Acids and bases have been known for a long time. When Robert Boyle characterized them in 1680, he noted that acids dissolve many substances, change the color of certain natural dyes (for example, they change litmus from blue to red), and lose these characteristic properties after coming into contact with alkalis (bases). In the eighteenth century, it was recognized that acids have a sour taste, react with limestone to liberate a gaseous substance (now known to be CO₂), and interact with alkalis to form neutral substances. In 1815, Humphry Davy contributed greatly to the development of the modern acid-base concept by demonstrating that hydrogen is the essential constituent of acids. Around that same time, Joseph Louis Gay-Lussac concluded that acids are substances that can neutralize bases and that these two classes of substances can be defined only in terms of each other. The significance of hydrogen was reemphasized in 1884 when Carl Axel Arrhenius defined an acid as a compound that dissolves in water to yield hydrogen cations (now recognized to be hydronium ions) and a base as a compound that dissolves in water to yield hydroxide anions.

Acids and bases are common solutions that exist everywhere. Almost every liquid that we encounter in our daily lives consists of acidic and basic properties, with the exception of water. They have completely different properties and are able to neutralize to form H₂O, which will be discussed later in a subsection. Acids and bases can be defined by their physical and chemical observations (Table 1).

Acids and bases in aqueous solutions will conduct electricity because they contain dissolved ions. Therefore, acids and bases are electrolytes. Strong acids and bases will be strong electrolytes. Weak acids and bases will be weak electrolytes. This affects the amount of conductivity.

The Arrhenius Definition of Acids and Bases

Historically, the first chemical definition of an acid and a base was put forward by Svante Arrhenius, a Swedish chemist, in 1884. An Arrhenius acid is a compound that increases the H⁺ ion concentration in aqueous solution. The H⁺ ion is just a bare proton, and it is rather clear that bare protons are not floating around in an aqueous solution. Instead, chemistry has defined the hydronium ion (H₃O⁺) as the actual chemical species that represents an H⁺ ion. H⁺ ions and H₃O⁺ ions are often considered interchangeable when writing chemical equations (although a properly balanced chemical equation should also include the additional H₂O). This process is represented in a chemical equation by adding H₂O to the reactants side.

[latex]\text{HCl}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\longrightarrow {\text{H}}^{\text{+}}\left(aq\right)+{\text{Cl}}^{-}\left(aq\right)[/latex]

In this reaction, hydrochloric acid (HCl) dissociates into hydrogen (H+) and chlorine (Cl⁻) ions when dissolved in water, thereby releasing H⁺ ions into solution. Formation of the hydronium ion equation:

[latex]\text{HCl}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\longrightarrow \text{H}_{3}\text{O}^{\text{+}}\left(aq\right)+{\text{Cl}}^{-}\left(aq\right)[/latex]

Classic Arrhenius acids can be considered ionic compounds in which H⁺ is the cation. Table 2 lists some Arrhenius acids and their names.

An Arrhenius base is a compound that increases the OH⁻ ion concentration in aqueous solution. Ionic compounds of the OH⁻ ion are classic Arrhenius bases (Table 3).

All of the bases listed in the table are solids at room temperature. Upon dissolving in water, each dissociates into a metal cation and the hydroxide ion.

[latex]\text{NaOH}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\longrightarrow {\text{Na}}^{\text{+}}\left(aq\right)+{\text{OH}}^{-}\left(aq\right)[/latex]

Nomenclature of Acids and Bases

Acids have their own nomenclature system based off of the type of acid, a binary acid or oxoacid.

Binary Acids

If an acid is composed of only hydrogen and one other element, the name is hydro- + the stem of the other element + -ic acid. For example, the compound HCl(aq) is hydrochloric acid, while H₂S(aq) is hydrosulfuric acid. (If these acids were not dissolved in water, the compounds would be called hydrogen chloride and hydrogen sulfide, respectively. Both of these substances are well known as molecular compounds; when dissolved in water, however, they are treated as acids.)

Oxoacids

If a compound is composed of hydrogen ions and a polyatomic anion, then the name of the acid is derived from the stem of the polyatomic ion’s name (Figure 1). Typically, if the anion name ends in -ate, the name of the acid is the stem of the anion name plus -ic acid; if the related anion’s name ends in -ite, the name of the corresponding acid is the stem of the anion name plus -ous acid. Table 4 lists the formulas and names of a variety of acids that you should be familiar with. You should recognize most of the anions in the formulas of the acids.

Figure 1. Naming Oxoacids