21.1. Acid-Base Definitions http://www.ck12.org
NaOH(s)
H 2 O
→Na+(aq)+OH−(aq)Sodium hydroxide is a very caustic substance also known as lye. Lye is used as a rigorous cleaner and is an ingredient
in the manufacture of soaps. Care must be taken with strong bases like sodium hydroxide, as exposure can lead to
severe burns (Figure21.4).
FIGURE 21.4
This foot has severe burns due to prolonged contact with a solution of
sodium hydroxide, also known as lye.Sodium belongs to the group of elements called the alkali metals. An alkaline solution is another name for a solution
that is basic. All alkali metals react readily with water to produce the metal hydroxide and hydrogen gas. The
resulting solutions are basic.
2K(s) + 2H 2 O(l)→2KOH(aq) + H 2 (g)Bases that consist of an alkali metal cation and the hydroxide anion are all very soluble in water. Compounds of the
Group 2 metals (the alkaline earth metals) are also basic. However, these compounds are generally not as soluble in
water. Therefore the dissociation reactions for these compounds are shown as equilibrium reactions.
Mg(OH) 2 (s)H⇀ 2 O
↽Mg^2 +(aq)+2OH−(aq)These relatively insoluble hydroxides were some of the compounds discussed in the context of the solubility product
constant (Ks p). The solubility of magnesium hydroxide is 0.0084 g per liter of water at 25°C. Because of its low
solubility, magnesium hydroxide is not as dangerous as sodium hydroxide. In fact, magnesium hydroxide is the
active ingredient in a product called milk of magnesia, which is used as an antacid or a mild laxative (Figure21.5).
Brønsted-Lowry Acids and Bases
The Arrhenius definition of acids and bases is somewhat limited. There are some compounds whose properties
suggest that they are either acidic or basic, but which do not qualify according to the Arrhenius definition. An
example is ammonia (NH 3 ). An aqueous solution of ammonia turns litmus blue, reacts with acids, and displays
various other properties that are common for bases. However, it does not contain the hydroxide ion. In 1923,
a broader definition of acids and bases was independently proposed by Danish chemist, Johannes Brønsted (1879-
1947) and English chemist, Thomas Lowry (1874-1936). ABrønsted-Lowry acidis, a molecule or ion that donates
a hydrogen ion in a reaction. ABrønsted-Lowry baseis a molecule or ion that accepts a hydrogen ion in a reaction.
Because the most common isotope of hydrogen consists of a single proton and a single electron, a hydrogen ion (in
which the single electron has been removed) is commonly referred to as a proton. As a result, acids and bases
are often called proton donors and proton acceptors, respectively, according to the Brønsted-Lowry definition. All
substances that are categorized as acids and bases under the Arrhenius definition are also defined as such under the
Brønsted-Lowry definition. The new definition, however, includes some substances that are left out according to the
Arrhenius definition.