Biology (Holt)

Acids and Bases
While the bonds in water molecules are strong, at any given time a
tiny fraction of those bonds might break, forming a hydrogen ion,
H+, and a hydroxide ion, OH–:

H 2 O →H++ OH–
As a result, pure water always has a low concentration of hydro-
gen ions and hydroxide ions, which are present in equal numbers.
Compounds that form hydrogen ions when dissolved in water are
called. When an acid is added to water, the concentration of
hydrogen ions in the solution is increased above that of pure water.
In contrast, compounds that reduce the concentration of hydro-
gen ions in a solution are called .Many bases form hydroxide
ions when dissolved in water. Such bases lower the concentration of
hydrogen ions because hydroxide ions react with hydrogen ions to
form water molecules.
The pH scale shown in Figure 7measures the concentration of
hydrogen ions in solutions. All solutions have a pH value between 0
and 14. Pure water has a pH value of 7. Acidic solutions have pH val-
ues below 7, and basic solutions have pH values above 7. Each
whole number represents a factor of 10 on the scale. A solution with
a pH value of 5, for example, has 10 times as many hydrogen ions as
one with a pH value of 6.

bases

acids

SECTION 2Water and Solutions 33

Reading Effectively

As you read, you may

encounter the terms alkaline

or alkalinity.Basic solu-

tions—whose pH is above

7—are often called alkaline

solutions. Solutions with

pH values below 7 are

usually referred to as

acidic solutions.

Figure 7 The pH scale.

The pH scale measures the

concentration of hydrogen ions

in a solution.

More acidic Neutral More basic

pH 0 12 3 456 7 8 9 10 11 12 13 14

Lemon Vinegar Milk Antacid

Hand

soap

Household

ammonia

Section 2 Review

Distinguishbetween adhesion and cohesion.

Identifya substance that would not dissolve well

in water. Explain why.

Differentiatebetween acids and bases.

Critical ThinkingInferring Relationships

When salt is added to water, the freezing point of

the water decreases. Explain why this occurs.

The pH of solution Ais 2. The

pH of solution Bis 4. How does the concentration

of hydrogen ions in A([H+]A) compare with the

concentration of hydrogen ions in B([H+]B)?

A[H+]A 2 [H+]B. C[H+]A 100 [H+]B.

B[H+]B 2 [H+]A. D[H+]B 100 [H+]A.

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