MPSC 2025 Prelims Paper-I · Question 56 of 100

Correct — option (1), Viscosity increases. Viscosity in a gas arises from momentum transfer between adjacent layers of gas molecules moving at different bulk speeds: molecules constantly cross between layers, carrying their layer's momentum with them, and this exchange tends to equalise the layers' speeds, which is exactly what a viscous drag force does. According to the kinetic theory of gases, raising the temperature increases the average molecular speed, which increases how frequently and how vigorously molecules cross between adjacent layers to exchange momentum. More frequent, higher-momentum crossings mean a stronger drag effect between layers, so a gas's viscosity increases as temperature rises — this is the opposite of what happens in a liquid, where viscosity is dominated by intermolecular cohesive forces that weaken as temperature rises, so liquid viscosity decreases with temperature.

• (2)Viscosity decreases — This is the correct temperature dependence for a liquid, not a gas — liquid viscosity is governed mainly by intermolecular cohesive forces that weaken as molecules gain thermal energy and move further apart, so liquids do get 'thinner' when heated. Gas viscosity works by an entirely different mechanism (momentum transfer between molecules crossing between layers), which strengthens rather than weakens as temperature rises, so applying the liquid rule to a gas gives the wrong direction.
• (3)Viscosity remains constant — Kinetic theory predicts, and experiments confirm, a definite temperature dependence for gas viscosity — it is not a temperature-independent property. The commonly cited approximate relationship is that viscosity scales with roughly the square root of the absolute temperature for an ideal gas of hard-sphere molecules, which is a real, measurable increase, not a constant value.
• (4)Viscosity becomes negative — Viscosity is a measure of a fluid's internal resistance to relative motion between its layers, and it is physically defined to be a non-negative quantity — a negative viscosity would imply the fluid actively accelerates relative motion between layers rather than resisting it, which does not correspond to any ordinary gas behaviour under changing temperature.

Viscosity is a fluid's internal resistance to flow, arising from friction-like forces between adjacent layers of fluid moving at different velocities. In liquids, viscosity is dominated by intermolecular cohesive (attractive) forces holding neighbouring molecules together, which weaken as thermal agitation increases with temperature — so liquid viscosity falls as temperature rises. In gases, molecules are far apart and cohesive forces are negligible; viscosity instead arises from momentum transfer as fast-moving molecules dart between adjacent layers moving at different bulk speeds, exchanging momentum and equalising their velocities. Since kinetic theory ties average molecular speed directly to temperature, a hotter gas has molecules crossing between layers more often and with more momentum to exchange, so gas viscosity rises with temperature — the reverse of the liquid case.

MPSC's science section tests this fact because it is a classic example of the same macroscopic property (viscosity) behaving in opposite directions in two different states of matter (liquid versus gas) for physically distinct microscopic reasons, and examiners use it to check whether a candidate understands the underlying mechanism rather than having memorised a single rule for 'viscosity and temperature' that only applies to one phase.

• Gas viscosity increases with rising temperature, because kinetic theory ties higher temperature to faster molecular speeds and hence more vigorous momentum exchange between adjacent gas layers.
• Liquid viscosity decreases with rising temperature, because liquid viscosity is dominated by intermolecular cohesive forces that weaken as thermal energy increases.
• For an ideal gas of hard-sphere molecules, viscosity is often approximated as increasing roughly with the square root of absolute temperature.
• Viscosity is defined as a non-negative measure of a fluid's internal resistance to relative motion between its layers.

• Applying the liquid rule (viscosity decreases with temperature) to a gas, when the two phases have opposite temperature dependence for physically different reasons
• Assuming viscosity is a fixed material constant unaffected by temperature
• Confusing viscosity with density, which does decrease for both liquids and gases as temperature rises (for most substances) due to thermal expansion

MPSC's science section frequently tests whether a property behaves the same way across different states of matter, and gas-versus-liquid viscosity is a recurring example precisely because the answer is counter-intuitive if a candidate assumes both phases behave alike.

No directly related past PYQ was found.

• practice — not a real PYQ

  What is the effect of an increase in temperature on the viscosity of a liquid such as water ?

  • (a)Viscosity increases
  • (b)Viscosity decreases
  • (c)Viscosity remains unchanged
  • (d)Viscosity first increases, then decreases

  Answer(b) Viscosity decreases — liquid viscosity is dominated by intermolecular cohesive forces, which weaken as temperature rises, unlike gas viscosity, which increases with temperature.

• practice — not a real PYQ

  The viscosity of a gas arises mainly from which mechanism ?

  • (a)Intermolecular cohesive (attractive) forces between gas molecules
  • (b)Momentum transfer as molecules move between adjacent layers of differing bulk velocity
  • (c)Electrostatic repulsion between charged gas molecules
  • (d)Gravitational interaction between gas molecules

  Answer(b) Momentum transfer as molecules move between adjacent layers of differing bulk velocity — this mechanism strengthens as temperature rises, since higher temperature means faster, more frequent molecular crossings between layers.