What is naturally occurring pure rainwater?

Pure rainwater is weakly acidic with a pH near 5.6 because dissolved carbon dioxide forms carbonic acid.

This mild acidity comes only from atmospheric carbon dioxide dissolving into the drops.

It is not weakly alkaline, as no natural base raises its pH above neutral.

It is not neutral, since the carbonic acid keeps it slightly below pH 7.

It is not very acidic, that level appears only in polluted acid rain carrying sulphuric and nitric acids.

Rain is termed acid rain when its pH falls below about 5.6.