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Examples of bases include solutions of sodium hydroxide For example, hydrogen chloride dissolved in water has the fol-
(NaOH), which has a common name of lye or caustic soda, lowing reaction:
and potassium hydroxide (KOH), which has a common name
of caustic potash. These two bases are used in products known

as drain cleaners. They open plugged drains because of their HCl(l) H 2 O(l) H 3 O (aq) Cl (aq)
caustic action, turning grease, hair, and other organic “plugs”
into soap and other soluble substances that are washed away. The dotted circle and arrow were added to show that the
A weaker base is a solution of ammonia (NH 3 ), which is often hydrogen chloride donated a proton to a water molecule. The
+
used as a household cleaner. A solution of magnesium hydrox- resulting solution contains H 3 O ions and has acid properties,
+
ide, Mg(OH) 2 , has a common name of milk of magnesia and is so the solution is called hydrochloric acid. It is the H 3 O ion
sold as an antacid and laxative. that is responsible for the properties of an acid.
Many natural substances change color when mixed with The bases listed in Table 11.3 all appear to have a hydroxide
–
–
acids or bases. You may have noticed that tea changes color ion, OH . Water solutions of these bases do contain OH ions,
slightly, becoming lighter, when lemon juice (which contains but the definition of a base is much broader. A base is defined as
citric acid) is added. Some plants have flowers of one color when any substance that is a proton acceptor when dissolved in water,
grown in acidic soil and flowers of another color when grown in increasing the hydroxide ion concentration. For example, am-
basic soil. A vegetable dye that changes color in the presence of monia dissolved in water has the following reaction:
acids or bases can be used as an acid-base indicator. An indica-
tor is simply a vegetable dye that is used to distinguish between
acid and base solutions by a color change. Litmus, for example, NH 3 (g) H OH(l) (NH 4 ) OH
is an acid-base indicator made from a dye extracted from cer-
tain species of lichens. The dye is applied to paper strips, which The dotted circle and arrow show that the ammonia molecule
turn red in acidic solutions and blue in basic solutions. accepted a proton from a water molecule, providing a hydrox-
–
ide ion. The resulting solution contains OH ions and has basic
properties, so a solution of ammonium hydroxide is a base.
CONCEPTS Applied Carbonates, such as sodium carbonate (Na 2 CO 3 ), form
basic solutions because the carbonate ion reacts with water to
Cabbage Indicator produce hydroxide ions.
2– – –

To see how acids and bases change the color of certain (CO 3 ) (aq) +H 2 O(l) → (HCO 3 ) (aq) + OH (aq)
vegetable dyes, consider the dye that gives red cabbage
Thus, sodium carbonate produces a basic solution.
its color. Shred several leaves of red cabbage and boil
Acids could be thought of as simply solutions of hydronium
them in a pan of water to extract the dye. After you have a
ions in water, and bases could be considered solutions of hy-
purple solution, squeeze the juice from the cabbage into
the pan and allow the solution to cool. Add vinegar in small droxide ions in water. The proton donor and proton acceptor
amounts as you stir the solution, continuing until the color definition is much broader, and it does include the definition of
changes. Add ammonia in small amounts, stirring until the acids and bases as hydronium and hydroxide compounds. The
color changes again. Reverse the color change again by broader, more general definition covers a wider variety of reac-
adding vinegar in small amounts. Will this purple cabbage tions and is therefore more useful.
acid-base indicator tell you if other substances are acids The modern concept of acids and bases explains why the
or bases?
properties of acids and bases are neutralized, or lost, when
acids and bases are mixed together. For example, consider the
hydronium ion produced in the hydrochloric acid solution and
the hydroxide ion produced in the ammonia solution. When
EXPLAINING ACID-BASE PROPERTIES these solutions are mixed together, the hydronium ion reacts
Comparing the lists in Tables 11.2 and 11.3, you can see that acids with the hydroxide ion, and
and bases appear to be chemical opposites. Notice in Table 11.2 + –
H 3 O (aq) + OH (aq) → H 2 O(l) + H 2 O(l)
that the acids all have an H, or hydrogen atom, in their formulas.
–
In Table 11.3, most of the bases have a hydroxide ion, OH , in Thus, a proton is transferred from the hydronium ion (an acid),
their formulas. Could this be the key to acid-base properties? and the proton is accepted by the hydroxide ion (a base). Water
The modern concept of an acid considers the properties of is produced, and both the acid and base properties disappear or
+
acids in terms of the hydronium ion, H 3 O . As was mentioned are neutralized.
+
earlier, the hydronium ion is a water molecule to which an H
ion is attached. Since a hydrogen ion is a hydrogen atom with-
out its single electron, it could be considered as an ion consist- STRONG AND WEAK ACIDS AND BASES
+
ing of a single proton. Thus, the H ion can be called a proton. Acids and bases are classified according to their degree of
An acid is defined as any substance that is a proton donor when ionization when placed in water. Strong acids ionize com-
dissolved in water, increasing the hydronium ion concentration. pletely in water, with all molecules dissociating into ions.

288 CHAPTER 11 Water and Solutions 11-14