PHASE CHANGESPHASE CHANGES

SECTION 3.3SECTION 3.3

CHARACTERISTICS OF PHASE CHARACTERISTICS OF PHASE CHANGESCHANGES

A.A. A phase change is the reversible A phase change is the reversible physical change that occurs when a physical change that occurs when a substance changes from one state substance changes from one state of matter to anotherof matter to another

B.B. Melting, freezing, vaporization, Melting, freezing, vaporization, condensation, sublimation, and condensation, sublimation, and deposition are six common phase deposition are six common phase changeschanges

C.C. Temperature and Phase ChangesTemperature and Phase Changes1.1. The temperature of a substance does not The temperature of a substance does not

change during a phase changechange during a phase change

2.2. When a solid changes to a liquidWhen a solid changes to a liquida.a. At first the temperature rises as a solid warms At first the temperature rises as a solid warms

upup

b.b. At the melting point, the temperature stops At the melting point, the temperature stops rising and remains the same until the melting rising and remains the same until the melting is completeis complete

c.c. The temperature will begin to rise againThe temperature will begin to rise again

3. When a liquid changes to a solid3. When a liquid changes to a solid

a. At first the temperature of the liquid a. At first the temperature of the liquid

will drop until it reaches the freezing will drop until it reaches the freezing pointpoint

b. At the freezing point, the temperature b. At the freezing point, the temperature will will remain at that temperature until all of remain at that temperature until all of the the liquid freezesliquid freezes

c. After the freezing is complete, the c. After the freezing is complete, the temperature will begin to drop againtemperature will begin to drop again

4. The temperature at which a substance 4. The temperature at which a substance freezes (its freezing point) is identical to freezes (its freezing point) is identical to the temperature at which it melts (its the temperature at which it melts (its melting point)melting point)

5. When a liquid changes to a gas5. When a liquid changes to a gas

a. The temperature keeps rising until a. The temperature keeps rising until it it reaches the boiling pointreaches the boiling point

b. The temperature then remains the b. The temperature then remains the same same until the boiling is completeuntil the boiling is complete

D.D. Energy and Phase ChangeEnergy and Phase Change1.1. Energy is either absorbed or released Energy is either absorbed or released

during a phase changeduring a phase change

2.2. During an endothermic change, the During an endothermic change, the system absorbs energy from its system absorbs energy from its surroundingssurroundingsa.a. Melting, vaporization, and sublimation are Melting, vaporization, and sublimation are

all examples of endothermic changesall examples of endothermic changes

b.b. The heat of fusion is the amount of energy The heat of fusion is the amount of energy a substance must absorb in order to a substance must absorb in order to change from a solid to a liquid. Fusion is change from a solid to a liquid. Fusion is another term for meltinganother term for melting

3. One gram of water releases 334 3. One gram of water releases 334 joules of energy to its surroundings joules of energy to its surroundings as it freezes.as it freezes.

a. This is the same amount of a. This is the same amount of energy energy absorbed when one gram of absorbed when one gram of ice meltsice melts

b. Farmers use this release of b. Farmers use this release of energy energy to protect their cropsto protect their crops

4. During an exothermic change, the 4. During an exothermic change, the system releases energy to its system releases energy to its surroundings. Freezing, deposition, surroundings. Freezing, deposition, and condensation are examples of and condensation are examples of exothermic reactionsexothermic reactions

MELTING AND FREEZINGMELTING AND FREEZING

A.A. The arrangement of molecules in The arrangement of molecules in water becomes water becomes less orderlyless orderly as water as water melts and melts and more orderlymore orderly as water as water freezesfreezes

B.B. MeltingMelting1.1. In ice, the In ice, the attractionsattractions between water between water

molecules keep the molecules in fixed molecules keep the molecules in fixed positionspositions

2.2. When ice is removed form the freezer, When ice is removed form the freezer, heat flows from the heat flows from the airair to the to the iceice

3. As the ice gains 3. As the ice gains energyenergy, the , the molecules molecules vibrate more quickly.vibrate more quickly.

4. At 4. At the melting pointthe melting point of water of water some some molecules gain enough molecules gain enough energy and energy and move from their fixed move from their fixed position.position.

