The pH of a solution is a measure of its ratio of hydrogen atoms to hydroxide radicals, which are molecules composed of one oxygen and one hydrogen atom. If the ratio is one-to-one, the solution is neutral, and its pH is 7. A low-pH solution is acidic and a high-pH solution is basic. Ideally, distilled water is neutral, with a pH of 7.
The Process of Distillation
In order to distill water, you boil it and direct the steam into a condensing coil, where it cools and returns to the liquid state. In the process, all salts and particulate matter are left behind, and the fluid collected is, in most cases, nothing but pure water. Distillation doesn't remove some solutes that evaporate along with water -- such as alcohol. If these are present, other refinement methods may be needed to purify the water.
Distilled Water Absorbs Carbon Dioxide
The pH of pure, distilled water in a vacuum is 7; it is neither acidic nor alkaline. Immediately after distillation, however, water begins absorbing carbon dioxide from the atmosphere, and it forms carbonic acid -- a weak acid. This absorption continues until the carbon dioxide concentration in the water is the same as it is in the atmosphere. At this point, the pH of the distilled water is around 5.8.