Question

What is the pH of normal rain water?

Answer 1

To know the pH of rainwater, we need to first understand the scaling of the pH scale and which numbers denote that the rain water is acidic and which numbers denote that the rain water is basic. Also, we need to know the process to determine the acidic or basic nature of the rain water.

Complete answer:
-The pH number of any solution can be ranging from $0$ to $14$ and this is the range of a pH scale. The pH values which tend to fall towards zero are acidic in nature and the pH values which tend to bend towards fourteen are basic in nature. Those lying in the centre are neutral in nature.
-Rain water incorporates and dissolves carbon dioxide from the atmosphere which reacts with the water molecules to form carbonic acid. Carbonic acid generally reacts with water and forms hydronium ions.
-That is why rainwater is slightly acidic even if the pH of pure water is $7$ (which means it is neutral). Thus, the pH of rain water is about $5$ and in volcanic areas like Hawaii, the pH of rain water ranges from $4\cdot 5$ to $5$.
-To test the acidic nature of rain water, we can use the litmus paper and notice that the litmus paper turns slightly red.

Note:
Apart from the litmus test, there exist many technologically influenced devices which are used to measure the pH and such devices also act as proof for the theoretical claims of the acidic and basic nature of a solution.