Acid rain forms when water reacts with sulfur dioxide or nitrogen dioxide.
Explanation:
Acid rain forms when water molecules in the atmosphere react with sulfur dioxide or nitrogen oxides to form an acidic compound. The resulting compound has a lower pH.
The chemical reactions that occur when water molecules mix with sulfur dioxide and nitrogen dioxide are below:
SO2+12O2+H2O→H2SO4SO2+12O2+H2O→H2SO4^-$$
Nitrogen oxides can be produced naturally by lightening strikes and sulfur dioxides is formed when volcanoes explode, but acid rain can also be caused by emissions due to humans.