Question

What is viscosity? What causes viscosity in liquids and in gases? Is dynamic viscosity typically higher for a liquid or for a gas?

Short answer

Answer: Viscosity is a measure of a fluid's resistance to flow, reflecting its internal friction during movement. In liquids, viscosity is mainly caused by cohesive forces between molecules like hydrogen bonds, van der Waals forces, and dipole-dipole interactions. In gases, viscosity is due to collisions and momentum transfer between gas molecules. Typically, the dynamic viscosity of liquids is higher than that of gases, primarily because cohesive forces in liquids are generally stronger than molecular collisions in gases.

Step-by-step solution

Definition of Viscosity

Viscosity is a measure of a fluid's resistance to flow. It is a property that describes the internal friction of the fluid when it is subjected to deformation or movement. The higher the viscosity, the thicker and slower a fluid will flow. Viscosity can be classified into two types: dynamic viscosity and kinematic viscosity. In this exercise, we will focus on dynamic viscosity.

Viscosity in Liquids

In liquids, viscosity is primarily caused by the cohesive forces between the molecules. These forces include hydrogen bonds, van der Waals forces, and dipole-dipole interactions, among others. Due to these forces, the molecules in a liquid experience resistance when they flow past one another. As a result, liquids with higher cohesive forces exhibit greater resistance to flow, resulting in higher viscosity.

Viscosity in Gases

Viscosity in gases is caused by the collisions and transfer of momentum between the gas molecules. When a gas is subjected to deformation or movement, the molecules collide with each other, as well as with the walls of a container if present, leading to a resistance to flow. Unlike liquids, the viscosity of gases is mostly affected by the temperature and not their cohesive forces. As the temperature of the gas increases, the gas molecules move faster and collide more frequently, resulting in higher viscosity.

Comparison of Dynamic Viscosity in Liquids and Gases

Typically, dynamic viscosity is higher for liquids than for gases. This is because the cohesive forces acting in liquids are generally stronger than the molecular collisions in gases. In addition, liquids are more closely packed compared to gases, leading to more significant interactions between the molecules. As a result, liquids generally demonstrate a higher resistance to flow or deformation compared to gases, which leads to a higher dynamic viscosity.