Dissociation

Acids are molecules that can donate protons (hydrogen or H⁺ ions) to other molecules. An alternate view is that an acid is a substance that will cause an increase in the concentration of hydrogen ions in a solution.

The dissociation of a strong acid (such as hydrochloric acid, HCl) is essentially 100%.

In this case, nearly every HCl molecule is dissociated (separated into ions). When any substance dissociates, both positive and negative ions will be formed. In this case, the positive ion (cation) is a proton, and the negative ion (anion) is the chloride ion. A strong acid is a strong electrolyte and a good conductor of an electric current. In the case of a strong base, nearly 100% of the molecules are dissociated as well, and strong bases (such as sodium hydroxide, NaOH) are also strong electrolytes.

A weak acid, such as hydrofluoric acid is only slightly dissociated. Many more of the molecules exist in the molecular (undissociated or unionized) form than in the ionized form. Since it forms fewer ions, a weak acid will be a weak electrolyte.

In the case of a weak base, such as aluminum hydroxide, Al(OH)_3, only a small percent of molecules ionize, producing few ions, and making weak bases weak electrolytes as well.

Figure 1. Illustration of the solvation process, in which the negative end of a water molecule faces the positive sodium ion and the positive end faces the negative ion. Illustration by Hans & Cassidy. Courtesy of Gale Group.

In any dissociation reaction, the total charges will mathematically cancel each other out. The case above has a positive three charge on the aluminum ion and a negative one charge on each of the three hydroxide ions, for a total of zero.