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Overview:

As of my knowledge from metallurgical thermodynamics a phase is a thermodynamically stable and homogeneous entity made up of different species of a given concentration.

e.g = 'Austenite' is a phase of steel with a given (Fe, C) concentration (%wt). So, we can call the Iron and carbon to be species.

But we may observe that 'Austenite' is nothing but Fe&C atoms arranged in a particular structure, which happens to be FCC (Face centered cubic). In that sense, a phase is nothing but a homogeneous aggregate of species at a different length scale(a larger length scale).

My questions related to the fluid system are.

  • What's the difference between a species and a phase in a solution (in a fluid)?
  • Given a mixture of water+salt+sugar, with the conditions that 'salt' is homogeneously mixed in the water, while 'sugar' is inhomogeneously mixed in water. May you please define the phase and species respectively for this system?
  • And if what does the transport of species in a fluid phase mean?
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Air, fresh water, sea water, whiskey are fluid phases, aside of solid phases.

A phase is a space region of homogenous properties of matter, possibly with gradient of theses properties (like sea water profile, or solution of statically dissolved solid), with strong discontinuity of these properties at the region boundary.

The gradient can be at special cases questioned, as it can be property of the phase boundary, if much stronger then elsewhere. The phase interior has minimal gradient of the considered gradient.

By other words, if you underlayer water by a thick sugar solution, you will get 2 liquid phases, with more or less sharp boundary and with the property gradient becoming suddenly steep. If you let it diffuse statically, or stirr it just a little, the solution properties with get about stable gradient and the liquid can be considered a single phase.

By even other words, there are 2 phases, if you can distinguish the boundary between them. If once sharp boundary becomes diffuse, occupying both once separated phases, it forms just a single gradient phase.


Nitrogen, oxygen, argon, water, sodium chloride, calcium bucarbonate, sugar are species. Species are chemical elements or compounds, consisting of the same kind of their smallest parts - atoms, molecules, ions.


If concentration of species within a phase is not homogenous, they can be subject of redistribution via diffusion controlled by Fick's laws and convection ( thermal or mechanically forced )

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  • $\begingroup$ Is a phase-only made up of its corresponding species? Like saline, water is a phase, and its species are water and salt. Now if we somehow remove the water, does this forms a new phase that's only made up of salt $\endgroup$ – Anshuman Sinha Jan 16 at 1:26
  • $\begingroup$ @Anshuman Sinha Yes, It is and it does. If you evaporate enough water from saline, the is the phase of saturated salt solution and the phase of the solid salt. $\endgroup$ – Poutnik Jan 16 at 5:24

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