Question

why do rain water become impure?

Answer 1

there are large amount of gases and dust particles

Answer 2

This is completely pure water in its original form, without any modifications made to it. Such types of water can be used to purify and cleanse oneself for ritual activities. For instance all of the 7 types of water mentioned above are pure water and are purifiers. One can use such water to remove literal impurity, and purify oneself for worship.