1. Ch. 13.2 The Nature of Liquids

2. A Model for Liquids In kinetic theory, there are no attractions between particles of a gas. Particles of a liquid are attracted to each other. So liquids have a definite volume. In kinetic theory, there are no attractions between particles of a gas. Particles of a liquid are attracted to each other. So liquids have a definite volume. The interplay between the disruptive motions of particles in a liquid and the attractions among the particles determines the physical properties of liquids. The interplay between the disruptive motions of particles in a liquid and the attractions among the particles determines the physical properties of liquids.

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

4. Evaporation Vaporization—the conversion of a liquid to a gas or vapor. Vaporization—the conversion of a liquid to a gas or vapor. Evaporation—when the conversion occurs at the surface of a liquid that is not boiling. Evaporation—when the conversion occurs at the surface of a liquid that is not boiling. During evaporation, only those molecules with a certain minimum kinetic energy can escape from the surface of the liquid. During evaporation, only those molecules with a certain minimum kinetic energy can escape from the surface of the liquid. Lower energy molecules are left behind, so the liquid’s temperature decreases. Lower energy molecules are left behind, so the liquid’s temperature decreases.

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

6. Vapor Pressure When evaporation occurs in in liquid in a closed container, no particles can escape into the outside air. When evaporation occurs in in liquid in a closed container, no particles can escape into the outside air. Vaporized particles collide with the walls of the sealed container. Vaporized particles collide with the walls of the sealed container. Vapor pressure--A measure of the force exerted by a gas above a liquid. Vapor pressure--A measure of the force exerted by a gas above a liquid. Particles of vapor can condense and return to the liquid state (condensation). Particles of vapor can condense and return to the liquid state (condensation).

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

8. Vapor Pressure and Temperature Change Vapor pressure increases with increasing temperature because particles in the warmed liquid have increased kinetic energy. Vapor pressure increases with increasing temperature because particles in the warmed liquid have increased kinetic energy. Vapor pressure is measured with a manometer. Vapor pressure is measured with a manometer.

9. Boiling Point Heating allows a greater number of particles at the liquids surface to overcome the attractive force that keeps them in the liquid state. Heating allows a greater number of particles at the liquids surface to overcome the attractive force that keeps them in the liquid state. When a liquid is heated to a temperature at which particles throughout the liquid have enough kinetic energy to vaporize, the liquid begins to boil. When a liquid is heated to a temperature at which particles throughout the liquid have enough kinetic energy to vaporize, the liquid begins to boil. Boiling point—the temperature at which the vapor pressure of the liquid is equal to the external pressure on the liquid. Boiling point—the temperature at which the vapor pressure of the liquid is equal to the external pressure on the liquid.

10. Boiling Point and Pressure Changes Boiling point temperatures of liquids decrease at lower pressures. Boiling point temperatures of liquids decrease at lower pressures. Because atmospheric pressure is lower at higher altitudes, boiling points decrease at higher altitudes. Because atmospheric pressure is lower at higher altitudes, boiling points decrease at higher altitudes. Boiling is also a cooling process…the particles with the highest kinetic energy escape first. Boiling is also a cooling process…the particles with the highest kinetic energy escape first. The temperature of a boiling liquid never rises above its boiling point. The temperature of a boiling liquid never rises above its boiling point.

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