Page 47 - Environment: The Science Behind the Stories

ion concentration. Thus, a substance with a pH
of 6 contains 10 times as many hydrogen ions
as a substance with a pH of 7 and 100 times as
many hydrogen ions as a substance with a pH of 8.
Figure 2.6 shows pH for a number of common
Ice
substances.
Most biological systems have a pH between 6
and 8, and substances that are strongly acidic (bat-
tery acid) or strongly basic (sodium hydroxide)
are harmful to living things. The acidification of
soils and water from acid rain (pp. 491–493) and
from acidic mine drainage (p. 657) are examples
of how pH changes caused by human activities
can affect ecosystems. The oceans worldwide are
Liquid water also turning more acidic as seawater absorbs excess
Figure 2.5 Ice floats in liquid water because ice is less carbon dioxide in the air from fossil fuel emissions
dense. In ice, molecules are connected by stable hydrogen (pp. 446–447, 506).
bonds, forming a spacious crystal lattice. In liquid water, hydrogen
bonds frequently break and reform, and the molecules are closer
together and less organized. Matter is composed of organic
and inorganic compounds
ion (OH ). The product of hydrogen and hydroxide ion concen- Beyond their need for water, living things also depend on
–
trations is always the same; as one increases, the other decreases. organic compounds, which they create and of which they are
Pure water contains equal numbers of these ions. Solutions in created. Organic compounds consist of carbon atoms (and gen-
which the H concentration is greater than the OH concentra- erally hydrogen atoms) joined by covalent bonds, and they
+
–
tion are acidic, whereas solutions in which the OH concentra- may also include other elements, such as nitrogen, oxygen,
–
tion exceeds the H concentration are basic, or alkaline. sulfur, and phosphorus. Inorganic compounds, in contrast,
+
The pH scale (Figure 2.6) quantifies the acidity or alka- lack carbon–carbon bonds.
linity of solutions. It runs from 0 to 14; pure water has a Carbon’s unusual ability to bond together in chains, rings,
hydrogen ion concentration of 10 and a pH of 7. Solutions and other structures to build elaborate molecules has resulted
–7
with a pH less than 7 are acidic, and those with a pH greater in millions of different organic compounds. One class of such
than 7 are basic. Because the pH scale is logarithmic, each compounds that is important in environmental science is
step on the scale represents a 10-fold difference in hydrogen hydrocarbons, which consist solely of bonded atoms of carbon
and hydrogen (although other elements may enter these com-
pounds as impurities). Fossil fuels and the many petroleum
products we make from them (Chapter 19) consist largely of
14 NaOH (sodium hydrocarbons.
hydroxide)
13 Basic The simplest hydrocarbon is methane (CH ), the key
4
component of natural gas; it has one carbon atom bonded to
12
Ammonia four hydrogen atoms (Figure 2.7a). Adding another carbon
11 atom and two more hydrogen atoms gives us ethane (C H ),
6
2
10 Soft soap the next-simplest hydrocarbon (Figure 2.7b). The smallest (and
9
8 Seawater H H
pH 7 Neutral Pure water H H H H C C C C C H
6 H C H H C C H
Normal rainwater C C C
5 H H H H C C H
4 Acid rain H H
3
Lemon juice (a) Methane, (b) Ethane, (c) Naphthalene,
2 CH 4 C 2 6 C 10 8
H
H
Stomach acid
1 Acidic
Car battery acid Figure 2.7 Hydrocarbons have a diversity of chemical
0
structures. The simplest hydrocarbon is methane (a). Many
Figure 2.6 The pH scale measures how acidic or basic hydrocarbons consist of linear chains of carbon atoms with hydro-
(alkaline) a solution is. The pH of pure water is 7, the midpoint gen atoms attached; the shortest of these is ethane (b). Volatile
of the scale. Acidic solutions have a pH less than 7, whereas basic hydrocarbons with multiple rings, such as naphthalene (c), are
46 solutions have a pH greater than 7. called polycyclic aromatic hydrocarbons (PAHs).

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