The Foundations of Chemistry

DISSOLUTION OF LIQUIDS IN LIQUIDS (MISCIBILITY)

In science, miscibilityis used to describe the ability of one liquid to dissolve in another.

The three kinds of attractive interactions (solute–solute, solvent–solvent, and solvent–

solute) must be considered for liquid–liquid solutions just as they were for solid–liquid

solutions. Because solute–solute attractions are usually much weaker for liquid solutes

than for solids, this factor is less important and so the mixing process is often exothermic

for miscible liquids. Polar liquids tend to interact strongly with and dissolve readily in

other polar liquids. Methanol, CH 3 OH; ethanol, CH 3 CH 2 OH; acetonitrile, CH 3 CN; and

sulfuric acid, H 2 SO 4 , are all polar liquids that are soluble in most polar solvents (such as

water). The hydrogen bonding between methanol and water molecules and the dipolar

interaction between acetonitrile and water molecules are depicted in Figure 14-3.

Because hydrogen bonding is so strong between sulfuric acid, H 2 SO 4 , and water, a

large amount of heat is released when concentrated H 2 SO 4 is diluted with water (Figure

14-4). This can cause the solution to boil and spatter. If the major component of the

mixture is water, this heat can be absorbed with less increase in temperature because of

the unusually high specific heat of H 2 O. For this reason, sulfuric acid (as well as other mineral

acids) is always diluted by adding the acid slowly and carefully to water. Water should never be

added to the acid.If spattering does occur when the acid is added to water, it is mainly water

that spatters, not the corrosive concentrated acid.

Nonpolar liquids that do not react with the solvent generally are not very soluble in

polar liquids because of the mismatch of forces of interaction. Nonpolar liquids are,

however, usually quite soluble in other nonpolar liquids. Between nonpolar molecules

(whether alike or different) there are only dispersion forces, which are weak and easily

overcome. As a result, when two nonpolar liquids are mixed, their molecules just “slide

between” one another.

14-3

Hydrogen bonding and dipolar
interactions were discussed in Section
13-2.

548 CHAPTER 14: Solutions

When water is added to concentrated
acid, the danger is due more to the
spattering of the acid itself than to the
steam from boiling water.

The nonpolar molecules in oil do
not attract polar water molecules, so
oil and water are immiscible. The
polar water molecules attract one
another strongly—they “squeeze
out” the nonpolar molecules in the
oil. Oil is less dense than water, so it
floats on water.

Figure 14-3 (a) Hydrogen bonding in methanol–water solution. (b) Dipolar interaction in

acetonitrile–water solution. Each polar molecule is labeled with regions of highest negative

charge () and regions of highest positive charge (
). Attractive forces are shown.

Molecules tend to arrange themselves to maximize attractions by bringing regions of

opposite charge together while minimizing repulsions by separating regions of like charge.

H

(b)

C

δ–

δ+

C

H

H

N

δ+

δ–

• •


• •
  H O

H

H

H

δ–

δ+

O

• •

(a)

H

H

H

H

C

O

• •
  
  
  • •
    H
  
  
  
  

H

δ–

δ+ δ–

• • • •
  O

δ–

δ+

δ+

δ+

• •
  
  
  • •
  
  
  
  

H H

O

H

H

H

H

C

O

• •
  
  
  • •δ–
  
  
  
  

δ+

• •


• •