The viscosity of a fluid is directly related to
the pumping power needed to transport a
fluid in a pipe or to move a body through a
fluid.
Viscosity is caused by the cohesive forces
between the molecules in liquids and by
the molecular collisions in gases, and it
varies greatly with temperature.
In a liquid, the molecules possess more
energy at higher temperatures, and they
can oppose the large cohesive
intermolecular forces more strongly. As a
result, the energized liquid molecules can
move more freely.
In a gas, the intermolecular forces are
negligible, and the gas molecules at high
temperatures move randomly at higher
velocities. This results in more molecular
collisions per unit volume per unit time and
therefore in greater resistance to flow.