Tap water may have a variety of dissolved substances including naturally occurring substances as well as those added by the “water company”. Distilled water has only trace amounts of materials that may come from the still itself. Both tap water and distilled water will have some dissolved gases. Any gas in the air, which is in contact with the water, can be dissolved in the water, although, most gases in the air aren’t all that soluble in water. The two most important gases which are found in water are carbon dioxide and oxygen. Of the gases in air, CO2 is the most soluble in water, which is why both natural water and distilled water are slightly acidic. CO2 + H2O <==> H+ + HCO3- Oxygen is slightly soluble in water, enough so that fish and other aquatic life can extract enough to stay alive.