Acid Deposition and
Its Effects on Materials
As we have seen, much of the rain, mist, fog, and snow in the United States is more
acidic than would be expected from carbon dioxide dissolution alone. In the worst
cases, rain, fog, and dew can have a pH of 3.0 or lower. Does this matter? To
answer this question, we need to know something about the effects of acid deposition
and how serious they are.
National studies can help us. During the 1980s the U.S. Congress funded a
national research effort called the National Acid Precipitation Assessment Program
(NAPAP). Over 2000 scientists were involved, with a total expenditure of $500 million.
The project was completed in 1990, and the participating scientists prepared a 28-volume
set of technical reports (NAPAP, State of the Science and Technology, 1991). Some of
the material in the remainder of this chapter is drawn from the NAPAP Report. Other
information is drawn from the proceedings of a conference in 2001 entitled “Acid Rain:
Are the Problems Solved?” sponsored by the Center for Environmental Information.
The purpose of that conference was “to put the acid rain problem squarely back on the
The forefront of the public agenda.”
And we agree—that acid rain should remain on the public agenda. One reason is the
damage that acid rain causes (Table 6.2). In this section, we describe the effects of acid
rain on metals, statues, and buildings. The effects of acid deposition on living things
are explored in the section that follows.
Recall from Chapter 1 that about 80% of the elements on the periodic table are
metals. Many of these metals are vulnerable to damage from acid precipitation. Metals
typically are shiny and silvery; well, before they become tarnished or rusted
Although acid rain (pH 3–5) does not affect all metals, unfortunately, iron is one
that it does. Bridges, railroads, and vehicles of all kinds depend on iron and the steel
that is made from it. Rods of steel is used to strengthen concrete buildings and roadways.
In many parts of the country, decorative iron fences and latticework both ornament
and protect city and rural homes.
The problem with iron is that it rusts, as represented by this chemical equation.
4 Fe(s) _ 3 O2(g) 2 Fe2O3(s) [6.26]
Rusting is a slow process. Iron combines rapidly with oxygen only if you heat or ignite
it, such as with a sparkler or fireworks. But at room temperature, the rusting of an iron is
a two-step process that requires the presence of hydrogen ions. Equation 6.26 is the
overall equation for the process. The role of H 1 is evident in equation 6.27, the fi rst
step of the process. In this step, the iron metal dissolves.
4 Fe(s) _ 2 O2(g) _ 8 H_(aq) 4 Fe2_(aq) _ 4 H2O(l) [6.27]
Even pure water (pH 5 7) has a sufficient concentration of H 1 to promote slow rusting.
In the presence of an acid, the rusting process is greatly accelerated. In the second step,
the aqueous Fe 21 further reacts with oxygen.
4 Fe2_(aq) _ O2(g) _ 4 H2O(l) 2 Fe2O3(s) _ 8 H_(aq) [6.28]
Adding the two steps together gives equation 6.26. The solid product, Fe 2 O 3, is the
familiar reddish-brown material that we call rust.
Because iron is inherently unstable when exposed to the natural environment,
billions of dollars are spent annually to protect exposed iron and steel in bridges,
cars, buildings, and ships. Paint is the most common means of protection, but even
paint degrades, especially when exposed to acidic rain and gases. Coating iron with
a thin layer of a second metal such as chromium (Cr) or zinc (Zn) is another means
of protection.
Automobile paint can be spotted or pitted by acid deposition. To prevent this
damage, automobile manufacturers now use acid-resistant paints. It is ironic that
automobiles emit the very chemical that mars their paint. Follow the NO from your
car’s tailpipe and you may find that its reaction product (nitric acid) eventually ends
up in droplets hitting the hood of your car.
Acidic rain also damages statues and monuments made of marble. For example,
those in the Gettysburg National Battlefield have suffered irreparable damage. Figure 6.22
shows a recognizable, but much-deteriorated statue of George Washington in New York
City. Marble limestone, composed mainly of calcium carbonate, CaCO 3, slowly dissolves
in the presence of hydrogen ions.
CaCO3(s) _ 2 H_(aq) Ca2_(aq) _ CO2(g) _ H2O(l) [6.29]
Visitors to the Lincoln Memorial in Washington, D.C., learn that the huge stalactites
growing in chambers beneath the memorial is the result of acid rain eroding
the marble, again a material containing either calcium carbonate or magnesium carbonate
(or both). Other monuments in the eastern United States are suffering similar
fates. Some limestone tombstones are no longer legible. Worldwide, many priceless
and irreplaceable marble statues and buildings are being attacked by airborne acids
(Figure 6.23). The Parthenon in Greece, the Taj Mahal in India, and the Mayan ruins
at Chichen Itza, all show signs of acid erosion. Ironically, some of the acid deposition
at these sites is due to the NO x produced by the tour buses and vehicles with minimal
emissions control