Is rain always fresh water or is it sometimes salt water?

It is fresh. Because salt does not evaporate into the air. But there can be other elements in rain other than water because the rain can dissolve gases, dust, volcanic ash or smoke as it falls. Rain is normally slightly acidic as it dissolves carbon dioxide. Acid rain is due to the dissolving of oxides of nitrogen or sulfur and has a pH less than around 5.5.