Principles and Practice of Engineering (PE) Environmental Practice Exam

The presence of carbon dioxide in surface water leads to the formation of carbonic acid. When carbon dioxide (CO2) dissolves in water, it reacts to form carbonic acid (H2CO3). The process can be represented by the following chemical equation:

CO2 (gas) + H2O (liquid) ⇌ H2CO3 (aqueous)

This reaction is significant in natural waters because carbonic acid can influence the pH of the water, which in turn affects aquatic life and the solubility of minerals. The formation of carbonic acid plays a crucial role in the carbon cycle, buffering the pH of surface waters and contributing to the overall health of aquatic ecosystems.

The other acids mentioned in the options are formed through different chemical processes and do not directly relate to the presence of carbon dioxide in water. For example, sulfuric acid is typically formed from the oxidation of sulfide minerals or from atmospheric sulfur dioxide. Acetic acid is often a product of microbial metabolism and fermentation processes, while nitric acid forms mainly from the oxidation of nitrogen compounds, specifically in the presence of atmospheric oxygen and water. Hence, they are not products of carbon dioxide in water.