(i) The coal and oil burned by power plants release `SO_2` into the air.

(ii) Automobile exhaust puts `NO_2` in the air.

(iii) Both `SO_2` and `NO_2` are converted to acids (`H_2SO_4` and `HNO_3` respectively) when they combine with water vapour in the presence of `O_2` in the atmosphere. These acids return to the earth as

(A) `NO + O_3 rightarrow NO_2 + O_2`

`NO_2 + O_3 rightarrow NO_3 + O_2`

`NO_2 + NO_3 rightarrow N_2O_5`

`N_2O_5 + H_2O rightarrow 2HNO_3`

(B) `2SO_2 + O_2 oversettext(no. soot or Metal ions) (rightarrow) 2SO_3`

(iv) The pure rain has a `pH` of about `5.`6 while the acid rain has `pH` below `5.6`.

(v) Acid rain is actually a mixture of `H_2SO_4` and `HNO_3` (usually `60-70%` `H2SO_4` and `30-40%` `HNO_3`)

(vi) Acid rain affects the ability of the trees to tolerate cold temperatures and the weakened trees are killed by cold conditions or become more susceptible to diseases.

(vii) Acid rain leaches lead, mercury and calcium, from the soils and rocks and discharges them into rivers and lakes. The metals may become concentrated in fish and then passed on to people through food chain.

(viii) Acid rain also damages building materials, including steel, paint, plastics, cement and marble.