The pH of very pure water is 7.
Understanding pH and Pure Water
pH is a measure of how acidic or basic a solution is. It ranges from 0 to 14, with 7 being neutral. Values below 7 indicate acidity, while values above 7 indicate basicity (alkalinity).
Pure water, by definition, contains equal concentrations of hydrogen ions (H+) and hydroxide ions (OH-). This balance results in a neutral pH of 7 at standard conditions (25°C or 77°F).
The Equilibrium of Water
Water undergoes a slight auto-ionization, where it dissociates into H+ and OH- ions:
H₂O ⇌ H⁺ + OH⁻
At 25°C, the concentration of both H+ and OH- ions in pure water is approximately 1.0 x 10⁻⁷ mol/L. The pH is calculated as the negative logarithm (base 10) of the hydrogen ion concentration:
pH = -log[H+]
Therefore, for pure water at 25°C:
pH = -log(1.0 x 10⁻⁷) = 7
Temperature Dependence
It's important to note that the pH of pure water is temperature-dependent. As temperature increases, the auto-ionization of water also increases, leading to a higher concentration of H+ and OH- ions. While the concentrations of H+ and OH- remain equal (so the water remains neutral), the pH value decreases slightly. For instance, at higher temperatures, the pH of pure water might be closer to 6.5. However, even at a pH slightly below 7, the water is still considered neutral because the [H+] and [OH-] are still equal.
Why Pure Water is Neutral
The neutrality of pure water arises from the perfect balance between acidic (H+) and basic (OH-) ions. Even though the water molecules dissociate into these ions, they do so in equal amounts, resulting in a pH of 7 at standard temperature. Any slight variations from 7 are due to temperature changes affecting the equilibrium constant of water's auto-ionization.
