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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?
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.
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).
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.
Different states are best demonstrated on water. You have three different states:
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:
The portrait of different phases dependent on external conditions (f.e. pressure and temperature) is called phase diagram. This one shows phases of iron:
Image is from https://en.wikipedia.org/wiki/Allotropes_of_iron.
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.
