Physics of Liquids: Surface Tension and Viscosity

Surface Tension

• Defined as the measure of the elastic force in a liquid's surface
• Illustrated by the image of molecules in a liquid, with stronger attractive forces between surface molecules

Cohesive and Adhesive Forces

• Cohesive forces: Attractive forces between molecules within a liquid
• Adhesive forces: Attractive forces between molecules of a liquid and the surface of another substance (e.g., a container)
• Water exhibits a concave meniscus in a tube due to stronger adhesive forces between water molecules and the glass surface, pulling the liquid upward
• Mercury exhibits a convex meniscus due to stronger cohesive forces within the mercury, causing the liquid to curve downward

Capillary Action

• The rise of liquids up narrow tubes
• Result of competition between cohesive and adhesive forces
• Also known as capillarity

Viscosity

• The measure of a liquid's resistance to flowing
• Increased viscosity corresponds to decreased speed of flow
• Viscosity is related to intermolecular forces (IMFs)
• Increased viscosity results from stronger IMFs and lower temperatures
• Unit of viscosity: poise (P), or Pa-s (Pascals-seconds)

Viscosity of Liquids (Examples)

• Viscosity values are provided for several liquids at 20°C, including acetone, benzene, carbon tetrachloride, diethyl ether, ethanol, glycerol, mercury, and water in Ns/m² units

Viscosity and Molecular Structure

• The structure of glycerol (CH2-OH-CH-OH-CH2-OH) influences its viscosity due to its multiple hydrogen bonding sites and molecular shape

Vapor Pressure of Liquids

• Open container: Some water molecules escape into the air
• Closed container: Water molecules vaporize and condense, eventually reaching equilibrium
• At equilibrium in a closed container, the rate of evaporation equals the rate of condensation
• This equilibrium state creates a vapor pressure within the container
• Vapor pressure is independent of the container's volume
• An increase in the number of gas particles leads to a rise in vapor pressure

Equilibrium Vapor Pressure

• Pressure exerted by a gas in equilibrium with a liquid in a closed container at a specific temperature
• Molecules in the gaseous state above the liquid contribute to the pressure
• Increased number of gas particles leads to increased vapor pressure
• Represents the maximum vapor pressure of a liquid at a given temperature

Vapor Pressure and Temperature

• Vapor pressure increases with temperature
• Molecules gain kinetic energy at higher temperatures, overcoming intermolecular forces and escaping into the gas phase more readily

Vapor Pressure and IMF

• Liquids with high vapor pressure have weak intermolecular forces
• Liquids with low vapor pressure have strong London dispersion forces and strong dipole-dipole forces

Vapor Pressure and IMF (Examples)

• Vapor pressures of pentane, acetone, ethyl alcohol, and water at 25°C are given along with the dominant intermolecular forces.
• The stronger the intermolecular forces, the lower the vapor pressure.