Phase Changes

1
Phase Changes
2
Four Phases of Matter

• Solid
• Liquid
• Gas
• Plasma
• Well only talk about the first three today!

3
Solid

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

4
Liquid

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

5
Gas

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

6
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 2nd law of thermodynamics

7
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)

8
Temperature of a Substance as Heat is Added at a
Constant Rate
Temperature
e
d
c
b
a
Heat Being Added
9
Temperature of a Substance as Heat is Added at a
Constant Rate

1. A solid will experience a temperature increase
2. Then, at its melting point, the solid will melt.
3. After melting is complete, a liquid will
   experience an increase in temperature.
4. Then, at its boiling point, the liquid will boil.
5. After boiling (or vaporizing) is complete, the
   gas will continue to experience an increase in
   temperature

10
Temperature of a Substance as Heat is Released at
a Constant Rate
11
Temperature of a Substance as Heat is Released at
a Constant Rate

1. A gas will experience a decrease in temperature.
2. At its points of condensation (same temperature
   as boiling), the gas will condense and become a
   liquid.
3. Once condensation is complete, the liquid will
   experience a decrease in temperature.
4. At its freezing point (same temperature as
   melting), the liquid will freeze.
5. Once freezing is complete, the solid will
   experience a decrease in temperature.

12
Quick Summary
13
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.

14
Melting

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

15
Vaporization

• A change of phase from liquid to gas.
• Two types
• Boiling
• Evaporation

16
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

17
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.

18
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

19
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

20
Freezingcont.

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

21
Sublimation

• A change of phase from solid to gas.
• Example Dry Ice

22
Deposition

• A change of phase from gas to solid.
• Example Snow