Why we can't drink rainwater?

We can't drink a rain water because the scientists figured that it is acid that is bad for our health.

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Rhubarb! Yes, rain water itself is very slightly acid, because it dissolves some atmospheric carbon-dioxide into itself (look up 'karst geology' if you want to know what that can do!). However, if you worry about that tiny CO2 fraction then you would never drink any carbonated beverage, whose CO2 proportion is higher than in rain-water!