PHASE CHANGES

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PHASE CHANGES

• SECTION 3.3

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CHARACTERISTICS OF PHASE CHANGES

1. A phase change is the reversible physical change
   that occurs when a substance changes from one
   state of matter to another
2. Melting, freezing, vaporization, condensation,
   sublimation, and deposition are six common phase
   changes

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• Temperature and Phase Changes
• The temperature of a substance does not change
  during a phase change
• When a solid changes to a liquid
• At first the temperature rises as a solid warms
  up
• At the melting point, the temperature stops
  rising and remains the same until the melting is
  complete
• The temperature will begin to rise again

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• 3. When a liquid changes to a solid
• a. At first the temperature of the liquid
• will drop until it reaches the freezing point
• b. At the freezing point, the temperature will
  remain at that temperature until all of the
  liquid freezes
• c. After the freezing is complete, the
  temperature will begin to drop again

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• 4. The temperature at which a substance freezes
  (its freezing point) is identical to the
  temperature at which it melts (its melting point)
• 5. When a liquid changes to a gas
• a. The temperature keeps rising until it
  reaches the boiling point
• b. The temperature then remains the same
  until the boiling is complete

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• Energy and Phase Change
• Energy is either absorbed or released during a
  phase change
• During an endothermic change, the system absorbs
  energy from its surroundings
• Melting, vaporization, and sublimation are all
  examples of endothermic changes
• The heat of fusion is the amount of energy a
  substance must absorb in order to change from a
  solid to a liquid. Fusion is another term for
  melting

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• 3. One gram of water releases 334 joules of
  energy to its surroundings as it freezes.
• a. This is the same amount of energy absorbed
  when one gram of ice melts
• b. Farmers use this release of energy to
  protect their crops

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

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MELTING AND FREEZING

• The arrangement of molecules in water becomes
  less orderly as water melts and more orderly as
  water freezes
• Melting
• In ice, the attractions between water molecules
  keep the molecules in fixed positions
• When ice is removed form the freezer, heat flows
  from the air to the ice

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• 3. As the ice gains energy, the molecules
  vibrate more quickly.
• 4. At the melting point of water some
  molecules gain enough energy and move from
  their fixed position.
• 5. When all of the molecules gain energy
  melting is complete.

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• Freezing
• 1. When liquid water is placed in a freezer,
  energy flows from the water to the air in the
  freezer
• 2. This process is the reverse of the melting
  process.

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VAPORIZATION CONDENSATION

1. Vaporization is the phase change in which a
   substance changes from a liquid into a gas.
2. A substance must absorb energy in order to change
   from a liquid to a gas.
3. The heat of vaporization for water is equal to
   2261 joules of energy.

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• There are two vaporization processes boiling
  and evaporation.
• Evaporation takes place at the surface of a
  liquid.
• Evaporation is the process that changes a
  substance form a liquid to a gas at temperatures
  below the boiling point.
• A vapor is the gaseous phase of a substance
• 3. Vapor pressure is the pressure caused by the
  collisions of vapor and the walls of the
  container.

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• Boiling
• 1. When vapor pressure becomes equal to
  atmospheric pressure, water boils.
• 2. The temperature at which this happens is the
  boiling point of water.
• 3. As temperature increases, water molecules
  move faster and faster.

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• 4. When the temperature reaches 100C, some
  molecules below the surface have enough kinetic
  energy to overcome the attraction of neighboring
  molecules.
• 5. Because water vapor is less dense than
  water, the bubbles rise to the surface.

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• 6. The boiling point of a substance depends on
  the atmospheric pressure.
• a. At higher elevations the atmospheric
  pressure is lower
• b. This will cause the boiling point to occur
  at a lower temperature.
• c. Cooking will take longer at higher
  elevations

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

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SUBLIMATION DEPOSITION

1. Sublimation is the phase change in which a
   substance changes from a solid to a gas or vapor
   without changing to a liquid first.
2. Example, at room temperature, dry ice can
   directly change from a solid to a colorless gas.

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• As dry ice sublimes, the cold carbon dioxide
  vapor causes water vapor in the air to condense
  and form clouds.
• D. Deposition is the phase change that occurs
  when a gas or vapor changes directly into a solid
  without first changing to a liquid.
• E. Deposition causes frost to form on windows