Some toxic gases in the atmosphere like sulpher dioxide combines with the water molecules and the ph of water decreases and makes it slight acidic. This water when comes down as rain is known as acid rain.

It causes diseases ans damages the historical monuments.
Acid rain is a rain that is unusually acidic due to the presence of acids such as sulphuric acid and nitric acid.Acid rain usually have a pH value less than 5.6.It can have harmful effects on plants,aquatic animals and infrastructure,and even human life .
When sulphuric acid and nitrogen oxide dissolve in rain water forms an acid.the acid of that rain is known as acid rain. It will damage plants,trees,habitates of animals etc,,.
the precipitation of dilute solutions of mineral acids from the atmosphere as rain , snow , fog , dew along with dust is known as acid rain.

acid rain destroys the habitats of wild animals .
it causes harm to buildings and statues made of marble , limestone etc.
it adds to the corrosion of metals.
acid rain affects vegetation by suppressing their growth.
it damages leaves of plants and trees.