Heat Transfer and Change of Phase Phases of Matter Solid Liquid Gas Plasma Molecules break into ions and electrons Evaporation Liquid to Gas phase change surface effect Cooling Process During bumping in a liquid some water molecules gain kinetic energy while others lose kinetic energy Molecules at the surface that gain enough kinetic energy can break free from the liquid The molecules with more kinetic energy leave the liquid leaving behind molecules with less kinetic energy The average kinetic energy of the liquid decreases and thus the temperature decreases When we sweat and our sweat evaporates our bodies cool down Boiling Liquid to Gas Boiling is a cooling process Pressure cooker and boiling freezing Condensation Gas to Liquid phase change Warming process Boiling Point Depends on the type of material Water s boiling point at atmospheric pressure 100 C Depends on the pressure of the liquid At high altitudes the boiling point of water is lower than 100 C Melting Freezing Solid Liquid You can change the freezing point of water Antifreeze lowers the freezing point of water The freezing point of water is also dependent on pressure Molecules in the solid phase are bond together by molecular forces As the molecules gain energy they begin to vibrate and move more rapidly It is harder to remain bound together and sometimes molecules break free Energy and Phase Change Whenever a substance changes phases a transfer of heat occurs At standard atmospheric pressure substances change phase at a given temperature Ice melts at 0 C water boils at 100 C Other materials melt at given temperatures and boil at other temperatures The temperature of a substance does not change during a phase change even though heat is added or removed Latent Heat Heat required to change phase Q m L Latent Heat of Fusion L f The amount of energy needed to change a substance from solid to a liquid Water L f 334 J g 334 000 J kg It requires 334 J of energy for every gram of water to convert water to ice or vice versa at 0 C Latent Heat of Vaporization L v The amount of energy needed to change a substance from liquid to a gas Water L v 2256 J g 2 256 000 J kg It requires 2256 J of energy for every gram of water to convert water to steam or vice versa at 100 C Heat for Phase Change Q m L Q Heat added or removed J m mass kg L Latent heat J kg Example How much heat must be added to 200 grams of ice in order to completely melt the ice at standard atmospheric pressure Assume the ice starts at 10 C m 200 g T i 10 C T 10 c ice 0 9 cal gC L f 80 cal g 1 Ice must warm to 0 C T f in order to melt 2 Energy must be added to change ice from 10 C to 0 C Q mc T 3 Reached 0 C for phase change Q m L f Q m c T Q 200 0 9 10 Q 1800 cal Q m L f Q 200 80 Q 16 000 cal Q total 17 800 cal How much heat must be added to 1 kg of ice at 0 C to change it to steam at 100 C 1 Ice must melt phase change solid to liquid Q m L f L f water 333 000 J kg m 1 kg Q m L f Q 1 333 000 Q 333 000 J 2 Water must change temperature 0 100 C Q m c T c water 4186 J kgC m 1 kg T 100 C Q m c T Q 1 4186 100 Q 418 600 J 3 Water must boil phase change liquid to gas Q m L v L v 2 260 000 J kg m 1 kg Q m L v Q 1 2 260 000 Q 2 260 000 J Q total 333 000 418 600 2 260 000 Q total 3 011 600 J