Before you decide if distilled water or deionized water is the best choice, you need to understand the differences between the two. Deionized (DI) water is water that has been treated to remove all ions – typically, that means all of the dissolved mineral salts. Distilled water has been boiled so that it evaporates and then re-condensed, leaving most impurities behind.
Distillation is one of the oldest methods for creating pure water. Filtered water is heated until it has evaporated, turning into steam. This steam is collected in a sterile container, where it condenses and becomes water again. Because water has a lower boiling point than most contaminants (including minerals), they are left behind when the water turns into steam. The resulting water is, therefore, very pure. In addition, some water is double or triple distilled, with the condensed water being boiled and condensed a second or third time.
Organic materials and inorganic minerals are the most common impurities found in water. The organics can typically be removed via filtering methods, including physical filters, carbon filters, and reverse osmosis (RO) membranes. After this pre-treatment, the water is sent through a DI system, which contains two types of resin: cation and anion. These two resins attract positive and negative ions, respectively, replacing them with H+ and OH-. H+ combined with OH- becomes H2O – water. The combination of filters and DI resins can remove nearly all contaminants.