Water is a great mixer, always ready to dissolve all kinds of minerals and chemicals. The water from streams and rivers has seeped through the soil or run over the rocks and gathered up all kinds of impurities on the way. Rain water is made from misty little droplets in the clouds high above the ground. Up there in the clean, clear air it does not get a chance to gather up so many chemicals and other debris. It is much purer or cleaner than the water which has been on or in the ground.

However, even rain water manages to gather up a few impurities on the way down. The falling raindrops may gather a few fine fragments of dust or even salt from the clouds. A little carbon dioxide gas may be dissolved from the air by the falling drops and if the air is dirty, the rain water may contain a few specks of soot and grime when it reaches the ground. There are always a few impurities mixed with the purest of rain water.