- 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
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
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
I agree that the wording on these two pages needs to be clarified. First, Allotropy usually refers to forms of pure elements and not other substances, in which case it is called polymorphism. Second, for the case of solids, each allotrope forms a unique phase. This is because each allotrope has distinct physical properties, so a mixture of two allotropes would not have homogenous physical properties, but would consist of distinct regions with properties corresponding either to one form or the other. Therefore, at least for the case of solids, each allotrope is a unique phase. For the case of a gas it gets more confusing. For example, according to the article H and H2 are allotropes. However, in this case since both are in the gas phase, a mixture of the two would constitute a single phase (gases always mix freely to form a single phase).
I think the real distinction is that allotropy refers to differences in the chemical bond structure between the atoms. Physical phase changes from solid to liquid to gas, do not affect the chemical bond structure, so two such phases are not allotropes. However, changes from one solid form (i.e. graphite) to another solid form (i.e. diamond) does change the chemical bonding between atoms, so in this case each form constitutes both an allotrope and a unique phase.
I hope the above discussion makes sense, I will try to incorporate some of the above into the Allotropy page. -- Matt Stoker
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