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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 |
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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 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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