Why rain water is the pure form of water explain?

It is because of the heat that evaporates only the pure water and not the salt's from the ocean

Answer:

Rainwater is not necessarily pure even though it starts out as "distilled" water vapor. Each rain drop starts out by condensing around a bit of dust or pollen, it then absorbs gases from the atmosphere (carbon dioxide for all rain and sulfurous and nitrous oxides for acid rain). Once falling it washes out dust and other particulates suspended in the air (some rains are red from dust they pick up)