Definition: a substance used to show visually by change of color the acidity or basicity of an aqueous solution.
pH Hydrion paper RED indicates an acidic solution, i.e.,a pH of less than 5; ORANGE indicates a weakly acidic or neutral solution, i.e., a pH between 6 and 7; GREEN indicates a weakly basic or neutral solution, i.e., a pH between 7 and 8; BLUE indicates a basic solution, i.e., a pH of greater than 9. Litmus paper RED indicates an acidic solution, i.e., a pH of less than 6; BLUE indicates a basic solution, i.e., a pH of greater than 8.