Prentice-Hall Chapter 13.2 Dr. Yager The Nature of Liquids Prentice-Hall Chapter 13.2 Dr. Yager

Objectives Identify factors that determine the physical properties of liquids Define evaporation in terms of kinetic energy Describe the equilibrium between a liquid and its vapor Identify the conditions at which boiling occurs

Model for Liquids All particles have kinetic energy. Liquids flow and conform to the shape of their container. Liquid particles are attracted to each other.

Substances that can flow are referred to as fluids Substances that can flow are referred to as fluids. Both liquids and gases are fluids.

Key Idea The interplay between the kinetic energy of a liquid’s particles and the attractive forces between them determine the liquid’s physical properties.

In an open container, molecules that evaporate can escape from the container.

Evaporation vaporization - the conversion of a liquid to gas evaporation - vaporization at a surface without boiling Both occur because particles have the minimum kinetic energy to break away from the surface. Some particles rebound back to the liquid. Both increase with heat.

In a closed container, the molecules cannot escape In a closed container, the molecules cannot escape. They collect as a vapor above the liquid. Some molecules condense back into a liquid.

Vapor Pressure Vapor pressure is the force exerted by a gas above a liquid. Some particles collide and condense back to liquid.

Key Idea In a system at 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.

Vapor Pressure and Temperature Change An increase in temperature causes an increase in vapor pressure. The particles in the warmed liquid have increased kinetic energy. As a result, more of the particles will have the minimum kinetic energy necessary to escape the surface of the liquid.

Vapor Pressure and Temperature Change

Vapor Pressure Measurement Manometer

Boiling Point As heating increases so does the kinetic energy of the particles. Boiling occurs when the liquid has enough kinetic energy throughout the liquid to vaporize. Bubbles of vapor form throughout the liquid.

Boiling Point boiling point: the temperature where the vapor pressure of the liquid just equals the external pressure. At a lower external pressure, the boiling point decreases. At a higher external pressure, the boiling point increases.

Sometimes in a chemistry lab we put glass boiling beads in a liquid as we heat it. Why? The beads absorb some of the kinetic energy by bouncing around and prevent the liquid from boiling too quickly.

Altitude and 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 a pressure of 101.3 kPa.

Boiling Versus Evaporation Both are cooling processes (remove kinetic energy). During both the highest kinetic energy particles escape first. Increased heat increases rate of boiling. Boiling liquid temperature never rises above the boiling point. More heat means more kinetic energy. Vapor can have more heat (potential energy) than the liquid.

Evaporation and Sweating As liquids evaporate the temperature of the remaining liquid decreases. When you perspire water molecules absorb heat from your body when they leave your skin surface, making it cooler.

Evaporation and Cooling

In liquids, the attractive forces are very weak compared with the kinetic energies of the particles. strong enough to keep the particles confined to fixed locations in the liquid. strong enough to keep the particles from evaporating. strong enough to keep particles relatively close together.

In liquids, the attractive forces are very weak compared with the kinetic energies of the particles. strong enough to keep the particles confined to fixed locations in the liquid. strong enough to keep the particles from evaporating. strong enough to keep particles relatively close together.

2. Which one of the following is a process that absorbs energy? freezing condensation evaporation solidifying

2. Which one of the following is a process that absorbs energy? freezing condensation evaporation solidifying

3. In a sealed gas-liquid system at constant temperature eventually there will be no more evaporation. the rate of condensation decreases to zero. the rate of condensation exceeds the rate of evaporation. the rate of evaporation equals the rate of condensation.

3. In a sealed gas-liquid system at constant temperature eventually there will be no more evaporation. the rate of condensation decreases to zero. the rate of condensation exceeds the rate of evaporation. the rate of evaporation equals the rate of condensation.

4. Where must particles have enough kinetic energy to vaporize for boiling to occur? at the surface of the liquid at the bottom of the container along the sides of the container throughout the liquid

4. Where must particles have enough kinetic energy to vaporize for boiling to occur? at the surface of the liquid at the bottom of the container along the sides of the container throughout the liquid

The boiling point of a liquid increases at higher altitudes. decreases at higher altitudes. is the same at all altitudes. decreases as the pressure increases.

The boiling point of a liquid increases at higher altitudes. decreases at higher altitudes. is the same at all altitudes. decreases as the pressure increases.