Section 12.4 Phase Changes (cont.) melting point vaporization evaporation vapor pressure boiling point freezing point condensation deposition phase diagram triple point Matter changes phase when energy is added or removed. Section 12-4

Phase Changes That Require Energy: Melting, Vaporization, Sublimation Heat is the transfer of energy from an object at a higher temperature to an object at a lower temperature. Melting occurs when heat flows into a solid object. Section 12-4

Phase Changes That Require Energy (cont.) When ice is heated, the temperature of ice will increase up until the ice reaches its melting point. At its melting point, the energy absorbed by the ice goes into breaking the hydrogen bonds that hold the water molecules together. Section 12-4

When the bonds break, the particles move apart and ice melts into water. This process will continue until all the ice is melted. The melting point of a crystalline solid is the temperature at which the forces holding the crystal lattice together are broken and it becomes a liquid.

Phase Changes That Require Energy (cont.) Particles with enough energy escape from the liquid and enter the gas phase. Section 12-4

Phase Changes That Require Energy (cont.) Vaporization is the process by which a liquid changes to a gas or vapor. Evaporation is vaporization only at the surface of a liquid. Boiling: vaporization that occurs throughout the liquid. Section 12-4

In an open container of water, all the molecules of water will eventually evaporate. In a closed container, water vapor will collect over the liquid.

Phase Changes That Require Energy (cont.) In a closed container, the pressure exerted by a vapor over a liquid is called vapor pressure. Section 12-4

Phase Changes That Require Energy (cont.) The boiling point is the temperature at which the vapor pressure of a liquid equals the atmospheric pressure. At this point, molecules throughout the liquid have the energy to vaporize. Section 12-4

Phase Changes That Require Energy (cont.) Sublimation is the process by which a solid changes into a gas without becoming a liquid. What do you know that goes through sublimation? Section 12-4

Phase Changes That Release Energy: Freezing, Condensation, Deposition As heat flows from liquid water to the surroundings, the particles lose energy. As they lose energy, the molecules go back into their fixed positions. Freezing is the reverse of melting. The freezing point is the temperature at which a liquid is converted into a crystalline solid. Section 12-4

Phase Changes That Release Energy (cont.) As water vapor loses energy, the velocity of the particles decreases. Water molecules bond, releasing energy as they do so. The process by which a gas or vapor becomes a liquid is called condensation Condensation is the reverse of evaporation. Section 12-4

Deposition is the process by which a gas or vapor changes directly to a solid, and is the reverse of sublimation. Frost is an example of deposition. Snowflakes are also formed in the upper atmosphere by deposition of ice crystals.

Phase Diagrams A phase diagram is a graph of pressure versus temperature that shows in which phase a substance will exist under different conditions of temperature and pressure. Section 12-4

Phase Diagrams (cont.) The triple point is the point on a phase diagram that represents the temperature and pressure at which all three phases of a substance can coexist. Section 12-4

The phase diagram for different substances are different from water. Phase Diagrams (cont.) The phase diagram for different substances are different from water. http://www.youtube.com/watch?v=gbUTffUsXOM&feature=related Section 12-4

Kinetic Theory Heating Curve of a Liquid This type of graph is called a heating curve because it shows the temperature change of water as thermal energy, or heat, is added. Notice the two areas on the graph where the temperature does not change. At 0°C, ice is melting. At 100°C, water vaporizes.

Region (a) – part of curve where the solid is warming/cooling Region (b) – part of the curve where melting/freezing is occurring (temperature dose not change during a phase change) Region (c) – part of the curve where the liquid is warming/cooling Region (d) – part of the curve where vaporization/condensation is occurring (temperature dose not change during a phase change) Region (e) – part of the curve where the gas is warming/cooling

A B C D Section 12.4 Assessment The addition of energy to water molecules will cause them to ____. A. freeze B. change to water vapor C. form a crystal lattice D. move closer together A B C D Section 12-4

A B C D Section 12.4 Assessment The transfer of energy from one object to another at a lower temperature is ____. A. heat B. degrees C. conductivity D. electricity A B C D Section 12-4

Section 15.1 Energy Define energy. Distinguish between potential and kinetic energy. Relate chemical potential energy to the heat lost or gained in chemical reactions. Calculate the amount of heat absorbed or released by a substance as its temperature changes. temperature: a measure of the average kinetic energy of the particles in a sample of matter Section 15-1