Ch. 13.2 The Ch. 13.2 The Nature of LiquidsNature of Liquids

A Model for LiquidsA Model for Liquids

In kinetic theory, there are no In kinetic theory, there are no attractions between particles of a gas. attractions between particles of a gas. Particles of a liquid Particles of a liquid areare attracted to attracted to each other. So liquids have a definite each other. So liquids have a definite volume.volume.

The interplay between the disruptive The interplay between the disruptive motions of particles in a liquid and the motions of particles in a liquid and the attractions among the particles attractions among the particles determines the physical properties of determines the physical properties of liquids.liquids.

Because liquids and solids are much Because liquids and solids are much denser than gases, they are known denser than gases, they are known as as condensed condensed states of matter.states of matter.

EvaporationEvaporation VaporizationVaporization—the conversion of a liquid —the conversion of a liquid

to a gas or vapor.to a gas or vapor. EvaporationEvaporation—when the conversion —when the conversion

occurs at the surface of a liquid that is occurs at the surface of a liquid that is not boiling.not boiling.

During evaporation, only those molecules During evaporation, only those molecules with a certain minimum kinetic energy with a certain minimum kinetic energy can escape from the surface of the liquid.can escape from the surface of the liquid.

Lower energy molecules are left behind, Lower energy molecules are left behind, so the liquid’s temperature decreases.so the liquid’s temperature decreases.

This is why evaporation is a cooling This is why evaporation is a cooling process.process.

Vapor PressureVapor Pressure When evaporation occurs in in liquid When evaporation occurs in in liquid

in a closed container, no particles in a closed container, no particles can escape into the outside air.can escape into the outside air.

Vaporized particles collide with the Vaporized particles collide with the walls of the sealed container.walls of the sealed container.

Vapor pressureVapor pressure--A measure of the --A measure of the force exerted by a gas above a liquid.force exerted by a gas above a liquid.

Particles of vapor can condense and Particles of vapor can condense and return to the liquid state return to the liquid state (condensation).(condensation).

In a system at a constant vapor In a system at a constant vapor pressure, a dynamic equilibrium pressure, a dynamic equilibrium exists between the vapor and the exists between the vapor and the liquid. The system is in equilibrium liquid. The system is in equilibrium because the rate of evaporation of because the rate of evaporation of liquid equals the rate of liquid equals the rate of condensation of vapor (closed condensation of vapor (closed systems).systems).

Vapor Pressure and Vapor Pressure and Temperature ChangeTemperature Change

Vapor pressure increases with Vapor pressure increases with increasing temperature because increasing temperature because particles in the warmed liquid have particles in the warmed liquid have increased kinetic energy.increased kinetic energy.

Vapor pressure is measured with a Vapor pressure is measured with a manometermanometer..

Boiling PointBoiling Point Heating allows a greater number of Heating allows a greater number of

particles at the liquids surface to particles at the liquids surface to overcome the attractive force that keeps overcome the attractive force that keeps them in the liquid state.them in the liquid state.

When a liquid is heated to a When a liquid is heated to a temperature at which particles temperature at which particles throughout the liquid have enough throughout the liquid have enough kinetic energy to vaporize, the liquid kinetic energy to vaporize, the liquid begins to boil.begins to boil.

Boiling pointBoiling point—the temperature at which —the temperature at which the vapor pressure of the liquid is equal the vapor pressure of the liquid is equal to the external pressure on the liquid.to the external pressure on the liquid.

Boiling Point and Pressure Boiling Point and Pressure ChangesChanges

Boiling point temperatures of liquids Boiling point temperatures of liquids decrease at lower pressures.decrease at lower pressures.

Because atmospheric pressure is Because atmospheric pressure is lower at higher altitudes, boiling lower at higher altitudes, boiling points decrease at higher altitudes.points decrease at higher altitudes.

Boiling is also a cooling process…the Boiling is also a cooling process…the particles with the highest kinetic particles with the highest kinetic energy escape first.energy escape first.

The temperature of a boiling liquid The temperature of a boiling liquid nevernever rises above its boiling point. rises above its boiling point.

Normal Boiling PointNormal Boiling Point

Because a liquid can have various Because a liquid can have various boiling points depending on boiling points depending on pressure, the pressure, the normal boiling pointnormal boiling point is defined as the boiling point of a is defined as the boiling point of a liquid at 101.3 kPa.liquid at 101.3 kPa.