5. When all of the molecules gain 5. When all of the molecules gain energy energy melting is melting is completecomplete..

C.C. FreezingFreezing

1. When liquid water is placed in a 1. When liquid water is placed in a freezer, energy flows from the freezer, energy flows from the waterwater to to the the airair in the freezer in the freezer

2. This process is the reverse of the 2. This process is the reverse of the melting process.melting process.

VAPORIZATION & VAPORIZATION & CONDENSATIONCONDENSATION

A.A. Vaporization is the phase change in Vaporization is the phase change in which a substance changes from a which a substance changes from a liquid into a liquid into a gasgas..

B.B. A substance must A substance must absorbabsorb energy in energy in order to change from a liquid to a order to change from a liquid to a gas.gas.

C.C. The heat of vaporization for water is The heat of vaporization for water is equal to equal to 22612261 joules of energy. joules of energy.

D.D. There are two vaporization There are two vaporization processes – processes – boilingboiling and and evaporationevaporation..

E.E. Evaporation takes place at the Evaporation takes place at the surfacesurface of a liquid. of a liquid.

1.1. Evaporation is the process that changes Evaporation is the process that changes a substance form a liquid to a gas at a substance form a liquid to a gas at temperatures below the temperatures below the boilingboiling point. point.

2.2. A vapor is the A vapor is the gaseous gaseous phase of a phase of a substancesubstance

3. 3. Vapor pressureVapor pressure is the pressure caused is the pressure caused by the collisions of vapor and the walls by the collisions of vapor and the walls of the container.of the container.

F.F. BoilingBoiling

1. When vapor pressure becomes 1. When vapor pressure becomes equal to equal to atmospheric atmospheric pressure, water pressure, water boils.boils.

2. The temperature at which this 2. The temperature at which this happens is the happens is the boiling pointboiling point of water. of water.

3. As temperature 3. As temperature increasesincreases, water , water molecules move faster and faster.molecules move faster and faster.

4. When the temperature reaches 4. When the temperature reaches 100100°C, some molecules below the °C, some molecules below the surface have enough surface have enough kinetic energykinetic energy to overcome the attraction of to overcome the attraction of neighboring molecules.neighboring molecules.

5. Because water vapor is less 5. Because water vapor is less densedense than water, the bubbles rise to the than water, the bubbles rise to the surface.surface.

6. The boiling point of a substance 6. The boiling point of a substance depends on the depends on the atmosphericatmospheric

pressure.pressure.a. At higher elevations the a. At higher elevations the

atmospheric pressure is atmospheric pressure is lowerlowerb. This will cause the boiling point b. This will cause the boiling point

to to occur at a occur at a lowerlower temperature. temperature.c. Cooking will take longer at c. Cooking will take longer at

higher higher elevationselevations

G. G. CondensationCondensation is the phase change is the phase change in which a substance changes from in which a substance changes from a gas or vapor to a liquid. This a gas or vapor to a liquid. This process is responsible for the process is responsible for the dewdew on grass and the condensation on on grass and the condensation on bathroom mirrors.bathroom mirrors.

SUBLIMATION & SUBLIMATION & DEPOSITIONDEPOSITIONA.A. Sublimation Sublimation is the phase change in is the phase change in

which a substance changes from a which a substance changes from a solid to a gas or vapor without solid to a gas or vapor without changing to a changing to a liquidliquid first. first.

B.B. Example, at room temperature, dry Example, at room temperature, dry ice can directly change from a ice can directly change from a solidsolid to a colorless to a colorless gasgas..

C.C. As dry ice As dry ice sublimessublimes, the cold carbon , the cold carbon dioxide vapor causes water vapor in dioxide vapor causes water vapor in the air to condense and form clouds.the air to condense and form clouds.

D. D. DepositionDeposition is the phase change that is the phase change that occurs when a gas or vapor changes occurs when a gas or vapor changes directly into a solid without first directly into a solid without first changing to a liquid.changing to a liquid.

E. Deposition causes E. Deposition causes frostfrost to form on to form on windowswindows