The theoretical pH of distilled water is 7.0. In practice, however, most distilled water will have a pH that is slightly acidic (less than 7.0) due to the presence of carbon dioxide (CO2) that is absorbed from the atmosphere. Dissolved carbon dioxide reacts slowly with water to give the bicarbonate and hydronium ions.
(carbonic acid, H2CO3, is only formed in strongly acid solutions). During distillation, the dissolved CO2 will be driven out of the liquid. However, during condensation the water will re-absorb the CO2 again resulting in a pH that is less than 7.0.