只有当体系中两相都存在时，在液体和气体之间才有界限分明的界面。
Only when both phases are present in the system is there a clear-cut distinction between liquid and gaseous states.
它可以减少液体、固体和气体界面间的表面或界面张力，容易在水或其他液体中混合或扩散，因而广泛应用于环境保护等领域。
It can reduce the surface tension between liquid, solid or gas. Because of being mixed or diffused easily in liquid, it has been widely used in environmental protection.
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