Distilled water is created by boiling water into steam and condensing it back into a liquid, separating water molecules from nearly all dissolved solids and impurities. This purification results in a high-purity substance often used in laboratories, medical settings, and household appliances. This unique chemical profile raises the question of whether it is acidic, alkaline, or neutral, which depends on how its properties are measured and how environmental factors alter its chemistry.
What the pH Scale Measures
The pH scale, which stands for the potential of Hydrogen, is used to classify water and other liquids. It quantifies the concentration of hydrogen ions (H⁺) in a solution to determine its acidity or basicity. The scale is logarithmic, meaning a change of one whole number represents a tenfold change in acidity or alkalinity.
The pH range spans from 0 to 14. A value below 7.0 indicates an acidic solution (higher H⁺ concentration), while solutions above 7.0 are alkaline or basic (higher OH⁻ concentration). A reading of exactly 7.0 represents a perfectly balanced, neutral solution. This scale only applies to aqueous solutions, where water is the solvent.
Why Pure Distilled Water is Neutral
Theoretically, perfectly distilled water isolated from the atmosphere is perfectly neutral, resulting in a pH of exactly 7.0. Distillation effectively removes ions, salts, minerals, and other dissolved impurities that act as sources of acidity or alkalinity. Without these substances, the water consists only of H₂O molecules.
Water molecules naturally undergo auto-ionization, dissociating into equal, extremely small concentrations of hydrogen ions (H⁺) and hydroxide ions (OH⁻). Because these ion concentrations are perfectly balanced, the resulting pH is 7.0, defining the pure liquid as chemically neutral. This baseline purity is why distilled water is used in scientific experiments requiring a consistent, unreactive baseline.
The Practical Reality of Distilled Water’s pH
While pure distilled water is neutral, the liquid rarely maintains a perfect pH of 7.0 in practice. As soon as it is exposed to air, it immediately begins to absorb atmospheric carbon dioxide (CO₂). This absorption is the primary reason distilled water tests as slightly acidic when purchased or freshly made.
Once dissolved, carbon dioxide reacts with water molecules to form carbonic acid (\(H_2CO_3\)). This acid releases additional hydrogen ions into the solution, lowering the pH from its theoretical neutral point.
In practical conditions, the pH of distilled water typically settles between 5.5 and 6.5. The lack of dissolved solids also means distilled water has very little buffering capacity, making its pH highly susceptible to minor environmental changes. However, for almost all non-laboratory applications, distilled water is still considered effectively neutral because this slight change does not impact its intended use.