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CHEMISTRY IN FOCUS
Gum That Foams
Mad Dawg chewing gum is a practical joker’s
dream come true. It is noticeably sour when someone first starts to chew it, but the big surprise
comes about ten chews later when brightly colored
foam oozes from the person’s mouth. Although the
effect is dramatic, the cause is simple acid–base
chemistry.
The foam consists of sugar and saliva churned
into a bubbling mess by carbon dioxide released
from the gum. The carbon dioxide is formed when
sodium bicarbonate (NaHCO3) present in the gum
is mixed with citric acid and malic acid (also present in the gum) in the moist environment of the
mouth. As NaHCO3 dissolves in the water of the
saliva, it separates into its ions:
react. In the manufacture of the gum, colorings and
flavorings are applied as aqueous solutions. The
water caused the gum to explode in early attempts
to manufacture it. The makers of Mad Dawg obviously solved the problem. Buy some Mad Dawg
and cut it open to see how they did it.
NaHCO3 1s2 ¡ Na 1aq2 HCO3 1aq2
H2O
The bicarbonate ion, when exposed to H ions
from acids, decomposes to carbon dioxide and
water:*
H 1aq2 HCO3 1aq2 S H2O1l2 CO2 1g2
The acids present in the gum also cause it to be
sour, stimulating extra salivation and thus extra
foam.
Although the chemistry behind Mad Dawg is
well understood, the development of the gum into
a safe, but fun, product was not so easy. In fact,
early versions of the gum exploded because the
acids and the sodium bicarbonate mixed prematurely. As solids, citric and malic acids and sodium
bicarbonate do not react with each other. However,
the presence of water frees the ions to move and
Chewing Mad Dawg gum.
*This reaction is often used to power “bottle rockets” by adding
vinegar (dilute acetic acid) to baking soda (sodium bicarbonate).
c. HCl and H2O are not a conjugate acid–base pair because they are not
related by the removal or addition of one H. The conjugate base of HCl
is Cl. The conjugate acid of H2O is H3O. ■
Example 15.2 Writing Conjugate Bases
Write the conjugate base for each of the following:
a. HClO4
b. H3PO4
c. CH3NH3
Solution
To get the conjugate base for an acid, we must remove an H ion.
a. HClO4 S H ClO4
Acid
Conjugate base
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