Phase (fluid)

A physical substance consisting of one or more chemical components which share the same volume, same boundary with other phases and move with the same macroscopic velocity.

The contact between two phases provides the interface for exchanging of chemical components to reach the chemical equilibrium specific to a given temperature, pressure and the total mass of the components in all phases.

The total mass of chemical components in all adjacent phases remain constant over time but at certain temperature and pressure values some phases may disappear (due to migration of their chemical components to the bordering phases).

Phases are not always continuous. For example, gas bubbles are not connected to each other but may move at the same speed which is different to the hosting liquid speed and they do represent a separate phase with a inter-phase contact surface comprised of a sum of all bubbles' surfaces.