The Foundations of Chemistry

We have described many properties of liquids and discussed how they depend on inter-
molecular forces of attraction. The general effects of these attractions on the physical
properties of liquids are summarized in Table 13-6. “High” and “low” are relative terms.
Table 13-6 is intended to show only very general trends. Example 13-4 illustrates the use
of intermolecular attractions to predict boiling points.

EXAMPLE 13-4 Boiling Points Versus Intermolecular Forces

Predict the order of increasing boiling points for the following: H 2 S; H 2 O; CH 4 ; H 2 ; KBr.

Plan

We analyze the polarity and size of each substance to determine the kinds of intermolecular
forces that are present. In general, the stronger the intermolecular forces, the higher is the
boiling point of the substance.

Solution

KBr is ionic, so it boils at the highest temperature. Water exhibits hydrogen bonding and boils
at the next highest temperature. Hydrogen sulfide is the only other polar covalent substance
in the list, so it boils below H 2 O but above the other two substances. Both CH 4 and H 2 are
nonpolar. The larger CH 4 molecule is more easily polarized than the very small H 2 molecule,
so dispersion forces are stronger in CH 4. Thus, CH 4 boils at a higher temperature than H 2.

H 2 CH 4 H 2 SH 2 OKBr

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increasing boiling points

You should now work Exercise 24.

THE SOLID STATE

MELTING POINT

The melting point (freezing point)of a substance is the temperature at which its solid
and liquid phases coexist in equilibrium.

melting

solid3::::4liquid

freezing

The melting pointof a solid is the same as the freezing pointof its liquid. It is the temper-
ature at which the rate of melting of a solid is the same as the rate of freezing of its liquid
under a given applied pressure.
The normal melting pointof a substance is its melting point at one atmosphere pres-
sure. Changes in pressure have very small effects on melting points; they have large effects
on boiling points.

HEAT TRANSFER INVOLVING SOLIDS

When heat is added to a solid below its melting point, its temperature rises. After enough
heat has been added to bring the solid to its melting point, additional heat is required to
convert the solid to liquid. During this melting process, the temperature remains constant

13-11

13-10

13-11 Heat Transfer Involving Solids 503