McGraw-Hill Connect Biology – Air Quality SmartBook Practice Test 2026 - Free Biology Practice Questions and Study Guide

Normal, unpolluted rain typically has a pH of about 5.6, which indicates that it is slightly acidic. This acidity is primarily due to the presence of dissolved carbon dioxide in the atmosphere, which reacts with water to form carbonic acid. This natural process results in a lower pH compared to pure water, which has a neutral pH of 7.

Understanding the pH scale is crucial; it ranges from 0 to 14, where values below 7 are considered acidic, and values above 7 are alkaline. Since normal rainwater has a pH of 5.6, it falls into the acidic category but is not highly acidic compared to more polluted rain, which can have much lower pH values due to the presence of sulfuric and nitric acids from industrial pollution. This slight acidity in normal rain is an essential aspect of the natural water cycle and helps maintain ecosystem balance.