What pH value indicates acid precipitation?

Acid precipitation is characterized by a pH value that is lower than the neutral level of 7.0, indicating higher acidity. A pH of 5.0 or lower signifies that the precipitation is more acidic than normal rainfall, which typically has a pH value around 5.6 due to the natural presence of carbonic acid formed from atmospheric carbon dioxide.

When considering various values, a pH of less than 5.0 is explicitly recognized as indicative of acid precipitation, confirming that the water contains more hydrogen ions than what is found in neutral conditions. This elevated acidity can result from various pollutants, particularly sulfur dioxide and nitrogen oxides, which combine with atmospheric moisture to form acids like sulfuric and nitric acid.

In contrast, a pH of 5.6 is indicative of normal rainfall, while pH values of 6.5 or above suggest a neutral or basic condition, far from the acidic threshold associated with acid precipitation. Thus, the value that directly reflects acid precipitation is any measurement below 5.0, substantiating the conclusion that less than 5.0 correctly identifies acid rain.