Phase Changes

Four Phases of Matter Solid Liquid Gas Plasma We’ll only talk about the first three today!

Solid Molecules Wiggle Definite Volume Definite Shape Low Kinetic Energy (very little motion)

Liquid Molecules Slide by each other Definite Volume Indefinite Shape More Kinetic Energy (More motion)

Gas Random Motion Indefinite Volume Indefinite Shape Lots of Kinetic Energy (Lots of Motion.)

Review of Heat Heat is the transfer of thermal energy of one thing to another because of a temperature difference. The direction of energy transfer is always from a warmer thing to a neighboring cooler thing. This is the 2 nd law of thermodynamics

Review of Heat Transfer There are three methods of heat transfer: conduction, convection, and radiation. When a substance absorbs or releases thermal energy, two things can happen: – Increase or decrease in kinetic molecular energy (temperature change) – Phase change (change in the structure of the molecule)

Heat Being Added TemperatureTemperature a b c d e Temperature of a Substance as Heat is Added at a Constant Rate

a Heat Being Added TemperatureTemperature b c d e a.A solid will experience a temperature increase b.Then, at its melting point, the solid will melt. c.After melting is complete, a liquid will experience an increase in temperature. d.Then, at its boiling point, the liquid will boil. e.After boiling (or vaporizing) is complete, the gas will continue to experience an increase in temperature

Temperature of a Substance as Heat is Released at a Constant Rate Heat Being Released TemperatureTemperature a b c d e

Temperature of a Substance as Heat is Released at a Constant Rate a.A gas will experience a decrease in temperature. b.At its points of condensation (same temperature as boiling), the gas will condense and become a liquid. c.Once condensation is complete, the liquid will experience a decrease in temperature. d.At its freezing point (same temperature as melting), the liquid will freeze. e.Once freezing is complete, the solid will experience a decrease in temperature. Heat Being Released TemperatureTemperature a b c d e

Quick Summary

Four BIG ideas When a substance absorbs heat energy, it can experience a phase change OR a temperature change. And when a substance releases heat energy, it can experience a phase change OR a temperature change. Phase changes require heat transfers. The substance experiencing the phase change always remains at constant temperature. During a temperature change, only one phase of matter is present. During a phase change, two phases of matter must be present.

Melting A change of phase from solid to liquid. – Example: holding an ice cube in your hand.

Vaporization A change of phase from liquid to gas. Two types: – Boiling – Evaporation

Boiling A change of phase from liquid to gas that takes place beneath the surface of a liquid. The gas that forms below the surface forms bubbles and the bubbles are buoyed upwards to the surface where they escape. It is important to note that it is the high temperature of the water, not the boiling process, that cooks food. Boiling is a cooling process since the remaining water is cooled. This is why we must continually add heat from the stove top. – Example: Boiling water on the stove

Evaporation Change of phase from liquid to gas that takes place at the surface of a liquid. Molecules at the surface of a liquid may be bumped enough to give them enough kinetic energy to break free from the liquid. They can leave the surface and fly into the space above the liquid. They now comprise a vapor, molecules in the gas phase. This is a cooling process. The liquid that is left behind is cooler since the molecules that broke free had increased kinetic energy. – Example: Blowing on your soup. – Example: Stepping out of the shower.

Condensation The changing of a gas to a liquid. Vapor molecules collide with the slower moving molecules of a cooler surface. The vapor molecules give up so much kinetic energy that they can't stay in the gaseous phase. They condense. The surface gains kinetic energy so condensation is a warming process. – Example: Muggy Boston Days – Example: A Steam Burn

Freezing A change of phase from liquid to solid. When energy is continually withdrawn from a liquid, molecular motion slows until the forces of attraction between the molecules cause them to get closer to one another and to form a solid. – Example: Freezing ice cubes in the freezer

Freezing…cont. Impurities in water, such as salt, lower the freezing temperature since the foreign molecules get in the way of water's crystalline structure.

Sublimation A change of phase from solid to gas. – Example: Dry Ice

Deposition A change of phase from gas to solid. – Example: Snow