A substance that gives rise to hydrogen ions when dissolved in water. Strictly, these ions are hydrated, known as hydroxonium or hydronium ions, and are usually given the formula H₃O⁺. An acid in solution will have a pH below 7. This definition does not take into account the competitive behavior of acids in solvents and it refers only to aqueous systems. The Lowry– Brønsted theory defines an acid as a substance that exhibits a tendency to release a proton, and a base as a substance that tends to accept a proton. Thus, when an acid releases a proton, the ion formed is the conjugate base of the acid. Strong acids (e.g. HNO₃) react completely with water to give H₃O⁺, i.e. HNO₃ is stronger than H₃O⁺ and the conjugate base NO₃⁻ is weak. Weak acids (e.g. CH₃COOH and C₆H₅COOH) are only partly dissociated because H₃O+ is a stronger acid than the free acids and the ions CH₃COO⁻ and C₆H₅COO⁻ are moderately strong bases. The Lowry–Brønsted theory is named for the English chemist Thomas Martin Lowry (1874–1936) and the Danish physical chemist Johannes Nicolaus Brønsted (1879–1947).