The Phenomenon of acid rain was first introduced by Rober Angus smith in 1852. Is means presence of excessive acids in rain water?

Acid rain is a form of precipitation (rain, snow or hail) containing high levels of sulphuric and nitric acid. It is produced when sulphur dioxide (SO₂) and various nitrogen oxides (NOx) combine with atmospheric moisture and form like H₂SO₄ and HNO₃. The Ph of normal rain water is 5.6 when ph falls below this level then it becomes acid rain.

Chemical reactions of acid rain

Generally rain water contains H2SO3, H2SO4, HNO3 and H2CO3 which are formed by following reactions.

                                 _{1          NO}                                 

         (i)           SO₂ + - O₂             SO₃

                                  ^{2     root particles}

                      SO₃ + H₂O       H₂SO₄

                       SO₂ + H₂0     H₂SO₃

         (ii)           NO + O₃           NO₂ + O₂

                        NO₂ + O₃         NO₃ +O₂

                        NO₃ +NO₂         N₂O₅

                         N₂O₅ + H₂O       2HNO₃

         (iii)            CO₂ + H₂O       H₂CO₃                                  

         (iv)       Some HCI gas present in the atmosphere dissolves in moisture and produces   

         aqueous HCI.

                                 HCL (g) + H₂O     HCI (eq)

                     These Acid Droplets Are Partly Neutralized With Bases Such As Nh3 Etc.

                                 NH₃ + H+        NH⁺           

                                                        ⁴                       

Settling of dust particles containing sulphates and nitrate is known as dry deposition.

Uses of acid rain:

(i)  Emission of industries and power plant.

(ii)Automobile exhausts

(iii) Natural causes such as volcanic eruption and forest fire.

Harmful effects of acid rain

         Acid rain causes extensive damage to plants, animals, human beings, natural ecosystem, buildings etc. the damages caused by acid rain are:

1.      Crops and forests are damaged due to acidification of soil and alternative of seed germination eg.  Decline in the population of red spruce in north east is caused by acid rain.

2.      The activity of symbiotic nitrogen fixing bacteria present in nodules of leguminous family is inhibited; thereby destroying the fertility of soil.

3.      In human beings it causes burning to eyes and irritation to mucous membrane.

4.      It causes neurological diseases and can cause complication in respiratory system and digestive system.

5.      Acid rain affects buildings and structural materials. It causes intensive damage to historical, artistic buildings or monuments. Structures made of lime stone reacts with acid as follows:

                     CaCO₃ + H₂ SO₄   CaSO₄  + H₂O + CO₂

The attack on marble is called stone leprosy. The Taj Mahal in Agra is also suffering due to SO₂ and  H₂SO₄ fumes and  other pollutants released from Mathura refinery.

6.      It causes corrosion to statues, bridges, fences; railing and oil based which is costing the world 1450 million dollars a year.