A phase is a homogeneous part of a mixture: a region of uniform composition and properties separated from other regions by an identifiable boundary. The idea is useful because many systems contain several phases at once, and reactions and separations often occur at the boundaries between them. A glass of iced water contains two phases, solid ice and liquid water, with a clear boundary between them.

相是混合物中均匀的部分:组成和性质一致、并由可辨界面与其他区域分隔的区域。这一概念之所以有用,是因为许多体系同时含有多个相,反应和分离常常发生在相之间的界面上。一杯冰水含有两个相——固态冰和液态水,两者之间有清晰的界面。