Pure water is neutral. Consisting solely of H₂O molecules, it contains no dissolved substances, minerals, or gases that would alter its chemical balance.
Understanding the pH Scale
The pH scale is a system used to measure the acidity or alkalinity of an aqueous solution. This scale ranges from 0 to 14. A pH value below 7 indicates an acidic solution, while a value above 7 signifies a basic, or alkaline, solution. A pH of exactly 7 represents neutrality.
The pH value is a logarithmic measure related to the concentration of hydrogen ions (H⁺) in a solution. Each whole number step on the pH scale represents a tenfold difference in hydrogen ion concentration. For example, a solution with a pH of 5 is ten times more acidic than one with a pH of 6.
The Neutrality of Pure Water
Pure water maintains its neutral pH of 7 due to a natural process called autoionization, or self-ionization. In this process, a very small fraction of water molecules spontaneously dissociate into ions. Two water molecules react, with one donating a proton (H⁺) to the other.
This reaction forms a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻). In pure water, the concentrations of these positively charged hydrogen ions and negatively charged hydroxide ions are exactly equal. This equal balance of H⁺ and OH⁻ ions is what defines pure water as neutral, resulting in its pH of 7.
Why Water’s pH Can Vary
While pure water is neutral, natural water sources and even tap water rarely exhibit a precise pH of 7. This variation primarily stems from the presence of dissolved impurities and gases. Carbon dioxide (CO₂) from the atmosphere is a common factor influencing water’s pH. When CO₂ dissolves in water, it reacts to form carbonic acid (H₂CO₃), a weak acid.
The formation of carbonic acid releases hydrogen ions into the water, increasing its acidity and thus lowering its pH. For instance, rainwater is naturally slightly acidic, often having a pH between 5.0 and 5.5, due to absorbed atmospheric CO₂. Other minerals, such as calcium and magnesium from geological formations, can also dissolve in water, influencing its pH. These dissolved substances can make water either harder or softer, affecting its pH.