Phase Changes (Section 3.3)

Characteristics of Phase Changes Phase change is 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

Most substances can exist in 3 states, depending on temperature and pressure If energy is added or removed, the states can change

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)

Phase Changes that REQUIRE Energy (Endothermic)

Melting Melting Point: the temperature where the forces holding together the crystal lattice of a solid are broken, turning it into a liquid 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 Process of a liquid changing into a gas or vapor 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

Evaporation: when vaporization occurs only at the surface of a liquid Boiling Point: the temperature where the pressure of the liquid equals the external or atmospheric pressure 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 Process where a solid changes directly into a gas, without becoming a liquid 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 the fog

Phase Changes that RELEASE Energy (Exothermic)

Freezing When enough energy is removed, the bonds between molecules become frozen into set positions Freezing Point: the temperature at which a liquid turns into a crystalline solid 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 Process where a gas or vapor becomes a liquid Energy is released when new bonds are formed 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 Process of a gas or vapor changing directly into a solid Energy is released as crystals are formed 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