Pure water can act as both an acid and a base. This characteristic, known as amphoterism, is important for understanding water’s behavior. Its ability to donate or accept protons makes it fundamental in acid-base chemistry.
Understanding Acids and Bases
Acids and bases are defined by their ability to donate or accept protons. A Brønsted-Lowry acid is any substance that can donate a proton (H⁺ ion). Conversely, a Brønsted-Lowry base is any substance that can accept a proton. An acid-base reaction involves the transfer of a proton from an acid to a base.
Water’s Unique Amphoteric Nature
Water’s amphoteric nature arises from autoionization, where two water molecules react. One water molecule donates a proton to another, forming a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻). This is represented by: 2H₂O ⇌ H₃O⁺ + OH⁻.
The water molecule that donates a proton acts as a Brønsted-Lowry acid, while the water molecule that accepts the proton acts as a Brønsted-Lowry base. This autoionization results in the constant, though small, presence of both hydronium and hydroxide ions. At 25°C, the concentrations of these ions in pure water are equal, both at 1.0 x 10⁻⁷ mol/L.
The pH Scale and Water’s Neutrality
The pH scale measures the concentration of hydronium ions in a solution, indicating its acidity or alkalinity. This scale ranges from 0 to 14. A pH value below 7 indicates an acidic solution with a higher concentration of hydronium ions. A pH value above 7 indicates a basic (alkaline) solution, characterized by a higher concentration of hydroxide ions.
Pure water is neutral and has a pH of 7 at 25°C. This neutrality occurs because the autoionization process produces equal concentrations of hydronium (H₃O⁺) and hydroxide (OH⁻) ions.
Water in Everyday Life
The pH of water encountered daily often differs from pure water due to external factors. Dissolved minerals, such as limestone, can influence water’s pH, making it more alkaline. Carbon dioxide (CO₂) from the atmosphere dissolves in water to form carbonic acid, which can lower the pH of rainwater and surface water.
Human activities and pollutants also affect water pH. Industrial waste and agricultural runoff can introduce chemicals that alter pH levels. Acid rain, caused by sulfur dioxide and nitrogen oxides, contains sulfuric and nitric acids, leading to pH values below 5.6. Ocean water pH is also decreasing due to the absorption of atmospheric CO₂, a phenomenon known as ocean acidification.