why is unpolluted rain acidic?

Answer 1

even if the air isnt polluted, there is still carbon dioxide and water vapor in the air. When carbon dioxide(CO2) and water(H2O) react in air, they form Carbonic Acid (H2CO3). Since it is an acid, it is acidic

Answer 2

CO2 dissolved in the rain gives carbonic acid.
Or look at this below
Carbonic acid and rain water

A solution of carbon dioxide in water in equilibrium with the atmosphere (0.033% CO2) has a pH of 5.6. Rain water is normally not quite saturated in CO2, and has a pH of around 6 in the absence of atmospheric pollutants. This effect is separate from the phenomenon of acid rain, where industrial pollutants such as sulfur dioxide dissolve in rain water and lower its pH drastically. However, the acidity of rain water has important geological consequences for carbonate rocks such as chalk and limestone. An equilibrium is established between the calcium carbonate of the rock and calcium bicarbonate in solution:

CaCO3 + CO2 + H2O ⇌ Ca(HCO3)2

This can erode underground caverns around fault lines which water runs down. As the calcium-rich water evaporates, the calcium carbonate precipitates, often as stalactites and stalagmites. Water drawn from chalk aquifers contains dissolved calcium carbonate, and is described as "hard".

Answer 3

It dissolves carbon dioxide from the atmosphere to form carbonic acid.