Final answer:
The acidity of rain was first studied in the mid-1750s as part of broader acid-base research, using pH levels to measure its acidity. Acid rain has been an environmental issue, particularly linked to industrial activities and concentrated populations in areas like the U.S. Northeast.
Step-by-step explanation:
The acidity of rain was first studied in the mid-1750s. This discovery was part of a larger investigation into acids and bases that has historically included the work of significant figures such as Robert Boyle and Humphry Davy. Over the years, the impact of acid rain has been of great environmental concern, especially in the 1980s when it led to extensive ecological damage.
Acid rain is measured by its pH level, where pure water has a pH of 7.0, and rainwater is naturally slightly acidic with a typical pH of approximately 5.6 due to dissolved carbon dioxide forming weak carbonic acid. By 2000, the most acidic rain recorded in the U.S. had a pH around 4.3. Factors like power plants, vehicles, and industrial activities can greatly increase the acidity of rainwater, particularly in densely populated regions like the Northeastern United States.