I would be happy if someone explains me why the word "phase" and "state of matter" is used so interchangeably.Lets say we have HBr and another compound in the system both gases.Then am I to take two phases on a single phase when I plug in the Gibbs formula in Phase rule?
closed as off-topic by Mithoron, Todd Minehardt, a-cyclohexane-molecule, A.K., Tyberius Jul 23 '18 at 4:08
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From the wikipedia page for phase (matter)
a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform [...] The term phase is sometimes used as a synonym for state of matter, but there can be several immiscible phases of the same state of matter.
A simple description is that a phase is a region of material that is chemically uniform, physically distinct, and (often) mechanically separable. In a system consisting of ice and water in a glass jar, the ice cubes are one phase, the water is a second phase, and the humid air is a third phase over the ice and water. The glass of the jar is another separate phase.
In the above system you can see that the ice and the glass jar are the same state of matter, but are two distinct phases. It's the same with oil and water. So, people who are being precise don't use the two terms interchangeably.
We don't. Phase is more specific. A couple examples of the differences:
1) A slab of steel is heated in a furnace in preparation for being hot rolled to a strip. The iron that predominates forms a face-centered cubic phase at the temperature in the furnace, about 1250°C. By the time the slab has been rolled to a strip and cooled to a temperature, usually somewhere between 500 and 800°C, the steel has switched to the other low-pressure solid phase of iron, which is body-centered cubic. The phases are different, but the slab/strip never melts in this process, they are both in the solid state of matter. Often a single substance, like iron or water, forms more than one solid phase.
2) Water and oil are poured together into a glass. Only a very limited amount of water can dissolve into the oil and vice versa, so they remain separate phases -- indeed physically separated by Earth's gravity, as the oil phase in its liquid state of matter is less dense than the water phase in its liquid state of matter. When we have two or more substances the liquid state of matter, despite its freedom of molecular motion, may split into more than two phases.