Chp 3.3 - Phase Changes Pg. 84-91

Characteristics of Phase Changes When at least two states of a substance are present, each state is described as a phase Phase change- a reversible physical change that occurs when a substance changes from one state of matter to another Melting, freezing, vaporization, condensation, sublimation, and deposition are six common phase changes.

Temperature and Phase Changes The temperature of a substance does not change during a phase change Temperature remains the same until the phase change is complete The temperature at which a substance freezes (freezing point) is identical to the temperature at which it melts (melting point)

Energy and Phase Changes During a phase change energy is transferred between a substance and its surroundings Energy is either absorbed or released during a phase change Melting is an example of an endothermic change Endothermic change - the system absorbs energy

Energy and Phase Changes The amount of energy absorbed depends on the substance One gram of water absorbs 334 Joules of energy as it melts = its heat of fusion The heat of fusion Varies from substance to substance Heat of “melting” Fusion is another term for melting

Phase Changes that REQUIRE Energy (Endothermic) Melting Vaporization Sublimation

Melting The arrangement of molecules in water becomes less orderly as water melts and more orderly as water freezes Melting Attractions between molecules keep them in fixed positions Heat flows from air to ice Ice gains energy and molecules vibrate Some gain enough energy to overcome the attractions and move away When all can move melting is complete

Vaporization Vaporization - the phase change in which a substance changes from a liquid to a gas Vaporization is an endothermic process A substance must absorb energy in order to change from a liquid to a gas One gram of water gains 2261 joules of energy when it vaporizes This is its heat of vaporization Heat of vaporization varies from substance to substance

Vaporization Evaporation takes place at the surface of a liquid and occurs at temperatures below the boiling point Evaporation - the process that changes a substance from a liquid to a gas at temperatures below the substance’s boiling point Water in a closed container Water evaporates and vapor collects above the liquid Vapor Pressure - caused by collisions of the vapor and the walls of the container The vapor pressure increases as the temperature increases

Boiling Heat a pot of water Temperature and vapor pressure increase When vapor pressure and atmospheric pressure become equal the water boils As temperature increases water molecules move faster and faster When it reaches 100 degrees Celsius some molecules below the surface overcome the attraction forces and form bubbles of water vapor When the bubbles reach the surface they pop and release water vapor into the air Takes place throughout a liquid

Sublimation Sublimation - phase change in which a substance changes form a solid to a gas or vapor Endothermic Example is dry ice Goes from solid carbon dioxide to a gas The gas causes water vapor to condense and that’s how it forms clouds or fog

Phase Changes that RELEASE Energy (Exothermic) Freezing Condensation Deposition

Freezing Freezing Energy flows from the water to the air in a freezer and the water cools down As kinetic energy of molecules decrease, the move more slowly Attractions between molecule take effect When all molecules have been drawn into orderly arrangement freezing is complete

Condensation Condensation - the phase change in which a substance changes from a gas or vapor to a liquid An exothermic process Responsible for the morning dew on grass and the “cloud” on the mirror after you take a shower.

Deposition Deposition - phase change in which a gas or vapor changes directly into a solid without first becoming a liquid Exothermic Opposite of sublimation Causes frost on cold window glass

The End