Heating and Cooling Curves Phase Diagrams Changes of State Heating and Cooling Curves Phase Diagrams

Objectives Discuss the following: evaporation condensation molar heat of vaporization molar heat of fusion boiling point melting point supercooling sublimation deposition molar heat of sublimation phase changes critical temperature and pressure Sketch a typical heating curve and identify various aspects of it. Use phase diagrams to identify what phase(s) is/are present given specific conditions.

Phases A phase is a homogeneous part of a system in contact with other parts of the system but with a distinct boundary Vapor is the gaseous state of a substance that is normally a solid or liquid at that temperature or pressure In a closed system a dynamic equilibrium is reached between the vapor and the condensed phase As many molecules enter the gas phase as return to the condensed phase

Changes of State Solid Liquid Gas Deposition Evaporation Melting Boiling Solid Liquid Gas Freezing Condensation Sublimation

Heating Curve Boiling Melting

Nomenclature The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium These are pressure dependent! The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure. The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm.

Heating increases potential energy and for vaporization does PV work Heating increases kinetic energy, potential energy and vibrational and rotational energy Heating increases potential energy and for vaporization does PV work

Phase Diagram of Water A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas.

Phase Diagrams-Water Water is unique! Density of solid is less than density of liquid so ice floats As pressure increases, melting point decreases Triple point: Only combination of temperature and pressure where all three phases exist at one time At the critical point separate liquid and gas phases cease to exist

Triple Point Dry ice is dry because it sublimes at 1 atm pressure