BASES ========================================================================= | SUBSTANCE | PERCENTAGE DISSOCIATION IN | 0.1 EQUIVALENT SOLUTION _____________________________________________|___________________________ | KOH, NaOH | 86 | Ba(OH)_{ 2} | 75 | NH_{ 4}OH | 1.4 | =========================================================================
SALTS ========================================================================= | TYPE OF SALT | PERCENTAGE DISSOCIATION IN | 0.1 EQUIVALENT SOLUTION _____________________________________________|___________________________ | R^{ +}R^{ -} | 86 | R^{ ++}(R^{ -})_{ 2} | 72 | (R^{ +})_{ 2}R^{ --} | 72 | R^{ ++}R^{ --} | 45 | =========================================================================
The percentage dissociation is determined by studying the electrical conductivity of the solutions and by other physico-chemical methods, and the following general statements summarize the results:
!Salts!, as a class, are largely dissociated in aqueous solution.
!Acids! yield H^{ +} ions in water solution, and the comparative !strength!, that is, the activity, of acids is proportional to the concentration of the H^{ +} ions and is measured by the percentage dissociation in solutions of equivalent concentration. The common mineral acids are largely dissociated and therefore give a relatively high concentration of H^{ +} ions, and are commonly known as "strong acids." The organic acids, on the other hand, belong generally to the group of "weak acids."
!Bases! yield OH^{ -} ions in water solution, and the comparative strength of the bases is measured by their relative dissociation in solutions of equivalent concentration. Ammonium hydroxide is a weak base, as shown in the table above, while the hydroxides of sodium and potassium exhibit strongly basic properties.
Ionic reactions are all, to a greater or less degree, !reversible reactions!. A typical example of an easily reversible reaction is that representing the changes in ionization which an electrolyte such as acetic acid undergoes on dilution or concentration of its solutions, !i.e.!, HC_{ 2}H_{ 3}O_{ 2} <--> H^{ +} + C_{ 2}H_{ 3}O_{ 2}^{ -}. As was stated above, the ionization increases with dilution, the reaction then proceeding from left to right, while concentration of the solution occasions a partial reassociation of the ions, and the reaction proceeds from right to left. To understand the principle underlying these changes it is necessary to consider first the conditions which prevail when a solution of acetic acid, which has been stirred until it is of uniform concentration throughout, has come to a constant temperature. A careful study of such solutions has shown that there is a definite state of equilibrium between the constituents of the solution; that is, there is a definite relation between the undissociated acetic acid and its ions, which is characteristic for the prevailing conditions. It is not, however, assumed that this is a condition of static equilibrium, but rather that there is continual dissociation and association, as represented by the opposing reactions, the apparent condition of rest resulting from the fact that the amount of change in one direction during a given time is exactly equal to that in the opposite direction. A quantitative study of the amount of undissociated acid, and of H^{ +} ions and C_{ 2}H_{ 3}O_{ 2}^{ -} ions actually to be found in a large number of solutions of acetic acid of varying dilution (assuming them to be in a condition of equilibrium at a common temperature), has shown that there is always a definite relation between these three quantities which may be expressed thus: