In a gas, how does viscosity change as temperature increases?

• A. It increases
• B. It decreases
• C. It stays the same
• D. It becomes zero

Answer: (A)

Viscosity in gases comes from momentum transfer between adjacent layers as molecules collide and exchange momentum. When temperature rises, molecules move faster, so each collision conveys more momentum and transfers it across layers more effectively. Even though the gas becomes less dense at higher temperatures (if pressure is held constant), the increase in molecular speed dominates, leading to a higher rate of momentum transport and thus a higher viscosity. In kinetic theory for an ideal gas at constant pressure, viscosity increases roughly with the square root of temperature, reinforcing why higher temperature makes a gas more viscous.