Phase Diagrams

Zumdahl, page 774 -

Consider water which exists in three familiar phases:

SolidLiquidGas

Each of the phases has distinct physical propertiesand each of the phases can co-exist with the others.

We can observe phase boundaries such as the meniscus between a liquid and gas.

Consider water which exists in three familiar phases:

SolidLiquidGas

Each of the phases has distinct physical propertiesand each of the phases can co-exist with the others.

At atmospheric pressure:

Low T ---> 0oC ---> 100oC ---> high T solid | liquid | gas

You also know that, as you change the pressure, thetemperatures of the phase transitions will change.

For example, as you reduce the pressure on the liquid,the boiling point will be reduced. ( e.g. up a mountain )

As you increase the pressure on a sample of ice, youlower the melting point. ( e.g. under a skate blade )

You also know that, as you change the pressure, thetemperatures of the phase transitions will change.

For example, as you reduce the pressure on the liquid,the boiling point will be reduced. ( e.g. up a mountain )

As you increase the pressure on a sample of ice, youlower the melting point. ( e.g. under a skate blade )

We can summarize these Pressure vs. Temperaturerelationships on a Phase Diagram.

P

100 KPa

T373K273K

Log scale

P

100 KPa

T373K273K

Liquid

Gas

Solid

P

100 KPa

T373K273K

Liquid

Gas

Solid

T

C

Zumdahl, page 780

Two special points:

Triple Point solid, liquid, and gas ALL co-exist

Critical Point The phase boundary between liquidand gas vanishes.

Two special points:

Triple Point solid, liquid, and gas ALL co-exist

Critical Point The phase boundary between liquidand gas vanishes.

For water, the triple point is at: 273.16K and 611 Pacompared to the freezing point of 273.15 at 101 325 Pa.

The critical point is at 647.1K and 22.05 MPa.

Two special points:

Triple Point solid, liquid, and gas ALL co-exist

Critical Point The phase boundary between liquidand gas vanishes.

For the triple point, both T and P are fixed and this is the second fixed point for the Kelvin scale.

At temperatures greater than the critical temperaturea material is called a Supercritical Fluid.

Note that, at pressures below the triple point, water will not melt but will sublime instead. It transforms from solid to gas without a liquid being formed as the temperature is increased.

Strictly speaking, I am being a bit careless with the use of the word phasefor the solid. The solid STATE actually shows about ten different PHASES thatonly exist at high pressure.

Ice III for example exists at about 270Kat a pressure of about 200 megapascals.( 2000 atmospheres )The different PHASES have different arrangements of the water molecules.

The gaseous STATE is a single homogeneous PHASE.

The liquid STATE is a single homogeneous PHASE.

The SOLID state may contain twoor more crystalline PHASES inequilibrium with each other.