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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?

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    $\begingroup$ We don't.$\;\!$ $\endgroup$ – Ivan Neretin Jul 21 '18 at 20:43
  • $\begingroup$ Ok In Gibbs Phase rule I have C-P+2.Now for a reaction which has HBr and say a gasesous organic compound giving a single liquid product then why don't I use 3 in place of C during the reaction?Hence HBr and the gaseous compounds are treated as just one phase in the formula.Explain this problem. $\endgroup$ – gateprep Jul 21 '18 at 20:49
  • $\begingroup$ Because the third component is determined by the first two via the equilibrium constant. As for HBr and the gaseous compounds, they are treated as one phase because they are one phase. $\endgroup$ – Ivan Neretin Jul 21 '18 at 21:09
  • $\begingroup$ Aren't they having the same state of matter?And please elaborate on the equilibrium constant. $\endgroup$ – gateprep Jul 21 '18 at 21:35
  • $\begingroup$ @IvanNeretin Gibbs rule is not applicable except for equilibrium.Why then do we apply it even before the reaction starts/during the reaction/after the reaction?Secondly u said HBr and gaseous compounds are in same phase will that be the case if one of them would have been liquid? $\endgroup$ – gateprep Jul 22 '18 at 10:12
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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.

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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.

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  • $\begingroup$ What about two gases? $\endgroup$ – gateprep Jul 22 '18 at 5:47

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