Molecular formula :
nature : system (on line) have the same physical properties and chemical properties of some uniformity. The so-called uniform dispersion refers to its molecular size of magnitude. Between phase is a clear interface across this interface, there are some macroscopic properties (such as density, refractive index, composition, etc.) to mutations. For example, air (or solution) is the mixture, but internal totally homogeneous, there is a phase. Water and ice coexist, although its composition is H2O, but have different physical properties, there are two phases; Water, ice and steam are three coexisting phase. Two crystal sulfur is the same one phase, two different crystal-sulfur (such as sulfur and ramps to monoclinic sulfur) Phase 2.
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