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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 l.jpg

Chapter 17Changes of Phase

Physics 1 (Garcia) SJSU

Phases of matter l.jpg
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 l.jpg

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

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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 l.jpg

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

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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

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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

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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

Boiling l.jpg

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

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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

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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

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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 l.jpg

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

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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 l.jpg
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 l.jpg

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

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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

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Energy & Changes of Phase

Physics 1 (Garcia) SJSU

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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

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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