Chapter 23

Change of Phase

Phases of Matter:

• Plasma

• Gas

• Liquid

• Solid

Energy

The Atmosphere

Pressure and Temperature

According to Gay-Lussac’s Law, as the pressure of a gas decreases, there is a corresponding decrease in the temperature.

This is why the temperature of the air drops with increasing altitude.

Evaporation

• Evaporation is a change of phase from liquid to gas that takes place at the surface of a liquid.

• Temperature is related to kinetic energy and molecules in a liquid are constantly bumping into each other; some gain energy while others lose energy.

• Occasionally, some molecules gain enough energy to escape the surface of the liquid.

• They are now considered a vapor, molecules in the gaseous phase

• Evaporation is a cooling process.

• When water molecules escape the surface of liquid water, they leave behind molecules with less kinetic energy. This results in a net cooling of the liquid.

The Water Cycle

Condensation

• The opposite process to evaporation is condensation

• Condensation is the changing of a gas to a liquid

• Condensation is a warming process

• Condensation of water vapor forms clouds

Types of Clouds

Pyrocumulus Cloud formed by forest fire:

Relative Humidity

• Relative Humidity is the amount of moisture in the air compared to what the air can "hold“

• Warm air can hold more water vapor than cool air

Temperature

Thunderstorms

Thunderstorms

Thunderstorms can spawn severe weather

• High Winds• Lightning• Hail• Tornados

Tornado Preparedness

• Move to an interior portion of your home or school, away from windows during a tornado

• Do not open windows to equalize pressure! This is a myth!

• Have evacuation routes planned out ahead of time

• Do not stay in your car. Your car can become a flying projectile with you in it!

Boiling

• Evaporation takes place at the surface of a liquid• A change in phase from liquid to gas can also take place beneath the surface of a liquid• The gas that forms beneath the surface causes bubbles to form• These bubbles are buoyed upward to the surface where they escape, this change in phase is called boiling

Freezing

• When energy is withdrawn from a liquid, molecular motion slows down until the forces of attraction between the molecules causes them to get closer to one another

• The molecules then vibrate about a fixed position which causes a solid to form. This change in phase from liquid to solid is called freezing

• If sugar or salt is dissolved in the water, the freezing temperature will be lowered.

• These “foreign” molecules or ions get in the way of water molecules that ordinarily would join them together in the ice crystal structure