The Importance of Water as an Electrolytic Solvent

Salts are insoluble in most solvents. Gasoline, benzene, carbon disulfide, carbon tetrachloride, alcohol, ether—these substances are good solvents for grease, rubber, organic materials generally but they do not generally dissolve salts. 

The reasons that water is so effective in dissolving salts are that it has a veiy high dielectric constant (about 81 at room temperature) and its molecules tend to combine with ions, to form hydrated ions. Both of these properties are related to the large electric dipole moment of the water molecule. 

The force of attraction or repulsion of electric charges is inversely proportional to the dielectric constant of the medium surrounding the charges. This means that two opposite electric charges in water attract each other with a force only as strong as in air (or a vacuum). It is clear that the ions of a crystal of sodium chloride placed in water could 

A related effect that stabilizes the dissolved ions is the formation of hydrates of the ions. Each negative ion attracts the positive ends of the adjacent water molecules, and tends to hold several water molecules attached to itself. The positive ions, which are usually smaller than the negative ions, show this effect still more strongly each positive ion attracts the negative ends of the water molecules, and binds several molecules tightly about itself, forming a hydrate, which may have considerable stability, especially for the bipositive and terpositive cations. 

In a crystal such as FeS04 7H20, six of the water molecules are attached to the iron ion, in the complex Fe(OH2)6 . and the seventh occupies another position, being packed near a sulfate ion of the crystal. In alum, KAl(804)2 I2H2O, six of the twelve water molecules are coordinated about the aluminum ion and the other six about the potassium ion. 

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