Final answer:
Acid rain is precipitation with a pH below 5.6. Normal rain is slightly acidic due to the presence of carbon dioxide.
Step-by-step explanation:
Acid rain is precipitation with a pH below 5.6.
The lower a substance's pH, the more acidic it is.
Pure water has a pH of 7.0. However, normal rain is slightly acidic due to the presence of carbon dioxide (CO₂), which forms weak carbonic acid and gives it a pH of approximately 5.6.
As of 2000, the most acidic rain falling in the U.S. has a pH of about 4.3.