Changes of StateSolid to Liquid to Gas and Back

IntroductionThe change of state of a substance is a physical change - not a chemical change.

We have already talked about ...

condensation (gas to liquid)

boiling (liquid to gas)

melting (solid to liquid)

freezing (liquid to solid)

IntroductionThere is one more set of changes we need to consider.

sublimation (solid to gas)

•and

condensation (gas to liquid)

SublimationSublimation is the change of phase from a solid directly to a gas.

There is no intermediate liquid phase.

Sublimation occurs in solids with vapor pressures that exceed atmospheric pressure at or near room temperature.

Sublimation commonly occurs in ...

dry ice

solid air freshener

CondensationCondensation occurs when gas particles ...

encounter a solid of the same material ...

are attracted to the particles in the solid ...

and become a part of the solid.

This means that solids and gases may be in equilibrium by condensation and sublimation.

Summary of Phase Changes

Solid

Liquid

Gas

Condensa

tion

Boili

ng

CondensationSublimation

MeltingFreezin

g

Phase DiagramsThe relationships between the solid, liquid, and gas phases of a substance in a sealed container can be represented by a single graph called a phase diagram.

We plot the pressure versus the temperature and put the various phases on the diagram.

The conditions of pressure and temperature at which two phases exist in equilibrium are indicated by a line separating the two phases.

Phase Diagrams

For example, the phase diagram for water looks like:

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

Vapor

Phase Diagrams

The triple point is where all three phases exist together.

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

VaporTriple Point

Phase Diagrams

The triple point is 0.61 kPa at 0.016°C.

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

VaporTriple Point

0.6

1 k

Pa

0.016°C

Phase Diagrams

The critical point is the highest temperature a substance can be a liquid

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

Vapor

Critical Point

Phase Diagrams

The critical point of water occurs at 374°C and 22,064 kPa.

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

Vapor

Critical Point

22,0

64

kPa

374°C

Phase Diagrams

At any given pressure, the temperature where we go from solid to liquid is the melting point.

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

Vapor

melting point

Phase Diagrams

At any given pressure, the temperature where we go from liquid to vapor is the boiling point.

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

Vapor

boiling point

Phase Diagrams

The normal melting point and boiling point occurs at 101.3 kPa.

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

Vapor

101

.3 k

Pa

0°C 100°C

Phase DiagramsThe phase diagram of water is unlike most phase diagrams.

In water, as the pressure increases, the melting point decreases.

Phase Diagrams

Phase diagram of water

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

Vapor

melting point

Phase Diagrams

Phase diagram of water

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

Vapor

melting point

Phase DiagramsThe phase diagram of water is unlike most phase diagrams.

In water, as the pressure increases, the melting point decreases.

In almost all other systems, as the pressure increases, the melting point increases.

Phase Diagrams

Phase diagram of something other than water

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

Vapor

melting point

Phase Diagrams

Phase diagram of something other than water

pre

ssure

(kP

a)

temperature (°C)

Solid

Liquid

Vapor

melting point

Phase DiagramsThe phase diagram of water is unlike most phase diagrams.

In water, as the pressure increases, the melting point decreases.

In almost all other systems, as the pressure increases, the melting point increases.

This is not the last time we will see that water behaves differently from most other compounds.

SummaryThe phase changes from solid are ..

sublimation (to gas) and melting (to liquid)

The phase changes from liquid are ...

boiling (to gas) and freezing (to solid)

The phase changes from gas are ...

condensation (to liquid or solid)

SummaryThe relationships between the solid, liquid, and gas phases of a substance in a sealed container can be represented by a single graph called a phase diagram.

The triple point is where all three phases exist together.

The critical point is the highest temperature a substance can be a liquid.

At any given pressure, the temperature where we go from solid to liquid is the melting point and where we go from liquid to gas is the boiling point.