Heat: Phase Changes

Solids Have an orderly arrangement and fixed location of particles Particles “vibrate” in place, so they do not flow When heat (energy) is added, particles “vibrate” faster and begin to spread apart (melt) Most are crystalline (rigid), some are amorphous (lack an internal structure) Examples of amorphous solids: rubber, glass

Liquids Intermolecular attractions reduce movement of particles (more dense than gas) Vaporization: turning liquid to gas Evaporation: surface particles only Is a cooling process (sweat) Boiling: particles throughout liquid Vapor pressure = atmospheric pressure (BP) Higher altitudes=lower bp

Gases Kinetic Theory: Particles are small, hard spheres with an insignificant volume Most of particles is rapid, constant, and random All collisions are perfectly elastic (energy is transferred without loss) Gas Pressure: caused by collisions with an object SI unit: pascal (Pa) Standard Pressure: 1 atm = 760 mmHg = 101.3 kPa

Processes that decrease energy (exothermic): Freezing: changing liquid to solid Ex: Freezing Rain Condensation: changing gas to liquid Ex: Rain/water on the outside of a glass on a warm day Deposition: changing gas directly to solid Ex: Frost on the ground/Snow

Processes that Increase Energy (Endothermic) Melting: turning solid to liquid Ex: Snow melting on roads Vaporization: turning liquid to gas Ex: Steam rising from hot water/puddles evaporating Sublimation: turning solid directly to gas Ex: dry ice/loss of glaciers

Heat Flow Heat = energy transferred from one object to another due to a difference in temperature Will always flow from high to low (warmer objects to cooler objects) The transfer of heat follows the law of conservation of energy (energy is neither created nor destroyed) Flows between a “system” and “surroundings”

Endothermic Processes/Reactions System absorbs heat from the surroundings Chemical reactions will feel cool to the touch because it is absorbing the heat energy out of your hand q will be positive (heat was added to the system)

Exothermic Processes/Reactions System loses heat to the surroundings Chemical reactions will feel warm to the touch because they are releasing heat to your hands q will be negative (heat was taken from the system)