What is the effect of increase in temperature on viscosity of gases ?

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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.

Why the other options are wrong — (b) 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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2025 Q56 — Science & Technology PYQ with Answer

State PCS 2025 Q56 — Science & Technology PYQ with Answer