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Chapter 17 Changes of Phase. Phases of Matter. Four Phases of Matter: Solid Liquid Gas Plasma Change of phase occurs when we pass from one phase to another, such as water (liquid) boiling to change into vapor (gas). Water. Plasma. Ice. Steam. Evaporation.

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chapter 17 changes of phase

Chapter 17Changes of Phase

Physics 1 (Garcia) SJSU

phases of matter
Phases of Matter

Four Phases of Matter:

  • Solid
  • Liquid
  • Gas
  • Plasma

Change of phase occurs when we pass from one phase to another, such as water (liquid) boiling to change into vapor (gas).





Physics 1 (Garcia) SJSU


Evaporation is a change of phase from liquid to gas that takes place at the surface of a liquid.

A random molecule at the surface acquires enough energy to escape the attraction force among the molecules (which holds the liquid together).



Physics 1 (Garcia) SJSU

evaporative cooling
Evaporative Cooling

Because only the most energetic molecules can escape the surface, evaporation removes internal energy from the liquid, that is, evaporation cools.












Wet towel cools head

Wetness cools person

Wet tongue cools dog

Physics 1 (Garcia) SJSU


Condensation is the reverse of evaporation, a change of phase from gas to liquid that takes place at the surface of a liquid.

A random molecule from the gas strikes the surface and sticks instead of bouncing back into the gas.

Condensation heats.



Physics 1 (Garcia) SJSU

hot and humid
Hot and Humid

A 90 degree day in a dry climate, like San Jose, is more comfortable than a 90 degree day in a humid place like New Orleans.

In a dry climate you’re cooled by evaporation, in a wet climate you’re heated by condensation.

Heat index is the apparent temperature a person feels for a given humidity.

Physics 1 (Garcia) SJSU

demo wet dry bulb thermometer
Demo: Wet/Dry Bulb Thermometer

Pair of thermometers; one is kept dry while the other’s bulb is wrapped in wet cloth.

Difference of their temperatures gives relative humidity.

Wet bulb

Dry bulb

Large temperature difference indicates high or low humidity?

Low humidity; evaporative cooling is significant.

Physics 1 (Garcia) SJSU

fog clouds
Fog & Clouds

Warm air rises. As it rises, it expands. As it expands, it cools. As it cools, vapor molecules condense into water droplets. This forms a cloud (or fog if warm, moist air cools near the ground).

As vapor expands, it cools

and tiny, visible, water droplets (liquid) condense.


Water vapor

(gas) is


Warm breath feels cool when it expands


Physics 1 (Garcia) SJSU


When the temperature of a liquid is high enough that evaporation occurs everywhere, not just the surface, then the liquid boils.

The temperature required depends on the pressure; lower the pressure, the lower the boiling temperature (boiling point).

Tiny bubbles grow due to evaporation at their surface

Physics 1 (Garcia) SJSU

liquid nitrogen
Liquid Nitrogen

Liquid nitrogen boils at atmospheric pressure and room temperature.

Boiling point is -320 ºF and freezes at -346 ºF.

Physics 1 (Garcia) SJSU

demo slowing air molecules
Demo: Slowing Air Molecules

Balloon returns to its original state

Air molecules slow down and lose kinetic energy

Cool balloon using liquid nitrogen

Balloon slowly warms up, restoring energy

Physics 1 (Garcia) SJSU

demo low pressure boiling
Demo: Low Pressure Boiling

Water boils at room temperature if the pressure is low.

Cooking at high altitudes is difficult due to this effect; coffee brewed in the mountains always tastes lukewarm.

Physics 1 (Garcia) SJSU


Melting is the change of phase from solid to liquid.

Melting is a cooling process; the solid must absorb heat to melt.

Physics 1 (Garcia) SJSU


Sublimation is change of phase from solid to gas without passing through liquid phase.

Solid carbon dioxide (dry ice) sublimates at a chilly -109 °F.

Put dry ice into warm water to create dense fog of tiny water droplets.

Physics 1 (Garcia) SJSU

demo carbon dioxide
Demo: Carbon Dioxide

Carbon dioxide, released when dry ice sublimates, is heavier than air.

  • Burning candle
  • Extinguished under CO2 layer

Bubbles float on layer of dry ice.

(c) Scoop out some CO2 in a cup

(d) Pour it on candle to extinguish

Physics 1 (Garcia) SJSU


Freezing is the opposite of melting, that is, the change of phase from liquid to solid.

Heat must be removed from a liquid in order to freeze it into a solid.

Lava (liquid) freezes into rock (solid), heating the seawater.

Seawater (liquid) boils into vapor (gas), cooling the lava.

Physics 1 (Garcia) SJSU

demo freeze solid
Demo: Freeze Solid

Materials become brittle when frozen solid.

Organic materials appear solid but cells contain large amounts of liquid water.

Physics 1 (Garcia) SJSU

energy changes of phase
Energy & Changes of Phase

Physics 1 (Garcia) SJSU

heats of fusion vaporization
Heats of Fusion & Vaporization

100 cal

540 cal

720 cal

80 cal

Heat of


Heat of


Total Energy

Heat Capacity

Heating a gram of water

Physics 1 (Garcia) SJSU

c heck yourself
Check Yourself

Is boiling a cooling or a warming process?

Boiling is a cooling process.

So can you cool your hand by putting it in boiling water?

NO! Ouch!

So why is boiling a cooling process?

Because when a liquid boils it cools by itself releasing its most energetic molecules, just as with cooling by evaporation.

Physics 1 (Garcia) SJSU