On increasing temperature, the viscosity of gas:
Try it first: pick an option, then read the explanation.
Correct answer: C. increases
Asked in SSC JE 25-01-2018 (Morning Shift)
Detailed Explanation
In gases, the primary mechanism of viscous resistance is molecular momentum exchange across adjacent fluid layers rather than intermolecular cohesive attraction.
Why “increases” Is Correct
When gas temperature rises, average molecular speed increases (\(\bar{v} \propto \sqrt{T}\)), resulting in higher collision frequency and greater momentum transfer between faster and slower fluid layers. Consequently, gas viscosity increases.
Why the Other Options Are Wrong
- A. decreases: Viscosity decreases with temperature in liquids (where cohesion dominates), not gases.
- B. first increases and then rapidly decrease: Gas viscosity increases monotonically across normal engineering temperature ranges.
- D. not affected by temperature: Viscosity of gases depends noticeably on temperature, approximately as μ ∝ T^(0.5 to 0.75).
Key Point
Viscosity of gases increases with temperature due to increased molecular collision frequency and momentum transfer.
Key Concepts for Students
- Sutherland's formula models gas viscosity: μ = μ₀ (T/T₀)^(3/2) (T₀ + S) / (T + S).
- Kinematic viscosity of gases increases even faster with temperature because density decreases.
- Contrast: Liquid viscosity decreases with temperature because cohesive bonds weaken.
Practice more: this is question 42 in Properties of Fluid MCQs – Part 3. Try the whole set and check every answer instantly.