3.3 Phase Changes

What are we learning?Define phase changeExplain how temperature can be used to

recognize a phase changeExplain what happens to the motion,

arrangement and average kinetic energy of water molecules during phase changes

Describe each of the 6 phase changesIdentify phase changes as endothermic or

exothermic

Characteristics of a Phase ChangeIf 2 states of the same substance are present

at the same time, we describe each different state as a phase.Ex: an iceberg floating in the ocean- solid

phase and liquid phaseA phase change is the reversible physical

change that occurs when a substance changes from one state of matter to another

Melting, freezing, vaporization, condensation, sublimation and deposition are the 6 common phase changes

Temperature and Phase ChangesOne way to recognize phase changes-

measuring the temperature as it is heated or cooled

The temperature of a substance does not change during a phase change.

Ex NaphthaleneNaphthalene is used in

mothballs. This graph is of data

collected when it is heated.

The temperature rises as it warms up, until it hits about 80C. (the melting point)

It will remain at 80C until it has all melted .

Energy and Phase ChangesDuring a phase change, energy is transferred

between a substance and its surroundings.Energy is either absorbed or released

during a phase change.Ex: Melting- endothermic changeDuring an Endothermic change, the system

absorbs energy from its surroundings.The amount of energy absorbed varies

Ex: one gram of ice absorbs 334 joules of energy as it melts. (this is the heat of fusion of water) Fusion is another term for melting

Energy and Phase ChangesWhen water freezes, it releases the same

amount of energy (334J) to its surroundings. Freezing is an exothermic changeIn exothermic change the system releases

energy to its surroundings.

Melting and FreezingThe arrangement of molecules in water becomes

less orderly as water melts and more orderly as water freezes. (remember the BB’s)

Attractions between water molecules in ice keep the molecules in fixed positionsAs ice warms up the molecules vibrate more quicklyWhen ice gets near the melting point (OC), some

molecules gain enough energy to over come the attraction with other molecules and move from their fixed positions.

(melting is complete when all the molecules can move).

FreezingWhen water is placed in a freezer, energy

flows from the water to the air in the freezer, and the water cools down.

The kinetic energy decreases and the molecules move more slowly

As the molecules slow down, forces of attraction have a greater effect

When all the molecules have been drawn into an orderly arrangement, freezing is complete

Vaporization and CondensationVaporization is the phase change where a

substance changes from a liquid to a gasit’s an endothermic process which means,

that a substance must absorb energy in order to change from a liquid to a gas

One gram of water gains 2261 joules of energy- this is the heat of vaporization of water

There are 2 vaporization process, boiling and evaporation

EvaporationEvaporation occurs at the surface of a liquid

and at temperatures lower than the boiling point.

Evaporation is the process that changes a substance from a liquid to a gas at temp. below the boiling point.

When water evaporates, some molecules near the surface are moving fast enough to escape and become water vapor.The greater the surface area, the faster it

evaporatesVapor pressure is the pressure caused by the

collisions of water vapor and the walls of a closed container

Boiling As you heat up a pot of water, the

temperature and vapor pressure of water increases

The water boils when the vapor pressure becomes equal to the atmospheric pressure (the temp. at which this happens is the boiling point)

As the temp. increases during boiling, the molecules move faster

When the temp gets near 100C, some molecules below the surface have enough kinetic energy to overcome the attraction with other molecules

Bubbles rise to the surface, burst and release water vapor into the air

The boiling point depends on atmospheric pressure (higher altitude = lower BP)

CondensationCondensation is the phase change where a

substance changes from a gas/vapor to a liquid. (morning dew!)

Is an exothermic process (energy releasing)

Sublimation and depositionSublimation the phase change where a

substance goes from a solid to a gas/vapor without changing to a liquid first.

When a gas /vapor changes directly to a solid without going through the liquid phase it’s called deposition. (exothermic)Causes frost to form on windows