NOTES: 13.2-13.4Relationship Between Gases & Liquids;Phase Changes & Phase Diagrams

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…

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)

● VAPORIZATION: the conversion of a liquid to a gas, or vapor

● 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

● 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!

● 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)

● this process is called EVAPORATIVE COOLING

● important for living organisms & ecosystems in maintaining temperatures…including YOU!

● CONDENSATION: the change of state from gas to liquid (the opposite of vaporization)

● In order to understand boiling, you must first understand something called vapor pressure.

● Every liquid has a property associated with it called vapor pressure.

● It depends significantly

upon temperature…vapor

pressure increases

with increasing

temperature.

● Consider a liquid in a closed container.

● At first the liquid level goes down, then it stays constant.

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)

● 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.

● 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!

Heat Entering Water

Heat Leaving

Water

Boiling is a Cooling Effect!

Liquid stays at a constant

temp. (100°C)

110oC100oC

75oC

20oC

0oC

1074 mm Hg760

300

17

4.6

110oC100oC

75oC

20oC

0oC

1074 mm Hg760

300

17

4.6

Room Pressure

Room Temp

110oC100oC

75oC

20oC

0oC

1074 Torr760

300

17

4.6

Room Pressure

Room Temp

110oC100oC

75oC

20oC

0oC

1074 Torr760

300

17

4.6

Room Pressure

Room Temp

Boiling at Room Temperature

Pressure Cookers

110oC100oC

75oC

20oC

0oC

1074 Torr760

300

17

4.6

Pressure Cookers Cook Faster Because The Boiling Water is Hotter

● 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

● FREEZING: the change of state from liquid to solid.

● Freezing Point = temp. at which a liquid changes into solid

Boiling

Point

Melting

Point

Gas

Liquid

Solid

● SUBLIMATION: the change of a substance from a solid to a gas (vapor) without passing through the liquid state

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

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