1. Forces between particles
One factor that affects solubility is the nature of the intermolecular forces or interionic forces in both the solute and the solvent. In Chapter 10, we listed the kinds of forces operating in solids and liquids. When one substance dissolves in another, the attractive forces in both must be overcome. The dissolving solute must be able to break up the aggregation of molecules in the solvent (that is, the hydrogen bonds between molecules or the dispersion forces between molecules in a nonpolar solvent), and the molecules of the solvent must have sufficient attraction for the solute particles to remove them one by one from their neighbors in the undissolved solute. If the solute is ionic, only a very polar solvent like water provides enough interaction to effect dissolution (see Figure 7.17). In those ionic compounds like barium sulfate that are called insoluble, the interaction between the ions is greater than can be overcome by interaction with the polar water molecules. If the solute particles are polar molecules, polar solvents such as alcohols can usually effect dissolution. If the solute is nonpolar, it may dissolve only in nonpolar solvents, not because polar solvent molecules are unable to overcome the weak dispersion forces between the solute molecules, but because these dispersion forces are too weak to overcome the dipole-dipole interaction between the solvent molecules.
A general rule is that like dissolves like. Ionic and polar compounds are most apt to be soluble in polar solvents like water or liquid ammonia. Nonpolar compounds are most apt to be soluble in nonpolar solvents, such as carbon tetrachloride, and hydrocarbon solvents like gasoline.
The solubility of gases in water depends a great deal on the polarity of the gas molecules. Those gases whose molecules are polar are much more soluble in water than are nonpolar gases. Ammonia, a strongly polar molecule, is very soluble in water (89.9 g/100 g H2O); so is hydrogen chloride (82.3 g/100 g H2O). Helium and nitrogen are nonpolar molecules. Helium is only slightly soluble (1.8 X 10-4 g/100 g H2O), as is nitrogen (2.9 X 10-3 g/100 g H2O).
Table 11.1 shows specific examples of different kinds of compounds and their relative solubilities in water, a polar solvent; in alcohol, a less polar solvent; and in benzene, a nonpolar solvent.
| Solubility in | ||||
|---|---|---|---|---|
| Kinds of bonds | Example | Water | Alcohol | Benzene |
| ionic | sodium chloride | very soluble | slightly soluble | insoluble |
| polar covalent | sucrose (sugar) | very soluble | soluble | insoluble |
| nonpolar covalent | napthalene | insoluble | soluble | very soluble |
2. Temperature
Table 11.2 shows the solubility of several substances in water at 20°C and 100°C. Notice that the solubilities of solids and liquids usually increase as the temperature increases, but the solubilities of gases decrease with increasing temperature. This property of gases causes our concern for the fish population of lakes, oceans, and rivers threatened with thermal (heat) pollution. Fish require dissolved oxygen to survive. If the temperature of the water increases, the concentration of dissolved oxygen decreases, and the survival of the fish becomes questionable.
| Solubility (g/100mL water) | |||
|---|---|---|---|
| Compound | Type of bonding | At 20°C | At 100°C |
| sodium chloride | ionic | 35.7 | 39.1 |
| barium sulfate | ionic | 2.3 X 10-4 | 4.1 X 10-4 |
| sucrose | polar covalent | 179 | 487 |
| ammonia | polar covalent | 89.9 | 7.4 |
| hydrogen chloride | polar covalent | 82.3 | 56.1 |
| oxygen | nonpolar covalent | 4.5 X 10-3 | 3.3 X 10-3 |
3. Pressure
The pressure on the surface of a solution has very little effect on the solubilities of solids and liquids. It does have an enormous effect on the solubility of gases. As the partial pressure Section 9.6) of a gas in the atmosphere above the surface of a solution increases, the solubility of that gas increases. A carbonated beverage is bottled and capped under a high partial pressure of carbon dioxide so that a great deal of carbon dioxide dissolves. When the bottle is uncapped, the partial pressure of carbon dioxide above the liquid drops to that in the atmosphere, the solubility of carbon dioxide decreases, and the gaseous CO2 comes out of solution as fine bubbles.
This same phenomenon is of concern in scuba diving. As divers descend, they are under ever-increasing pressure due to the increasing mass of water above them. Each 33 ft of water exert a pressure of 1 atm. As the diver descends, the pressure on the air in the lungs increases rapidly, and the blood dissolves more than normal amounts of both nitrogen and oxygen. As the diver comes up, the pressure decreases and the gases come out of solution. If the change in pressure is too swift, the bubbles of gas are released into the blood stream and tissues, instead of into the lungs. The bubbles cause sharp pain wherever they occur, most notably in the joints and ligaments. This dangerous and sometimes fatal condition is called bends, the name deriving from the temporary deformities caused by the affected diver's being unable to straighten his or her joints. Because helium, at all pressures, is less soluble than nitrogen, divers often breathe a mixture of helium and oxygen instead of air, a nitrogen-oxygen mixture.
The term hyperbaric means greater-than-normal atmospheric pressure. In hyperbaric medical treatments, patients are subjected to a pressure greater than atmospheric. This pressure increases the amount of oxygen dissolved in the blood. Treatment in hyperbaric, or high-pressure, chambers is of particular value for patients who are suffering from severe anemia, hemoglobin abnormalities, or carbon monoxide exposure or undergoing skin grafts, for such treatment increases the amount of oxygen transported by the blood to the tissues.
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