Rainwater is often considered the purest form of water available on the Earth. The evaporation of the water by the sun allows the salts and other impurities to be left behind out of the water.
Although rainwater is considered the purest form of water, it is often much less than pure when it reaches the surface of the Earth. As rain moves through the atmosphere, it picks up particles and impurities that are in the air. Chemicals and pollutants are among the impurities that can be picked up by the rain. These chemicals can drastically change the purity of water as it reaches the Earth.
Water is generally neutral on an acid-to-base scale. It is a nearly perfect seven when it is in its most natural state. Rain that has passed through many chemicals or pollutants is much less than a seven and usually lies somewhere around a five. A five is closer to the acid side of the scale and helps garnish the name for acid rain. Acid rain is not necessarily harmful immediately, but the long-term effects of acid rain caused by pollution and chemicals can be detrimental to both humans and plant life on the Earth.