Essentials of Ecology

SUPPLEMENT 6 S41

Hydrochloric

acid (HCl)

1

0 2 3 4 5 6 7 8 9

10

11

12

13

14

Gastric fluid

(1.0–3.0)

Lemon juice,

some acid rain

Vinegar, wine,

beer, oranges

Tomatoes

Bananas

Black coffee

Bread

Typical rainwater

Urine (5.0–7.0)

Milk (6.6)

Pure water

Blood (7.3–7.5)

Egg white (8.0)

Seawater (7.8–8.3)

Baking soda

Phosphate detergents

Bleach, Tums

Soapy solutions,

Milk of magnesia

Household ammonia

(10.5–11.9)

Hair remover

Oven cleaner

Sodium hydroxide (NaOH)

100

10 –1

10 –2

10 –3

10 –4

10 –5

10 –6

10 –7

10 –8

10 –9

10 –10

10 –11

10 –12

10 –13

10 –14

Figure 5 The pH scale, representing the concentration of hydrogen ions (H) in one liter

of solution is shown on the righthand side. On the left side are the approximate pH values

for solutions of some common substances. A solution with a pH less than 7 is acidic, one

with a pH of 7 is neutral, and one with a pH greater than 7 is basic. A change of 1 on the

pH scale means a tenfold increase or decrease in H concentration. (Modified from Cecie

Starr, Biology: Today and Tomorrow, Pacific Grove, Calif.: Brooks/Cole, © 2005)

shows the approximate pH and hydrogen
ion concentration per liter of solution for
various common substances. Figure 14 on
p. S9 of Supplement 2 shows how the pH of
precipitation varies in the lower U,S. states as a
result of acidic air pollutants discussed in detail
on pp. 479–480.

THINKING ABOUT

pH

A solution has a pH of 2. How many times more

acidic is this solution than one with a pH of 6?

There Are Weak Forces
of Attraction between
Some Molecules

Ionic and covalent bonds form between the ions
or atoms within a compound. There are also
weaker forces of attraction between the mol-
ecules of covalent compounds (such as water)
resulting from an unequal sharing of electrons
by two atoms.
For example, an oxygen atom has a much
greater attraction for electrons than does a
hydrogen atom. Thus, in a water molecule the
electrons shared between the oxygen atom and
its two hydrogen atoms are pulled closer to the
oxygen atom, but not actually transferred to the
oxygen atom. As a result, the oxygen atom in
a water molecule has a slightly negative partial
charge and its two hydrogen atoms have a
slightly positive partial charge (Figure 6).
The slightly positive hydrogen atoms in one
water molecule are then attracted to the slightly
negative oxygen atoms in another water mol-
ecule. These forces of attraction between water
molecules are called hydrogen bonds (Figure 6).

H H

δ+

δ+

δ+ δ+

δ+

δ+ δ+

δ+

δ+

δ+ δ+

δ+

δ−

δ−

δ−

δ−

δ−

δ−

δ−

Slightly

positive

charge

Slightly

negative

charge

Hydrogen

bonds

O

O

O

O

O

O

O

H

H

H

H

H

H H

H

H

H

H

H

Figure 6 Hydrogen bond: slightly

unequal sharing of electrons in the

water molecule creates a molecule

with a slightly negatively charged

end and a slightly positively charged

end. Because of this electrical po-

larity, the hydrogen atoms of one

water molecule are attracted to

the oxygen atoms in other water

molecules. These fairly weak forces

of attraction between molecules

(represented by the dashed lines) are

called hydrogen bonds.