Acidity

Acidity is a chemical property describing the tendency of a substance to donate protons or otherwise increase the hydrogen ion activity in solution. In the Arrhenius framework, an acid is a substance that yields hydrogen ions (H+) when dissolved in water, forming hydronium ions (H3O+). The Brønsted–Lowry concept generalizes acidity to substances that donate protons to a base, while the Lewis definition extends it to electron-pair acceptance. Acidity is distinct from concentration and from whether a substance is a strong or weak acid.

Measuring acidity typically involves the pH scale, which indicates hydrogen ion activity; solutions with pH below 7 are acidic. Acidity can also be described by titratable acidity, determined by titration with a base to a specified endpoint and expressed in equivalents of a given acid per liter or per mass of sample. The strength of an acid relates to its equilibrium constant, not merely its concentration.

Common acids include hydrochloric, sulfuric, and acetic acids. Acidity plays roles in chemistry, biology, food, and industry. In biology, stomach acid (hydrochloric acid) aids digestion. In foods, organic acids such as citric, malic, and lactic acids contribute tartness and preservation. In industry, acids drive many reactions and processing steps.

Environmental and agricultural contexts address acidity in soils and waters. Soil acidity affects nutrient availability and microbial activity, often managed by liming. Acid deposition from pollution can lower rain and surface water pH, impacting ecosystems. Safety concerns relate to corrosivity and handling of strong acids.