Phy 102: Fundamentals of Physics II Chapter 17: States of Matter Lecture Notes

Changing States of Matter LiquidSolidGas MeltingEvaporation FreezingCondensation Energy is absorbed when change of phase is in this direction Energy is released when change of phase is in this direction

Liquid to Solid Transformations Freezing: A phase change from liquid to solid –Heat energy must be released Melting: A phase change from solid to liquid –Heat energy must be absorbed Regelation: A phase change from solid to liquid (under high pressure) that reverts back to solid (when pressure is removed) Example: the blade of an ice skate causes the ice beneath to melt under its pressure, the ice then re- freezes once the blade has passed over it

Evaporation: A phase change from liquid to gas Occurs only at the surface of the liquid Energy must be absorbed Condensation: A phase change from gas to liquid Energy is released Liquid to Gas Transformations

Boiling Evaporation that takes throughout a liquid instead of only at the surface Energy must be absorbed by a liquid to boil Actually a cooling process –it keeps liquid from exceeding its critical temperature, its boiling point All liquids have a critical temperature at which they boil (called the boiling point) Boiling point depends on pressure –Higher pressure higher boiling point –Lower pressure lower boiling point

Sublimation A phase change from solid to gas Energy must be absorbed Examples: –“freezer burn” occurs when ice crystals present in frozen food sublimate –Freeze drying is a more useful example of sublimation –Dry ice (solid CO 2 ) sublimates when it is placed in water producing “artificial” smoke SolidGas Sublimation

Latent Heat Heat is always absorbed or released when a substance undergoes a change of phase The quantity of heat energy gained/released by a substance (per mass) during a phase transformation is called the latent heat (L) There are 3 latent heats: –Latent heat of fusion (solid to liquid, vice versa), L f The latent heat of fusion for water is L f = 3.35x10 5 J/kg –Latent heat of vaporization (liquid to gas, vice versa), L v The latent heat of vaporization for water is L v = 2.26x10 6 J/kg The heat absorbed/released as an object changes phase is described by Q = m. L m is the mass of substance transformed (in kg) Example: How much energy must be absorbed to melt 0.2 kg of ice? Q = m. L = (0.2 kg)(33.5x10 4 J) = 67.0x10 3 K or 67.0 kJ