Phase Changes

Changing State:

Changing State The temperature of a substance helps determine what state (solid, liquid or gas) a substance will be Freezing – Process by which molecules lose heat energy and form an ordered, rigid structure Melting – Process by which molecules gain heat energy and take on a less ordered, less rigid structure For any pure substance: Freezing Point = Melting Point

Evaporation:

Evaporation In a closed container, only some of a liquid evaporates (molecules leave liquid phase and enter the gas phase) Area of the surface (and temp and humidity) affect the rate of evaporation Liquids that evaporate quickly are called volatile liquids As liquids evaporate, they cool At the surface Sufficiently high KE

Condensation:

Condensation Reverse of evaporation Vapor (gaseous) molecules lose KE, come close together Intermolecular forces take over and form a liquid Example – Dew on grass

Sublimation & Deposition:

Sublimation & Deposition Sublimation – process by which particles of a solid escape from the surface due to a high vapor pressure Example – Dry ice, mothballs Deposition – process by which particles of a gas come together to form a solid Example – Frost forming on grass

Vapor Pressure:

Vapor Pressure Liquid and vapor come to a dynamic equilibrium Vapor exerts a pressure on the liquid Vapor pressure – pressure of a substance in equilibrium with its liquid Vapor pressure indicates how easily something evaporates (High VP = Volatile) “Gas” vs. “Vapor”? The same; “vapor” used for gaseous state of substances that are liquids/solids at room temp

Boiling:

Boiling Point at which the vapor pressure equals the pressure exerted on the surface of the liquid Normal Boiling Point – temperature where P vapor = 101.325kPa Boiling Point is a function of pressure Can occur under the surface, not just on the surface (appear as bubbles)

Vapor Pressure & Boiling:

Vapor Pressure & Boiling * The pressure unit “torr” is the same thing as “mmHg”

Liquefaction of Gases:

Liquefaction of Gases “Liquefaction” is the condensation of substances that are normally gases Gases must be cooled below a certain temperature before liquefaction is possible; increased pressure is also necessary sometimes Every gas has a critical temperature (T c ) above which no amount of pressure will result in liquefaction