Chemistry 13.2

13.2 A Model for Liquids Substances that can flow are referred to as fluids. Both liquids and gases are fluids. Both liquids and gases can flow. The liquid on the left is colored water. The gas on the right is bromine vapor. If a gas is denser than air, it can be poured from one container into another. These pictures were taken in a fume hood because bromine is both toxic and corrosive. Predicting Over time, what will happen to the gas in the uncovered beaker? Explain.

The conversion of a liquid to a gas or vapor is called vaporization. 13.2 Evaporation The conversion of a liquid to a gas or vapor is called vaporization. When such a conversion occurs at the surface of a liquid that is not boiling, the process is called evaporation.

13.2 Evaporation In an open container, molecules that evaporate can escape from the container. The process of evaporation has a different outcome in an open system, such as a lake, than in a closed system, such as a terrarium. a) In an open container, molecules that evaporate can escape from the container. b) In a closed container, the molecules cannot escape. They collect as a vapor above the liquid. Some molecules condense back into a liquid.

13.2 Evaporation In a closed container, the molecules cannot escape. They collect as a vapor above the liquid. Some molecules condense back into a liquid. The process of evaporation has a different outcome in an open system, such as a lake, than in a closed system, such as a terrarium. a) In an open container, molecules that evaporate can escape from the container. b) In a closed container, the molecules cannot escape. They collect as a vapor above the liquid. Some molecules condense back into a liquid.

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

13.2 Vapor Pressure Vapor pressure is a measure of the force exerted by a gas above a liquid.

13.2 Vapor Pressure 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 13.2 Vapor Pressure Vapor Pressure and Temperature Change An increase in the temperature of a contained liquid increases the 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.

13.2 Vapor Pressure

Vapor Pressure Measurements 13.2 Vapor Pressure Vapor Pressure Measurements The vapor pressure of a liquid can be determined with a device called a manometer.

Manometer 13.2 Vapor Pressure The vapor pressure of a contained liquid can be measured in a manometer. The vapor pressure is equal to the difference in height of the mercury in the two arms of the U-tube. Calculating What is the difference in vapor pressure between ethanol at 0°C and ethanol at 20°C?

Boiling Point Under what conditions does boiling occur? 13.2

13.2 Boiling Point 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.

13.2 Boiling Point The temperature at which the vapor pressure of the liquid is just equal to the external pressure on the liquid is the boiling point (bp).

Boiling Point and Pressure Changes 13.2 Boiling Point Boiling Point and Pressure Changes Because a liquid boils when its vapor pressure is equal to the external pressure, liquids don’t always boil at the same temperature. At a lower external pressure, the boiling point decreases. At a higher external pressure, the boiling point increases.

Altitude and Boiling Point 13.2 Boiling Point Altitude and Boiling Point A liquid boils when the vapor pressure of particles within the liquid equals the atmospheric pressure. The boiling point varies with altitude.

13.2 Boiling Point 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 a pressure of 101.3 kPa.

13.2 Section Quiz. 13.2.

13.2 Section Quiz 1. 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.

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

13.2 Section Quiz 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.

13.2 Section Quiz 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

13.2 Section Quiz 5. 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.

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