-具有均勻的物理和化學性質的均質部分是一個相。

-相可以是混合物（如糖水混合物），也可以是純淨物（如固態糖）。

-相都是均勻的，題中所述的均相應該是指系統：有的系統是均相系統，有的是非均相系統。而非均相系統是由幾個均相系統組成的（如冰水=冰+水）。

（參考英文原文如下，歡迎指出不正之處）

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 of
H2O 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 their
respective 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）

物理化學中相（phase）指的是體系內部物理和化學性質完全均勻的部分稱為相。相與相之間在指定條件下有明顯的介面，在介面上宏觀性質的改變是飛躍式的。體系中相的總數稱為相數，用P表示。相關概念：

1、相律，是研究相平衡的基本規律，表示平衡物系中的自由度數，相數及獨立組分數之間的關係。（1）相與相之間有介面，各相可以用物理或機械方法加以分離，越過介面時性質會發生突變。（2）一個相可以是均勻的，但不一定只含一種物質。體系的相數P∶氣體：一般是一個相，如空氣組分複雜。液體：視其混溶程度而定，可有1、2、3…個相。固體：有幾種物質就有幾個相，如水泥生料。但如果是固溶體時為一個相。固溶體：固態合金中，在一種元素的晶格結構中包含有其它元素的合金相稱為固溶體。在固溶體晶格上各組分的化學質點隨機分佈均勻，其物理性質和化學性質符合相均勻性的要求，因而幾個物質間形成的固溶體是一個相。

2、相平衡體系∶在宏觀上沒有任何物質從一相轉移到另一相的體系。