Change of Phase

1
Change of Phase

• Physics 1010
• Dr. Don Franceschetti
• April 1, 2005

2
States (Phases) of Matter

• Solid
• Liquid
• Gas (vapor)
• Plasma
• Tricky technical point strictly speaking, a gas
  only exists above the critical temperature, at
  which liquefaction becomes impossible, no matter
  how high the pressure. Below the critical
  temperature the gaseous phase is called a vapor.

3
Evaporation

• Converts a liquid to gas phase
• Only molecules with enough kinetic energy can
  leave. Lower energy molecules left behind.
  Evaporation cools.
• Perspiration is meant to take advantage of this!
  Dogs have to pant, pigs must wallow.
• Sublimation is the evaporation of a solid

4
Condensation and Humidity

• Gas changes to liquid.
• Is a warming process. Steam can burn.
• Humidity is mass of water per volume of air
• Relative humidity is humidity divided by maximum
  humidity at that temperature.
• Saturation is 100 relative humidity
• Warm air rises, expands, cools, rain drops form.
  Discuss cloud seeding.

5
Boiling

• Vapor pressure is the pressure of the liquid in a
  closed space above the liquid. It goes up with
  temperature. When it equals atmospheric
  pressure, the liquid boils. AT high altitudes,
  water boils below 100 degrees C,
• Pressure cooker, water vapor goes above
  atmospheric and boiling point goes up.

6
Geysers

• Consist of a long, narrow, vertical hole into
  which underground streams seep. Heated by
  volcanic heat from below. Narrow shaft cuts off
  convection. Boiling begins near bottom, steam
  pushes out water above, lowering pressure,
  producing more boiling.

7
Geysers
8
Getting to the triple point
Boiling water freezes when the pressure is low
enough. Freeze dry coffee, et.c
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Freezing and Melting

• As atoms move faster they get unstuck from each
  other. Take away their random kinetic energy and
  they get restuck.
• Regelationmelting under pressure and then
  freezing again unique to water
• Phase transition temperatures are precise.
• Latent heat of fusion
• Latent heat of vaporization