Carbon dioxide is an essential component of atmosphere. However, when the concentration of `CO_2` increases in the atmosphere, it can lead to global warming through green house effect. Similarly, rain water is an essential source of water. It has pH of 5.6 due to the formation of `H^+` ions from the reaction of rain water with `CO_2` present in the atmosphere. When pH of the rain water falls below 5.6 due to the presence of acids in the atmosphere, it has very damaging effects.
Why does the rain water normally have a pH of about 5.6 ?

Explanation

Normally rain water has pH of about 5.6 due to the dissolution of `CO_2` of the atmosphere in it.
`H_2O (l) + CO_2 (g) iff H_2CO_3 (aq)`
`H_2CO_3 (aq) iff 2H^(+)(aq) + CO_3^(2-) (aq)`