Compounds that, upon ionization, yield only hydrogen ions (H⁺) as cations are classified as acids; alternatively, substances that dissolve in water and are capable of releasing protons to form H₃O⁺ (hydronium ions) are also defined as acids. The higher the concentration of H₃O⁺, the stronger the acidity of the solution. Even in pure water, H₃O⁺ ions are present at a concentration of 10⁻⁷ mol/L.
Acids have a wide range of applications and are utilized in numerous industrial and laboratory settings; commonly used examples include sulfuric acid, hydrochloric acid, and nitric acid. Many chemical reactions take place in aqueous solutions, where the pH value plays a critical role. For instance, when carbon dioxide is bubbled through a solution containing Ca²⁺ ions, whether or not a precipitate of calcium carbonate forms depends on the solution's pH. Certain reactions must be conducted at a constant pH level; to achieve this, solutions containing a weak acid (or base) and its corresponding salt are frequently employed as buffer solutions. The pH of normal human blood is approximately 7.4 (containing buffer systems such as HCO₃⁻/H₂CO₃ and HPO₄²⁻/H₂PO₄⁻); even slight deviations from this value can have adverse effects on health.
