AQUEOUS SOLUTIONS: AN INTRODUCTION
Approximately three fourths of the earth’s surface is covered with water. The body fluids
of all plants and animals are mainly water. Thus we can see that many important chem-
ical reactions occur in aqueous (water) solutions, or in contact with water. In Chapter 3,
we introduced solutions and methods of expressing concentrations of solutions. It is useful
to know the kinds of substances that are soluble in water, and the forms in which they
exist, before we begin our systematic study of chemical reactions.
1 Electrolytes and Extent of Ionization
Solutes that are water-soluble can be classified as either electrolytes or nonelectrolytes.
Electrolytesare substances whose aqueous solutions conduct electric current. Strong
electrolytesare substances that conduct electricity well in dilute aqueous solution. Weak
electrolytesconduct electricity poorly in dilute aqueous solution. Aqueous solutions of
nonelectrolytesdo not conduct electricity. Electric current is carried through aqueous
solution by the movement of ions. The strength of an electrolyte depends on the number
of ions in solution and also on the charges on these ions (Figure 4-2).
4-2
4-2 Aqueous Solutions: An Introduction 129Figure 4-2 An experiment to demonstrate the presence of ions in solution. Two copper
electrodes are dipped into a liquid in a beaker. When the liquid contains significant
concentrations of ions, the ions move between the electrodes to complete the circuit (which
includes a light bulb). (a) Pure water and sugar are nonelectrolytes. (b) A solution of a weak
electrolyte, acetic acid (CH 3 COOH); it contains low concentrations of ions, and so the bulb
glows dimly. (c) A solution of a strong electrolyte, potassium chromate (K 2 CrO 4 ); it contains
a high concentration of ions, and so the bulb glows brightly.
(a) (b) (c)H 2 O molecule Sugar molecule Hydrogen ion,
H+Acetic acid molecule,
CH 3 COOHAcetate ion,
CH 3 COO–Potassium ion,
K+Chromate ion,
CrO 4 2–