### Introduction

Acid rain is a topic of much concern in today’s world. As carbon dioxide gas, CO2, dissolves in water droplets of unpolluted air, the following reaction occurs:

${\text{C}}{{\text{O}}_{\text{2}}} + {\text{ }}{{\text{H}}_{\text{2}}}{\text{O}} \leftrightarrow {{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$

H2CO3 is a weak acid that causes the rain from unpolluted air to be slightly acidic. This source of “acid rain” is not usually considered to be a pollutant, since it is natural and usually does not alter the pH of rain water very much. Oxides of sulfur dissolve in water droplets to cause more serious problems. Sulfur dioxide dissolves to produce sulfurous acid, H2SO3, by the equation:

${\text{S}}{{\text{O}}_{\text{2}}} + {\text{ }}{{\text{H}}_{\text{2}}}{\text{O}} \leftrightarrow {{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{3}}}$

This source of sulfur dioxide can occur naturally, as from volcanic gases. More often, however, sulfur dioxide is considered a pollutant, since it is a by-product of fossil-fuel combustion.

The acidity of a solution can be expressed using the pH scale, which ranges from 0 to 14. Solutions with a pH above 7 are basic, solutions with pH below 7 are acidic, and a neutral solution has a pH of 7. In Part I of this experiment, you will study how the pH of water changes when CO2 is dissolved in water. In Part II, you will study the effect sulfuric acid has on the pH of different water types.

### Objectives

In this experiment, you will

• Measure pH.
• Study the effect of dissolved CO2 on the pH of distilled water.
• Study the effect on pH of dissolving H2SO4 in various waters.
• Learn why some bodies of water are more vulnerable to acid rain than others.