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substances are insoluble, not dissolving at all or at least not no-
1.00006
1.00003 ticeably. In general, the forces of attraction between molecules
1.00000 or ions of the solvent and solute determine if something will
0.99997 dissolve and if there are any limits on the solubility. These forces
Density (g/cm 3 ) 0.99991 sidered in the following examples.
0.99994
of attraction and their role in the dissolving process will be con-
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First, consider the dissolving process in gaseous and liquid
0.99985
solutions. In a gas, the intermolecular forces are small, so gases
0.99982
0.99979
0.99976 can mix in any proportion. Fluids that can mix in any propor-
tion without separating into phases are called miscible fl uids.
0.99973 Fluids that do not mix are called immiscible fluids. Air is a mix-
0.99970
0.99967 ture of gases, so gases (including vapors) are miscible.
Liquid solutions can dissolve a gas, another liquid, or a
0 1 2 3 4 5 6 7 8 9 10 solid. Gases are miscible in liquids, and a carbonated beverage
Temperature ( C)
O
(your favorite cola) is the common example, consisting of car-
bon dioxide dissolved in water. Whether or not two given liquids
FIGURE 11.4 The density of water from 0°C to 10°C. The
form solutions depends on some similarities in their molecular
density of water is at a maximum at 4°C, becoming less dense as
it is cooled or warmed from this temperature. Hydrogen bonding structures. The water molecule, for example, is a polar molecule
explains this unusual behavior. with a negative end and a positive end. On the other hand, car-
bon tetrachloride (CCl 4 ) is a molecule with covalent bonds that
are symmetrically arranged. Because of the symmetry, CCl 4 has
no negative or positive ends, so it is non polar. Thus, some liq-
arrangement of liquid water. As the liquid water is warmed from uids have polar molecules and others have nonpolar molecules.
0°C still more, hydrogen bonds break down and the density of the The general rule for forming solutions is like dissolves like. A
water steadily increases. At 4°C, the expansion of water from nonpolar compound, such as carbon tetrachloride, will dissolve
the increased molecular vibrations begins to predominate, and oils and greases because they are non polar compounds. Water, a
the density decreases steadily with further warming (Figure 11.4). polar compound, will not dissolve the nonpolar oils and greases.
Thus, water has its greatest density at a temperature of 4°C. Carbon tetrachloride at one time was used as a cleaning solvent
The heat of fusion, specific heat, and heat of vaporization because of its oil- and grease-dissolving abilities. Its use is no lon-
of water are unusually high when compared to other, chemically ger recommended because it causes liver damage.
similar substances. These high values are accounted for by the Some molecules, such as soap, have a part of the molecule
additional energy needed to break hydrogen bonds. that is polar and a part that is nonpolar. Washing with water
alone will not dissolve oils because water and oil are immiscible.
When soap is added to the water, however, the polar end of the
THE DISSOLVING PROCESS soap molecule is attracted to the polar water molecules, and the
A solution is a homogeneous mixture of ions or molecules of nonpolar end is absorbed into the oil. A particle (larger than a
two or more substances. Dissolving is the process of making molecule) is formed, and the oil is washed away with the water
a solution. During dissolving, the different components that (Figure 11.5).
make up the solution become mixed. For example, when sugar
dissolves in water, the molecules of sugar become uniformly
dispersed throughout the molecules of water. The uniform taste
of sweetness of any part of the sugar solution is a result of this CONCEPTS Applied
uniform mixing.
The general terms solvent and solute identify the compo- How to Mix
nents of a solution. The solvent is the component present in
the larger amount. The solute is the component that dissolves Obtain a small, clear water bottle with a screw-on cap. Fill
the bottle halfway with water, then add some food coloring
in the solvent. Atmospheric air, for example, is about 78 percent
and swirl to mix. Now add enough mineral or cooking oil to
nitrogen, so nitrogen is considered the solvent. Oxygen (about
almost fill the bottle. Seal the bottle tightly with the cap.
21 percent), argon (about 0.9 percent), and other gases make up
Describe the oil and water in the bottle. Shake the bottle
the solutes. If one of the components of a solution is a liquid, vigorously for about 30 seconds, then observe what happens
it is usually identified as the solvent. An aqueous solution is a when you stop shaking. Does any of the oil and water mix?
solution of a solid, a liquid, or a gas in water. Try mixing the oil and water again, this time after
A solution is formed when the molecules or ions of two adding a squirt of liquid dishwashing soap. Describe what
or more substances become homogeneously mixed. But the happens before and after adding the soap. What does this
process of dissolving must be more complicated than the sim- tell you about the structure of the oil and water molecules?
ple mixing together of particles because (1) solutions become How did soap overcome these differences?
saturated, meaning there is a limit on solubility, and (2) some

11-5 CHAPTER 11 Water and Solutions 279