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What is the difference between state of matter and phase? This site said that:

Phases are different from states of matter. The states of matter (e.g., liquid, solid, gas) are phases, but matter can exist in different phases yet the same state of matter.

I don't really understand this. Could anyone start from scratch and explain this? Or send me a link to go to?

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    $\begingroup$ Well the answer is a bit more complicated than either of the two answers below. For example we'd say that wood is a solid, but it can never have a solid phase. If you have a oil and vinegar salad dressing which is mixed into an emulsion we'd say that the salad dressing is a liquid, but it has two liquid phases. $\endgroup$ – MaxW Oct 29 '15 at 22:53
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A phase is a region of space where all physical and chemical properties are uniform. This means that for example the density, the chemical composition and the temperature is equal everywhere in the region. This will also mean that the state of matter (solid, liquid, gas, etc.) is equal everywhere.

A state of matter is a form that matter can take. Examples are solid, liquid, gas, plasma. There are some others that appear under extreme conditions.

For example, nitrogen and water at 50K are both solid, so they have the same state of matter. However the chemical composition is not the same and hence they don't have the same phase.

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  • $\begingroup$ So oil and water would be different phases because they have different chemical compositions? (Even though both are liquid). $\endgroup$ – Joe Academia Oct 4 '15 at 10:23
  • $\begingroup$ @JoeAcademia Yes: It's because they have different compositions, but also that even when you put them in contact with each other, they don't merge together into one indistinguishable liquid with a single composition. $\endgroup$ – hBy2Py Oct 29 '15 at 23:24
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    $\begingroup$ "A phase is a region of space where all physical and chemical properties are uniform." this isn't absolutely true. Think of a metal alloy with two components. As the metal solidifies the chemical composition can change. $\endgroup$ – MaxW Oct 29 '15 at 23:35
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    $\begingroup$ @MaxW: Metal alloys are great example for more phases in one state of matter. Steel is probably the most studied example. Of course, the composition of phases changes during annealing, quenching, and tempering, but the resulting (meta-)stable piece of iron is a multi-phase solid. $\endgroup$ – ssavec Jul 13 '16 at 18:40
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Vegetable oil such as olive oil is a liquid. Water is also a liquid. Thus, vegetable oil and water have the same "state".

But, as anyone who has tried to mix them together knows, they aren't the same "phase".

You could also add liquid mercury to the system, and probably also a fluorinated liquid such as perfluorodecalin. But the different liquid phases would not mix, despite all being the same state (liquid).

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  • $\begingroup$ This isn't really true. In air, I seal a 1 liter bottle with 250 ml of water and 250 ml of benzene. and shake it thoroughly. I let it sit. I now have 3 phases. (1) A gas phase with air/benzene/water (2) A liquid phase with mostly benzene and a little water and (3) a liquid phase with mostly water and a little benzene. So it is wrong to say that benzene and water don't mix - the implication being at all. $\endgroup$ – MaxW Oct 30 '15 at 0:40
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    $\begingroup$ I stand by my answer. I think you are drawing extreme implications from it and then attacking those implications. What sentence in my answer do you think is not true? $\endgroup$ – Curt F. Oct 30 '15 at 9:33
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The definition of both state and phase depend highly on the context of the discussion. Knowing the context of the discussion is the responsibility of anyone involved in the discussion.

State generally refers to a condition which the matter is exposed. When exposed to certain conditions matter may take on a particular form, which the form is sometimes referred to as a state depending on the context of the discussion.

Phase when discussing chemistry generally refers to portions of matter which are physically distinctive. Phase may also refer to a state within a cycle depending on the context of the discussion.

Phase and state can be completely synonymous, again, depending on the context of the discussion.

Example:

  • A chemist working on a batch reactor may refer to the state as the condition of the reactor and refer to the phase of the matter in the reactor.

  • Another chemist working on a similar batch reactor may refer to the phase of a given sequence in the reactor's operation and refer to the state of the matter in the reactor.

  • Yet another chemist with a similar batch reactor may interchangeably refer to the 'state' or 'phase' of matter in the reactor.

Alone, both 'state' and 'phase' can be ambiguous; however, this ambiguity can be avoided through clear communication of the context of the discussion.

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Intuitive answer

Different states are best demonstrated on water. You have three different states:

  • Solid = ice
  • Liquid = water
  • Gas = water vapour

Phase is more general term. These three states are also three phases, but you can have different phases within one state too!

For example iron has different properties under different conditions. Under atmospherical pressure it has these phases dependent on temperature:

  • $\alpha$-iron: from -273 K to 912 K
  • $\gamma$-iron: from 912 K to 1394 K
  • $\delta$-iron: from 1394 K to 1539 K

The portrait of different phases dependent on external conditions (f.e. pressure and temperature) is called phase diagram. This one shows phases of iron:

Pure iron phase diagram

Image is from https://en.wikipedia.org/wiki/Allotropes_of_iron.

Additional information

I think states and phases has good definition on Wikipedia.

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Below is a definition from the IUPAC Gold Book for a liquid crystal.

mesomorphic phase
Also contains definition of: nematic state Often formed by concentrated systems of surfactants. These are states of matter in which anisometric molecules (or particles) are regularly arranged in one (nematic state) or two (smectic state) directions, but randomly arranged in the remaining direction(s).

It seems that state and phase have specific definitions as well as the common understanding of solid, liquid and gas with interchangeable use of states or phase. The context is all.

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