The Acid Rain Problem. Carbon dioxide in the air can dissolve in rain water to form carbonic acid, H 2 CO 3. CO 2 + H 2 O H 2 CO 3. Carbonic acid is a weak acid. It partially ionises to form hydrogen ions. H 2 CO 3 H + + HCO 3 -. The hydrogen ions from carbonic acid give natural rain water a slightly acid pH value of 5.6.