Why does viscocity increase with temperature in gases? When they gain kinetic energy, they should be more free right?
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$\begingroup$ For more details refer: Temperature dependence of viscosity - Wikipedia $\endgroup$ – Guru Vishnu May 17 at 8:42
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From the basis of the kinetic theory of gases, viscosity in gases increases with temperature because the increase in kinetic energy will increase the frequency of collisions between the molecules. This increased number of collisions decreases the ability of the molecules to diffuse in the desired direction (i.e. the gas doesn't flow down the pipe to a reactor).
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A short, qualitative answer:
In gas, the addition of heat increases the average velocities of molecules, which means more intermolecular collisions, which means more intermolecular interactions that limits macroscopic mobility of fluid in certain direction.
In liquid it's the opposite - heating the system increases the average velocities of particles which means less intermolecular interactions with other molecules.
