-具有均匀的物理和化学性质的均质部分是一个相。
-相可以是混合物（如糖水混合物），也可以是纯净物（如固态糖）。-相都是均匀的，题中所述的均相应该是指系统：有的系统是均相系统，有的是非均相系统。而非均相系统是由几个均相系统组成的（如冰水=冰+水）。（参考英文原文如下，欢迎指出不正之处）A phase may be defined as a homogeneous portion of a system that has uniform physical and chemical characteristics. Every pure material is considered to be a phase; so also is every solid, liquid, and gaseous solution. For example, the sugar–water syrup solution just discussed is one phase, and solid sugar is another. Each has different physical properties (one is a liquid, the other is a solid); furthermore, each is different chemically (i.e., has a different chemical composition); one is virtually pure sugar, the other is a solution ofH2O and C12H22O11. If more than one phase is present in a given system, each will have its own distinct properties, and a boundary separating the phases will exist, across which there will be a discontinuous and abrupt change in physical and/or chemical haracteristics.When two phases are present in a system, it is not necessary that there be a difference in both physical and chemical properties; a disparity in one or the other set of properties is sufficient. When water and ice are present in a container, two separate phases exist; they are physically dissimilar (one is a solid, the other is a liquid) but identical in chemical makeup. Also, when a substance can exist in two or more polymorphic forms (e.g., having both FCC and BCC structures), each of these structures is a separate phase because theirrespective physical characteristics differ.Sometimes, a single-phase system is termed homogeneous. Systems composed of two or more phases are termed mixtures or heterogeneous systems. Most metallic alloys and, for that matter, ceramic, polymeric, and composite systems are heterogeneous. Typically, the phases interact in such a way that the property combination of the multiphase system is different from, and more desirable than, either of the individual phases.（引用自MATERIALS SCIENCE and ENGINEERING an introduction by William D. Callister, Jr. & David G. Rethwisch, Page 300）
