Liquid and Solid States: Qualitative Aspects

of Phase Changes

Lecture 13

Three states of mattereverywhere.

We consume water in three states

There are states of matter and phases.

Do not confuse them.

Two types of electrostatic forces at work:Intramolecular (bonding) forces exist within each molecule and influence the chemical properties of the substance;Intermolecular (nonbonding) forces exist between the molecules and influence the physical properties of the substance.

A macroscopic comparison:Gas conforms to shape and volume of container, has high compressibility and ability to flow.Liquid conforms to shape of container, its volume is limited by surface. It has very low compressibility and moderate ability to flow.Solid maintains its own shape and volume, has almost no compressibility and ability to flow.

Phase changes

Phase change definitionsMelting (fusion) is a process when a solid liquifies.Freezing (crystallization) is a process when a liquid solidifies.Vaporization is a process when a liquid vaporizes.Condensation is a process when a gas changes into a liquid.

Phase changes

Phase change definitionsSublimation is a process when a a solid becomes a gas without first becoming a liquid.Deposition is a process when a a gas becomes a solid without first becoming a liquid.

How enthalpy changes

Melting and vaporizing are endothermic processes.Condensing and freezing are exothermic processes.

Each phase change has a specific H.H2O(l) H2O(g); Hvaporization

H2O(g) H2O(l); Hcondensation

H2O(s) H2O(l); Hfusion

H2O(l) H2O(s); Hcrystallization

I2(s) I2 (g); Hsublimation

I2(g) I2 (s); Hdeposition

All the reverse processes have H of the same magnitude but opposite sign:Hvaporization= —Hcondensation

Hfusion= —Hcrystallization

Hsublimation= —Hdeposition

Enthalpy of phase changes

Enthalpy of phase changes

Heat of sublimation equals sum of the heats of fusion and vaporization:Solidliquid; Ho

fus

Liquidgas; Hovap

-----------------------------Solidgas; Ho

subl

Heating water

Phase changes of many substances

are reversible and reach equilibrium.

Liquid-gas equilibrium

When a system at equilibrium is

disturbed,it counteracts the

disturbance and eventually re-establishes a state of

equilibrium.

Molecular speed vs temperature

The higher the temperature is, the higher the vapor

pressure.The weaker the intermolecular forces are,

the higher the vapor pressure.

Vapor pressure vs temperature

Vapor pressure vs temperature

Rudolf Julius Emanuel Clausius (1822–1888), German scientist

Benoît Paul Émile Clapeyron (1799-1864), French scientist

The Clausius-Clapeyron equation:

A sample problem

on using the Clausius-Clapeyron equation.

The boiling point is the temperature

at which the vapor pressure equals the external pressure.The boiling point depends on

the applied pressure.

Solid, liquid, gas

Phase diagram of CO2

Features of phase diagram:Regions correspond to different phases of the substance.Lines between regions represent the phase transition curves.

Features of phase diagram:The critical point shows critical temperature and pressure, at which the phase boundary disappears.The triple point shows temperature and pressure, at which all phases co-exist.

Phase diagram of H2O

THE END