CONSTANT BOILING MIXTURES (azeotropic solutions, hilotropny mixes, mixes nerazdelenno boiling) — the solutions consisting of two or more components, boiling with this external pressure and at a constant temperature without change of structure. Constant boiling mixtures cannot be divided into the making components in the way usual distillations (see).
Constant boiling mixtures were discovered by the English scientist Dalton (J. Dalton, 1810); D. P. Konovalov gave them theoretical justification (1881).
Constant Boiling mixtures differ from pure liquids in the fact that at change of external pressure at them not only temperature of boiling, but also structure changes. So, e.g., Constant Boiling mixture — water + hydrogen chloride with a pressure of 760 mm of mercury. supports 20,24% of HCl, and with a pressure of 50 mm of mercury. — 23,2%.
Constant boiling mixtures happen two types: mixes with the minimum temperature of boiling and mix with the maximum temperature of boiling. In the first case Constant Boiling mixtures boil at a temperature below temperature of boiling of low-boiling component [e.g., mix of 95,57% of alcohol (t ° kip 78,5 °) and 4,43% of water (t ° kip 100 °) boils at t ° 78,15 °]. Constant boiling mixtures of the second type boil at a temperature over temperature of boiling of high-boiling component [e.g., mix of 68% nitric to - you (t°kip 86 °) and 32% of water (t ° kip 100%) boil at t ° 120,5 °]. Ampere-second. often meet in practice. Formation of Ampere-second. (e.g., waters with alcohol or waters with nitric acid etc.) is the reason that receiving absolute alcohol or 100% nitric to - you by distillation of their aqueous solutions is impossible. Knowledge of features of properties of Ampere-second. it is necessary in laboratory practice.
Bibliography: Daniels F. and Alberti R. Physical chemistry, the lane with English, page 181, M., 1967; The Course of physical chemistry, under the editorship of Ya. I. Gerasimov, t. 1, page 200, M., 1963; Horsley L. Tables of constant boiling mixtures, the lane with English, M., 1951.
V. P. Mishin.