The pH of normal, unpolluted rain is approximately ______.

Normal, unpolluted rain typically has a pH of approximately 5.6 due to the presence of dissolved carbon dioxide, which forms a weak acid (carbonic acid) when it reacts with water. This natural acidity occurs because carbon dioxide is a common gas in our atmosphere, and its dissolution in rainwater creates a slightly acidic environment, which is important for the overall ecosystem.

In contrast, a pH of 7 indicates a neutral solution, which is not characteristic of rainwater because it does not reflect the natural acidity from atmospheric gases. A pH lower than 5.6, such as 4, would indicate more acidic conditions often caused by pollution and the presence of other acid-forming compounds from human activities. A pH of 8.1 would signify a basic solution, which is also not typical for rainwater. Understanding the pH of rain is crucial for assessing the health of ecosystems and the impacts of air quality on environmental conditions.