Aim: I.O.SWBAT: 1)Define vapor pressure 2)Interpret phase change diagrams 3)Describe critical temperature and pressure 4)Draw a phase change diagram MOTIVATION: What happens ice at -25oC if your add heat to it?

Do-Now: How does the behavior of a liquid in a closed container differ from that in an open one? Vapor: Gas form of a substance that is a solid or a liquid at room temperature.

LIQUID GAS Dynamic equilibrium: Vaporization and condensation occur at the same rate.

As more molecules escape the liquid, the pressure they exert increases. Vapor pressure: partial pressure of the vapor over the liquid measured at equilibrium

At any temperature, some molecules in a liquid have enough energy to escape. As the temperature rises, the fraction of molecules that have enough energy to escape increases.

Boiling point: is the temperature at which the vapor pressure of a liquid equals the surroundings pressure.

Vapor Pressure Curves The normal boiling point is the temperature at which its vapor pressure is 760 torr.

Volatile liquids: Liquids with a high vapor pressure and weak intermolecular forces. (have low boiling points ) Volatility: The tendency of a substance to evaporate easily.

PRACTICE: 1)Why does the vapor pressure of a liquid depend on intermolecular forces? 2)Why does it take longer to hard boil an egg at high elevations? 3)How does a pressure cooker cook food faster?

Do-Now: Complete hand-out.

Phase diagram: Is a graphic representation of the relationships between the physical state of a substance and its pressure and temperature.

The AB line is the liquid-vapor interface. Each point along this line is the boiling point of the substance at that pressure.

Triple point (A): solid, liquid and gas are in nequilibrium. Critical point (B): Beyond this temperature gas cannot be liquefied by increasing pressure.

The AD line is the interface between liquid and solid. The melting point at each pressure can be found along this line.

Below A the substance cannot exist in the liquid state. Along the AC line the solid and gas phases are in equilibrium. (the sublimation point at each pressure is along this line)

Phase Diagram of Water The slope of the solid– liquid line is negative. –As the pressure is increased at a temperature just below the melting point, water goes from a solid to a liquid.

The high critical point of water is due to the strong van der Waals forces between water molecules.

Phase Diagram of Carbon Dioxide Carbon dioxide cannot exist in the liquid state at pressures below 5.11 atm. CO 2 sublimes at normal pressures. Isotherm: constant temperature Isobar: constant pressure

PRACTICE: Complete Hand-Out HW#44