Distilled water
Distilled water is water that has been purified by boiling it into vapor and condensing the vapor back into liquid in a separate container. Any impurities in the original water, such as non-volatile or mineral components, that do not boil below or near the boiling point of water remain in the original container. For example, water escaping as steam from a boiler of heating system or steam engine, leaves behind any dissolved materials, which leads to mineral deposits known as boiler scale.
In general, non-purified water can leave behind mineral deposits, or could cause or interfere with chemical reactions. Distillation is a method for removing impurities from water and other fluids. Distilled water has not been proven to be healthier for drinking than hard water.