notes: 13.2-13.4 relationship between gases & liquids; phase changes & phase diagrams
TRANSCRIPT
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NOTES: 13.2-13.4Relationship Between Gases & Liquids;Phase Changes & Phase Diagrams
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The Nature of Liquids
● like the particles in a gas, the particles in a liquid also have kinetic energy
● like gases, liquids have an indefinite shape (they conform to the shape of their containers)
● however, there is a key difference between gases and liquids…
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The Nature of Liquids
● the particles in a liquid are attracted to one another…this keeps the particles close together!
● as a result, liquids have a DEFINITE VOLUME (unlike gases!)
● liquids are MUCH more dense than gases…increasing the pressure on a liquid has hardly any effect on its volume (same is true for solids)
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● VAPORIZATION: the conversion of a liquid to a gas, or vapor
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● EVAPORATION: the conversion of a liquid to a gas at the surface of a liquid that is NOT boiling
● most molecules in a liquid don’t have enough KE to overcome the attractive forces of the liquid & escape into the gas state
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● a liquid evaporates faster when heated…WHY?
● heating a liquid increases the average KE of its particles…more particles are able to escape to the gas state!
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● the particles with the highest KE escape first…as a result…
● the particles left in the liquid have a lower average KE than the particles that escaped…so, evaporation is a COOLING PROCESS (the liquid left behind is cooler)
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● this process is called EVAPORATIVE COOLING
● important for living organisms & ecosystems in maintaining temperatures…including YOU!
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● CONDENSATION: the change of state from gas to liquid (the opposite of vaporization)
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● In order to understand boiling, you must first understand something called vapor pressure.
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● Every liquid has a property associated with it called vapor pressure.
● It depends significantly
upon temperature…vapor
pressure increases
with increasing
temperature.
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● Consider a liquid in a closed container.
● At first the liquid level goes down, then it stays constant.
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Time
Rate
Rate of Evaporation
Rate ofCondensation
● The pressure in the container at the equilibrium point (between liquid & vapor) is the vapor pressure.
Rates are equal.(Equilibrium Point)
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● BOILING = a rapid state of evaporation that takes place within the liquid as well as at it’s surface.
● Boiling takes place when the vapor pressure of the liquid equals the external (surrounding, or applied) pressure.
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● BOILING POINT = the temperature at which the vapor pressure of a liquid is just equal to the external (applied) pressure on the liquid…the temperature at which a liquid boils!
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Heat Entering Water
Heat Leaving
Water
Boiling is a Cooling Effect!
Liquid stays at a constant
temp. (100°C)
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110oC100oC
75oC
20oC
0oC
1074 mm Hg760
300
17
4.6
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110oC100oC
75oC
20oC
0oC
1074 mm Hg760
300
17
4.6
Room Pressure
Room Temp
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110oC100oC
75oC
20oC
0oC
1074 Torr760
300
17
4.6
Room Pressure
Room Temp
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110oC100oC
75oC
20oC
0oC
1074 Torr760
300
17
4.6
Room Pressure
Room Temp
Boiling at Room Temperature
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Pressure Cookers
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110oC100oC
75oC
20oC
0oC
1074 Torr760
300
17
4.6
Pressure Cookers Cook Faster Because The Boiling Water is Hotter
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● MELTING: the change of state from solid to liquid; energy (heat) is absorbed by the substance that is melting.
● Melting Point = temp. at which a solid changes into liquid
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● FREEZING: the change of state from liquid to solid.
● Freezing Point = temp. at which a liquid changes into solid
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Boiling
Point
Melting
Point
Gas
Liquid
Solid
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● SUBLIMATION: the change of a substance from a solid to a gas (vapor) without passing through the liquid state
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PHASE DIAGRAMS:
● a PHASE DIAGRAM gives the conditions of temperature & pressure at which a substance exists as solid, liquid, or gas (vapor).
● the conditions of pressure & temperature at which two phases exist in equilibrium are indicated on a phase diagram by a line separating the 2 regions representing the phases
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Phase Diagram Features:● BOUNDARY LINE: where 2 phases are in
equilibrium● TRIPLE POINT: temp / pressure at which
all 3 phases are in equilibrium● CRITICAL POINT: temp / pressure above
which a gas can no longer be condensed to liquid
**above the critical point, a substance is known as a supercritical fluid
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