Question

what happened if PH value
of rainwater is less than 5.5

Answer 1

Then it will become acid rain.

Answer 2

Rain water containing sulphuric acid has lower pH than rain water containing carbonic acid because sulphuric acid is a strong acid and carbonic acid is a weak acid. Presence of carbonic acid in rain water can lower pH to a minimum of 5.6 whereas sulphuric acid can lower pH below 5.