Raining vinegar
In some places in world the rain water has become so polluted with chemicals that it is like vinegar. This type of rain is called acid rain.
Pure water is neutral and has a pH of 7.
Natural rain water is slightly acidic mainly because of dissolved CO2 which produces carbonic acid or H2CO3
H2O(l) + CO2(g) <==> H2CO3(aq)
The pH of unpolluted rainwater ranges from pH 5 to 6.
Acid rain is rain water with a pH of less than 5.