Chemistry - A Molecular Science

Chapter 7 States of Matter and Changes in State

The

melting

or

freezing point

is the temperature at which the solid and liquid coexist

in a mixture called a

melt

. The melt remains at equilibrium so long as no heat is added or

removed; the melting and freezing processes continue, but they occur at the same rate;

i.e.

,

the solid melts at the same rate as the li

quid freezes. Two processes that continue in

opposite directions at the same rate and involve

no net change in the system are said to be

in

dynamic equilibrium

. Almost all chemical processes reach dynamic equilibrium.

Dynamic equilibria are represented with double

arrows to indicate that the forward and

reverse processes continue at equilibrium. T

hus, the solid-liquid e

quilibrium is expressed

as solid

U

liquid.

The melting point is a measure of the therma

l energy required to overcome the forces

holding the particles in their fixed positions in

the solid,* so it is an indication of the

strength of those forces. We conclude that

substances with high melting points interact

strongly with one another.

For example, ionic compounds have high melting points

because the forces that must be overcome to

break down the solid structure are ionic

bonds, which are very strong interactions.

Temperature

Liquid Solid

Pressure

Solid

Liquid

(a)

(b)

Figure 7.14 Solid-liquid equilibrium The more dense phase is always above the pressure versus temperature line for the equilibr

ium. Increasing the pressure

freezes the substance in (a),

but it melts it in (b).

†

One theory holds that ice skating is

possible because ice melts when

pressure is applied. Consider that an ice cube that has just been removed from the freezer is not slippery, but it becomes so only after melting slightly. The slick surface is the result of a thin layer of water on the ice, not the ice itself. Thus,

the pressure that is exerted from

the force of a skater’s weight c

oncentrated on the thin blades

produces a pressure that is suffici

ent to melt the ice and produce the

thin layer of water, which makes the surface slippery.

* The forces between particles can be intermolecular forces or ionic or

covalent bonds.

The

heat of fusion

, Δ

Hfus

, is the amount of heat required to melt a substance at its

melting point. It is the potential energy difference between the solid and liquid states (Figure 7.13a). Heat added to a melt is used

to melt the solid, and as long as there is solid

present, there is no temperature rise,

i.e.

, the heat is used to increase the potential energies

of the molecules not their kinetic energies.

A solid can also be made to melt or a liquid made to freeze by pressure changes, but

large pressure changes are typically requi

red to produce small changes in the melting

point. The sign of the slope of the pressure versus temperature line (Figure 7.14) indicates the relative densities of the solid and liquid states because

increasing the pressure on an

equilibrium mixture involving different states

always moves the mixt

ure toward the more

dense state

. The solid is denser than the liquid

for most substances, so increasing the

pressure usually freezes the liquid to produce

the denser solid phase (Figure 7.14a).

However, the liquid is more dense than the solid in some compounds, such as water, so increasing the pressure of a solid-liquid mi

xture of these compounds causes the solid to

melt, producing the more dense liquid phase (Figure 7.14b).

†^

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Carolina

State

University