Phase Changes Physical change of matter from one phase to another due to a transfer of energy.

Evaporation Liquid – gas at surface of a liquid. Molecules gain KE to become a vapor. Molecules left behind lose KE. Cooling process – molecules left behind lose KE (cooler).

Condensation Gas – liquid. Gas molecules ↓KE when collide with cold surface. Condense to liquid phase. Warming process – KE lost by condensing gas molecules warms the surface they strike.

Evaporation-Condensation Rates Depends upon environment, if moist: Condensation > evaporation (warming) Evaporation > condensation (cooling) Equilibrium – state of balance. Evaporation and condensation occurring at equal rates.

Boiling Liquid – gas beneath the surface of a liquid. Bubbles of vapor form beneath the surface – rise – break free to the vapor phase. ↑atm. pressure - ↑boiling pt. Cooling process – the water is being cooled relative to the ↑temp it would attain otherwise. Because of cooling, it remains 100 °C instead of getting hotter.

Freezing Liquid - solid. Energy is extracted, molecules slow down and the molecular attraction overcomes the KE. Warming process – when you make ice cubes, you put liquid water in the freezer. The freezer cools the water, taking energy out – so it must give off energy.

Melting Solid – liquid. Heat energy is added until KE is greater than the molecular attraction. Cooling process – the source of energy is the object or material around the stuff that is melting. The source cools as energy goes to the melting object.

Sublimation Solid - gas. Example: mothballs that “evaporate” without leaving a liquid, and when snow on the ground “evaporates” skipping the liquid phase altogether.

Phase Change Graph

Phase Change Diagram Temperature °C Heat Energy Heat of vaporization C.P Condenses Heat of fusion Temperature °C 100 Boiling F.P B.P Freezing O Melting M.P Heat Energy

Phase Changes Phase Changes Melting Freezing Vaporization Condensation From what Phase-to-Phase does change occur Absorb or Release Heat Energy? Cooling or Warming Process Melting Freezing Vaporization Condensation Sublimation solid-liquid Absorb Cooling liquid-solid Release Warming liquid-gas Absorb Cooling gas-liquid Release Warming solid-gas Absorb Cooling

Heat of Fusion Amount of heat needed to change 1g substance from solid – liquid phase. 80 calories/g for water. Example: How many calories are needed to change 10g of ice at 0 °C to 10g of water at 0 °C? 800

Heat of Vaporization Amount of heat needed to change 1g substance from liquid – gas phase. 540 calories/g for water. Example: How many calories are needed to change 10g of water at 100 °C to entirely to water vapor? 5400

Sample Problem How much heat is required to change 1g of ice at -20°C to water vapor at 130°C? Q = mc∆t 1g · 0.5 cal/g-C · 20 °C = 10 cal. Hf 1g · 80 cal/g = 80 cal. Q = mc∆t 1g · 1 cal/g-C · 100 °C = 100 cal. Hv 1g · 540 cal/g = 540 cal. Q = mc∆t 1g. · 0.5 cal/g · 30 °C = 15 cal. 745 calories