Final answer:
The pH of rainwater in an unpolluted atmosphere is closest to 5.4, slightly acidic due to the formation of carbonic acid from dissolved CO₂.
Step-by-step explanation:
The pH of rainwater falling through an unpolluted atmosphere is closest to 5.6. This slight acidity is due to the dissolution of carbon dioxide (CO₂) in the water, which forms a weak carbonic acid, shifting the pH below neutral. As pure water has a pH of 7.0, and normal rainwater is slightly acidic, the closest answer to the pH of rainwater is option b, 5.4. This is based on typical atmospheric concentrations of CO₂, leading to the formation of rainwater with a slightly acidic character.