explanation: Silver can be beaten into very thin sheets because it is malleable. Its shiny appearance results from metallic lustre, which reflects visible light. Ductility instead refers to drawing wires, while sonority refers to producing a ringing sound.
Q42. What is the ability of rock or soil to allow fluids to pass through connected pores called?
[SSC MTS – October 2024]
(A) Permeability
(B) Plasticity
(C) Stability
(D) Density
Answer: (A) Permeability
Explanation: Permeability describes how readily a porous material transmits fluids through connected openings. Large or well-connected pores often increase fluid passage. Porosity measures the fraction of empty spaces and does not, by itself, establish how easily fluid can flow.
Q41. A pumpkin weighs 7.5 N and displaces 0.75 litre of water when fully immersed. Calculate its density, taking g = 10 m/s².
[UPSC NDA-II – 2024]
(A) 10 kg/m³
(B) 100 kg/m³
(C) 1000 kg/m³
(D) 10000 kg/m³
Answer: (C) 1000 kg/m³
Explanation: The pumpkin’s mass equals 7.5/10 = 0.75 kg. Its volume is 0.75 litre, or 0.00075 m³. Dividing mass by volume yields 1000 kg/m³. The displaced water volume equals its volume because the pumpkin is fully immersed.
