Squeeze a lemon, and the sharp tang on your tongue is citric acid at work. Open a can of soda, and the fizzy bite comes from carbonic acid formed when carbon dioxide dissolves in water. That burning sensation in your stomach when you are stressed? Hydrochloric acid, the powerful digestive acid your stomach produces at a pH of about 1.5 — acidic enough to dissolve a nail, yet your stomach lining regenerates fast enough to protect itself. Acids are defined by their willingness to release hydrogen ions (H⁺, essentially bare protons) into solution. The more readily they do this, the stronger the acid.
Acids range from mild to terrifyingly powerful. Acetic acid in vinegar (about 5% concentration) is safe to drizzle on your salad. Citric acid gives oranges and lemons their flavor. But sulfuric acid (H₂SO₄), the most produced chemical in the world at over 260 million tonnes per year, is viciously corrosive and can char organic matter on contact. Hydrofluoric acid (HF), though technically a weak acid by chemistry's definition, can dissolve glass and is one of the most dangerous substances in any laboratory. The strength of an acid depends not on how corrosive it is, but on how completely it dissociates — strong acids like HCl split entirely into ions, while weak acids like acetic acid only partially dissociate.
The pH scale, running from 0 to 14, measures how acidic or basic a solution is, with 7 being neutral. Each step on the scale represents a tenfold change in hydrogen ion concentration: a solution at pH 3 is ten times more acidic than one at pH 4, and a hundred times more acidic than pH 5. Acid rain, caused by sulfur dioxide and nitrogen oxides in polluted air dissolving in rainwater, typically has a pH around 4.2 — enough to damage marble statues, erode buildings, and harm aquatic ecosystems. Understanding acids is essential for chemistry, biology, environmental science, and everyday life.