ABSTRACT
In aqueous solutions (i.e., solutions in water), acids are conveniently dened as substances that release hydrogen ions. Since an atom of hydrogen has only one electron, and its nucleus is a single proton, the H+ ion is the same thing as a proton. Protons will not exist by themselves in solutions. Rather, when an acidic substance such as hydrogen chloride dissolves in water the H+ ions that would be released associate with one or more water molecules, forming the H3O+ ion (called the hydronium ion). The resulting solution has the characteristics that are normally associated with an acid, such as a sour taste and the ability to corrode metals, because of the presence of H3O+ ions. Often reactions of aqueous acids are written as if the active species were H+, but this is done for convenience or simplicity and the actual acidic species is H3O+.*
The acidity (or basicity) of any solution in water can be described in terms of the pH scale (Figure 27.1). Pure water is chemically neutral, with a pH of 7.00. pH values less than 7 represent acidic solutions; the smaller the number, the greater the acidity and the higher the concentration of H+ in solution. pH values greater than 7 represent basic (alkaline) solutions; the larger the number, the greater the basicity. The pH scale is logarithmic; pH is the negative of the logarithm of the H+ ion concentration in units of moles per liter. Because of the logarithmic nature of pH, every unit in the pH scale represents a change by a factor of ten from the previous unit. For example, a solution of pH 2 is not two times as acidic as one of pH 4 (i.e., a change of 4 − 2 = 2), it is a hundred times (i.e., 102) more acidic.
