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A Closer Look

Acid Rain
cid rain is a general term used to de- pollution led to the replacement of short
Ascribe any acidic substances, wet or smokestacks of about 60 m (about 200 ft)
dry, that fall from the atmosphere. Wet with taller smokestacks of about 200 m
acidic deposition could be in the form of (about 650 ft). This did reduce the local
rain, but snow, sleet, and fog could also be levels of pollution by dumping the exhaust
involved. Dry acidic deposition could in- higher in the atmo sphere where winds
clude gases, dust, or any solid particles that could carry it away. It also set the stage for
settle out of the atmosphere to produce an longer-range transport of SO x and NO x
acid condition. and their eventual conversion into acids.
Pure, unpolluted rain is naturally There are two main reaction pathways
acidic. Carbon dioxide in the atmosphere by which SO x and NO x are converted to
is absorbed by rainfall, forming carbonic acids: (1) reactions in the gas phase and
acid (H 2 CO 3 ). Carbonic acid lowers the pH (2) reactions in the liquid phase, such as in
of pure rainfall to a range of 5.6 to 6.2. De- water droplets in clouds and fog. In the gas
caying vegetation in local areas can provide phase, SO x and NO x are oxidized to acids,
more CO 2 , making the pH even lower. A pH mainly by hydroxyl ions and ozone, and the
range of 4.5 to 5.0, for example, has been acid is absorbed by cloud droplets and pre-
measured in remote areas of the Amazon cipitated as rain or snow. Most of the nitric
jungle. Human-produced exhaust emissions acid in acid rain and about one-fourth of
of sulfur and nitrogen oxides can lower the the sulfuric acid are formed in gas-phase
pH of rainfall even more, to a 4.0 to 5.5 reactions. Most of the liquid-phase reac-
range. This is the pH range of acid rain. tions that produce sulfuric acid involve BOX FIGURE 11.1 Natural rainwater
The sulfur and nitrogen oxides that the absorbed SO x and hydrogen peroxide has a pH of 5.6 to 6.2. Exhaust emissions
produce acid rain come from exhaust (H 2 O 2 ), ozone, oxygen, and particles of of sulfur and nitrogen oxides can lower the
emissions of industries and electric utili- carbon, iron oxide, and manganese oxide. pH of rainfall to a range of 4.0 to 5.5. The
exhaust emissions come from industries,
ties that burn coal and from the exhaust These particles also come from the exhaust
electric utilities, and automobiles. Not all
of cars, trucks, and buses (Box Figure 11.1). of fossil fuel combustion.
emissions are as visible as those pictured in
The emissions are sometimes called “SO x ” Acid rain falls on the land, bodies of wa-
this illustration.
and “NO x ,” read as “socks” and “knox.” ter, forests, crops, buildings, and people. The
The x subscript implies the variable concerns about acid rain center on its envi-
presence of any or all of the oxides, for ex- ronmental impact on lakes, forests, crops, aluminum ions. The aluminum ions disrupt
ample, nitrogen monoxide (NO), nitrogen materials, and human health. Lakes in dif- the water equilibrium of fine root hairs, and
dioxide (NO 2 ), and dinitrogen tetroxide ferent parts of the world, for example, have when the root hairs die, so do the trees.
(N 2 O 4 ) for NO x . been increasing in acidity over the past Human-produced emissions of sulfur
SO x and NO x are the raw materials of 50 years. Lakes in northern New England, and nitrogen oxides from burning fossil
acid rain and are not themselves acidic. the Adirondacks, and parts of Canada now fuels are the cause of acid rain. The heavily
They react with other atmospheric chemi- have a pH of less than 5.0, and correlations industrialized northeastern part of the
cals to form sulfates and nitrates, which have been established between lake acidity United States, from the Midwest through
combine with water vapor to form sulfuric and decreased fish populations. Trees, mostly New England, releases sulfur and nitrogen
acid (H 2 SO 4 ) and nitric acid (HNO 3 ). These conifers, are dying at unusually rapid rates in emissions that result in a precipitation pH
are the chemicals of concern in acid rain. the northeastern United States. Red spruce in of 4.0 to 4.5. This region is the geographic
Many variables influence how much Vermont’s Green Mountains and the moun- center of the nation’s acid rain problem.
and how far SO x and NO x are carried in tains of New York and New Hampshire has The solution to the problem is found in
the atmosphere and if they are converted been affected by acid rain, as have pines in (1) using fuels other than fossil fuels and
to acid rain or simply return to the surface New Jersey’s Pine Barrens. It is believed that (2) reducing the thousands of tons of SO x
as a dry gas or particles. During the 1960s acid rain leaches essential nutrients, such and NO x that are dumped into the atmo-
and 1970s, concerns about local levels of as calcium, from the soil and also mobilizes sphere per day when fossil fuels are used.

and magnesium ions as water is run through it. The softener is depends on the water hardness, and each cycle can consume from
regenerated periodically by flushing with a concentrated sodium 4 to 20 lb of sodium chloride per renewal cycle. In general, water
chloride solution. The sodium ions replace the calcium and mag- with less than 75 ppm calcium and magnesium ions is called soft
nesium ions, which are carried away in the rinse water. The soft- water; with greater concentrations, it is called hard water. The
ener is then ready for use again. The frequency of renewal cycles greater the concentration above 75 ppm, the harder the water.

292 CHAPTER 11 Water and Solutions 11-18