The structure of liquid and phase transitions

The problem of phase transitions in liquids is considered in a general form based on the Bogolyubov-Born-Green-Kirkwood-Ivon hierarchy and the Ornstei n-Zernike equation. The concept of the "structure of liquid" is formulated; it is shown that only two types of "physical" phase transitions (both coex isting phases have macroscopic dimensions) can occur in liquids, namely, fi rst-order phase transitions and critical phenomena (second-order phase tran sitions). In addition, "chemical" phase transitions with at least one phase having microscopic dimensions and mixed-type phase transitions, in which c lusters formed in a chemical phase transition coalesce into one macroscopic phase, are possible. This is an exhaustive list of phase transition types. The properties of metastable phases are analyzed and a "single-phase" theo ry of first-order phase transitions is constructed.