Pure water is neutral, defined by a pH of 7 at standard temperatures. It consists only of H₂O molecules, containing no dissolved minerals, salts, or atmospheric gases that could alter its chemical balance. Neutrality is the foundation of the pH scale, providing a reference point for measuring the acidity or basicity of other aqueous solutions.
Defining Neutrality on the pH Scale
The pH scale is a logarithmic measure used to specify the acidity or basicity of a water-based solution, ranging from 0 to 14. This scale is directly linked to the concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻). Solutions with a pH below 7 are acidic, indicating a higher concentration of hydrogen ions. Conversely, solutions with a pH above 7 are basic, or alkaline, meaning the concentration of hydroxide ions outweighs that of hydrogen ions. Neutrality is the point where the concentration of H⁺ ions is exactly equal to the concentration of OH⁻ ions, which corresponds to a pH of 7 at \(25^\circ\text{C}\).
The Self-Ionization of Water
Pure water maintains its neutral state through autoionization, or self-ionization, a reversible chemical reaction. In this process, two water molecules react, and one donates a proton to the other. This proton transfer results in the constant, simultaneous formation of a hydronium ion (H₃O⁺, often simplified as H⁺) and a hydroxide ion (OH⁻).
The reaction ensures that for every hydronium ion created, one hydroxide ion is also created, maintaining an exact one-to-one ratio of acidic and basic components. This dynamic equilibrium is governed by the ion product constant for water, known as \(K_w\). At \(25^\circ\text{C}\), \(K_w\) is approximately \(1.0 \times 10^{-14}\). This means the concentration of both H⁺ and OH⁻ ions is \(1.0 \times 10^{-7}\) moles per liter, and the negative logarithm of this concentration mathematically defines the pH of 7.
While pH is generally defined as 7 at neutrality, this value shifts with temperature because \(K_w\) changes. For example, at \(100^\circ\text{C}\), pure water’s neutral pH is \(6.14\) because the autoionization reaction produces more ions at higher temperatures. Despite the change in the numerical pH value, the water remains chemically neutral because the concentrations of H⁺ and OH⁻ ions are still equal.
Factors Affecting Real-World Water pH
While pure H₂O is always neutral, the water encountered in daily life is almost never exactly pH 7 due to external contaminants. The most common factor is the absorption of atmospheric carbon dioxide (CO₂). As CO₂ dissolves into water, it reacts to form a weak acid called carbonic acid (H₂CO₃).
The formation of carbonic acid releases additional hydrogen ions into the solution, which lowers the pH below 7. This is why even pristine rainwater typically registers a slightly acidic pH, often falling between 5.5 and 6.5. Other dissolved substances, such as salts and minerals found in groundwater or tap water, can also shift the pH balance.
The presence of dissolved minerals like calcium and magnesium, or pollutants from industrial runoff, further contributes to variations in pH. These impurities introduce ions that disrupt the balance of H⁺ and OH⁻ concentrations that characterizes pure water. Therefore, the pH of 7 serves as a theoretical and scientific benchmark for water entirely free of foreign substances.