Deionized water is water that has been passed through resins that remove electrically charged material from the water. If it has not come into contact with carbon dioxide, the pH of deionized water is 7. Its pH is approximately 5.6 if it has come into contact with atmospheric carbon dioxide.
Deionized water is pure. Therefore it tends to pick impurities, such as carbon dioxide, fast from the atmosphere. When this happens, the pH of water falls. Deionization systems do not remove molecular material from the product water, such as sugar and other organic matter. However, most of them have an activated charcoal column that removes such material.