How is rain slightly acidic?

1 Answer
Simon Moore
May 8, 2018

It combines with carbon dioxide in the atmosphere to form carbonic acid......

Explanation:

Leaving aside the issue of "acid rain" caused by pollution in the atmosphere, natural "clean" rain is also slightly acidic because the water is able to mix with and absorb carbon dioxide naturally occurring in the atmosphere.
Carbon dioxide forms carbonic acid when mixed with water.
This is a weak acid (i.e. not strongly ionized) but will still give rain a pH below 7 (typically between 5 and 6).

#H_2O (l) + CO_2 (g) ⇌ H_2CO_3 (aq)#

Carbonic acid then ionizes (albeit weakly):

#H_2O (l) + H_2CO_3 (aq) ⇌ HCO_3^− (aq) + H_3O^+ (aq)#

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