The pH of pure water at room temperature is exactly 7. Since a small amount of atmospheric carbon dioxide dissolves in water and forms carbonic acid, distilled water is often slightly acidic, having a pH of approximately 5 to 6.
The pH of water is also temperature-dependent. At its freezing point, the pH of water is 7.47. At its boiling point, the pH of water is 6.14. This is because the dissociation of water into protons and hydroxide ions is an endothermic process. Finally, the presence of dissolved acids and bases strongly affects the pH of water. For example, a 1-molar solution of hydrochloric acid in water has a pH of zero.