A. Interaction between Polar Molecules
Polar molecules interact with one another. This interaction is governed by the fact that like charges repel and opposite charges attract. Figure 7.15 shows a collection of polar molecules aligned so that the partially positive end of one molecule is near the partially negative end of another.

FIGURE 7.15 Polar molecules, when free to move around in the liquid state, will align themselves, as in this diagram, with the positive end nearest the negative end of another molecule.

 

Polar molecules aligned by charge need not all be of the same compound. Figure 7.13 showed that water is a polar molecule. When another polar molecule such as ammonia (shown in Figure 7.13) is added to water, it dissolves because of the interaction between the two types of molecules. Figure 7.16 shows the polarity of water molecules and ammonia molecules and the interactions between them.

FIGURE 7.16 When ammonia (a polar molecule) is dissolved in water (another polar molecule) the two kinds of molecules orient themselves, negative end to positive end, as in this diagram.

B. Water Solutions of Ionic Compounds
When an ionic compound dissolves in water, the solution contains ions rather than neutral particles. For example, when sodium chloride, NaCl, dissolves in water, the solution contains sodium ions (Na⁺) and chloride ions (Cl^-), rather than neutral units of NaCl. A polyatomic ion does not break up into separate atoms in solution. Thus, a solution of sodium nitrate, NaNO₃, contains sodium ions and nitrate ions; no nitrogen ions or oxygen ions are present. A solution of ammonium sulfate, (NH₄)₂SO₄, contains ammonium ions and sulfate ions.

This dissolution of ionic compounds takes place because of interactions like that between ammonia and water. Figure 7.17 shows at the molecular level what happens when sodium chloride, an ionic compound, dissolves in water. Each ion of the solid crystal becomes surrounded by water molecules, with the negative end of the water molecules approaching closest to the positive sodium ions, and the positive end of the water molecules surrounding the negative chloride ions. The water molecules pull these ions, one by one, away from the rest of the crystal. Other ionic compounds that are virtually insoluble in water have such strong interactions between their ions that the pull of the polar water molecules is not strong enough to break the ions apart.

FIGURE 7.17 The dissolution of sodium chloride in water. Notice how the polar water molecules are oriented in one way around the positive sodium ions and in another way around the oppositely charged chloride ions.

 

Frequently, when a very polar molecule such as hydrogen chloride dissolves in water, there is sufficient interaction between the two molecules to cause one of them to break up into ions. With hydrogen chloride, the equation for this reaction is

To some extent the same process occurs when ammonia is dissolved in water. The equation for that reaction is

C. Electrolytes, Nonelectrolytes, and Weak Electrolytes
The presence of ions in a solution of an ionic compound can be demonstrated with an apparatus like the one shown in Figure 7.18. The apparatus consists of two electrodes--one connected to one pole of a power source and the other connected to a light bulb that is, in turn, connected to the other pole of the power source. If the two electrodes touch, an electric current flows through the completed circuit and the bulb lights (Figure 7.18b). If the two electrodes are separated and then immersed in water, no current flows (Figure 7.18c). Pure water cannot carry an electric current. However, if the pure water is replaced by an aqueous (water) solution of an ionic compound, the bulb lights, indicating a flow of electricity through the solution (Figure 7.18d). The electric current is carried through the solution by the dissolved ions.

FIGURE 7.18 Conductivity apparatus for showing the presence of ions in an aqueous solution: (a) electrodes apart, circuit broken; (b) electrodes touching, circuit complete; (c) electrodes in pure water, circuit broken; (d) electrodes in an ionic solution, circuit complete.

 

Compounds whose aqueous solutions conduct electricity are called electrolytes. This group of compounds includes hydroxides, salts (metal-nonmetal and metal-polyatomic ion compounds), and some acids (nitric, sulfuric, hydrochloric). Compounds that dissolve in water without forming a solution that carries an electric current are called nonelectrolytes. These compounds dissolve as molecules. Many compounds containing only nonmetals are nonelectrolytes. If a compound containing only nonmetals ionizes, its molecule will usually contain hydrogen bonded to a very electronegative atom like oxygen (as in acetic acid) or chlorine (as in hydrochloric acid).

Compounds that react somewhat but not completely with water to form ions are said to be partially ionized; their solutions are poor conductors of electricity. They are called weak electrolytes. Acetic acid is a weak electrolyte. In the apparatus in Figure 7.18, a solution of acetic acid causes the bulb to glow only faintly, showing that not many ions are present in solution.

D. Other Differences between Ionic and Covalent Compounds
Ionic compounds are usually solids at room temperature. They have high melting points. They dissolve only in very polar liquids like water. Some are only sparingly soluble in water.

Covalent compounds are found in all three states - solids, liquids, and gases - at room temperature. Those of low molecular weight and those that are polar may dissolve in water, but a covalent solid is much more apt to be soluble in a nonpolar (carbon tetrachloride) or slightly polar (ethyl alcohol) liquid.