Phases of matter/Talk

Is the density difference between oil and water really considered a phase difference?

Yes, if you add a third component, for example an alcohol, the alcohol will distribute itself between both phases, such ::that you have a specific phase equilibrium with different compositions in each phase

--Matt Stoker

Perhaps a more exact definition of phase would be helpful. For instance, I had always attributed the oil/water thing to

the fact that neither is soluable in the other and that they had a density difference. Is there some special difference

between the states of the molecules in different phases?

I think the definition is adequate, the oil and water phases each have unique chemical compositions and physical properties (eg, density, viscosity, etc...). In addition they are separated from each other by a boundary surface. But if you have a better definition or can reword the current definition to make it more clear then go for it! --Matt Stoker

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Could the person who wrote about polymorphism check the entry on allotropes. I have never heard the term polymorph so I can't tell if they are the same. --rmhermen

It appears that Allotropy and Polymorphism are definitely similar, but the term allotropy is usually reserved for pure elemental solids. From W.D. Callister, Materials Science and Engineering: An Introduction, John Wiley & Sons, Inc. 1991.

Allotropy - The possibility of existence of two or more different crystal structures for a substance (generally an elemental solid) (glossary).

Polymorphism - The ability of a solid material to exist in more than one form or crystal structure (glossary).

Some metals, as well as nonmetals, may have more than one crystal structure, a phenomenon known as polymorphism. When found in elemental solids, the condition is often termed allotropy.(Text on page 38)

--Matt Stoker

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This article seems to possibly contradict the one on allotropy -- which says that different physical states (phases) are not allotropes; but here it says the allotropes are different physical states. Is a phase a broader term than allotrope, or what? -- Simon J Kissane