It is often difficult to meet the experimental requirements by distilling water alone. Double-distilled water, triple-distilled water, and deionized water are needed in many experiments. Volatile impurities in water can be removed by distilling water for many times, but dissolved gaseous impurities (Mn2 +, Cu2 +, Zn2 +, Fe3 +, Mo (â…¥)) can not be completely removed.