the ph scale measures how acidic an object is. objects that are not very acidic are called basic. the scale has values ranging from zero (the most acidic) to 14 (the most basic). pure water has a ph value of 7; this value is considered neutral - neither acidic or basic. normal, clean rain has a ph value of around 5.5, which is slightly acidic. however, when rain combines with sulfur dioxide or nitrogen oxides - produced from power plants and automobiles - the rain becomes much more acidic.