Final answer:
Liquid viscosity decreases with rising temperature as molecules gain kinetic energy to overcome cohesive forces, while gas viscosity increases due to more frequent and energetic collisions among its molecules.
Step-by-step explanation:
The viscosity of a liquid decreases with an increase in temperature because the kinetic energy of the molecules increases, allowing them to overcome the cohesive forces that impede their flow. In liquids, viscosity is closely linked to the strength of intermolecular forces (IMFs). As the temperature rises, the additional kinetic energy allows the molecules to move more rapidly and break free from these forces more easily, resulting in decreased viscosity.
In contrast, the viscosity of a gas increases with temperature due to the increased frequency and energy of collisions between the atoms and molecules. As the temperature of a gas rises, the molecules move faster, collide more frequently, and exchange more energy with each other, increasing the resistance to flow, hence increasing the gas's viscosity.