For acid rain, the pH of rain water should be less than

Acid rain is defined by its pH level falling below a specific threshold. While pure water is neutral with a pH of 7.0, normal rainwater is naturally slightly acidic with a pH of approximately 5.6 [1]. This natural acidity occurs because atmospheric carbon dioxide reacts with water to form weak carbonic acid [3]. Consequently, precipitation is classified as acid rain when its pH value drops below this natural baseline of 5.6 [3]. This further acidification is primarily caused by the presence of sulfur dioxide and nitrogen oxides in the atmosphere, which react to form stronger sulfuric and nitric acids [2]. These pollutants, often from industrial sources and fossil fuel combustion, can lower the pH of rain to levels as low as 4.0 or even 3.0 in extreme cases [2].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Are plants and animals pH sensitive? > p. 26
2. [3] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 6: Environmental Degradation and Management > Acidification > p. 7
3. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 6: Geomorphic Movements > Anthropogenic Solution Weathering > p. 90