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When a substance changes its alloptropic form, is this considered a physical change or a chemical change?

Right now, I am thinking that it would depend on what the change was. For example, if graphite was changed to diamond under high pressure, I would say it is a chemical change since new bonds are formed. However the change of iron from a body-centred cubic structure (ferrite) to a face-centred cubic structure (austenite) at high temperatures would be a physical change because only the arrangement of the atoms are changed.

Am I correct?

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  • $\begingroup$ Why do you think a physical process is not a chemical process (or vice versa). The electronic structure of iron changes between the two phases, as does the crystal structure. The same can be said for graphite to diamond. You are splitting hairs at best. So, no, I do not believe you are correct. $\endgroup$ – Jon Custer Dec 13 '16 at 23:29
  • $\begingroup$ It is a bit of a tautology, what is a chemical change. A chemical change breaks bonds therefore bonds are the forces that get broken in a chemical change. A chemical change changes the substance therefore I will define this change as not being chemical, and define these two samples as being the same substance. Chemical changes change the characteristic properties of a sample-I will call this a physical change and therfore whatever properties changed, they were not "characteristic". $\endgroup$ – Joseph Hirsch Dec 13 '16 at 23:31
  • $\begingroup$ I would call your crystal change as chemical because the densities are quite different, but there are many forms of ice that have different densities www1.lsbu.ac.uk/water/ice_phases.html and density of water changes with temperature so density must not be a characteristic propery of $\ce{H2O}$ $\endgroup$ – Joseph Hirsch Dec 13 '16 at 23:33
  • $\begingroup$ but @JonCuster is right. The separation of physical and chemical changes is a bit simplistic. We should instead perhaps ask if it is a change that will spontaneously reverse with the addition or removal of heat. Maybe we could ask whether the fundamental base unit of the substance has changed. The base unit of austenite is different from the base unit of ferrite. Not true for forms of ice. Also, are minerals pure substances or approximately pure substances. $\endgroup$ – Joseph Hirsch Dec 13 '16 at 23:37
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Well, it's a matter of choice and tradition. We traditionally classify phase transitions (such as melting of ice or evaporation of water) as physical changes. The conversion of ferrite into austenite is a phase transition, so it should formally be classified as a physical change. But yes, it's hair splitting indeed, especially when you consider the phase change from diamond (giant covalent lattice, sigma-bonds only) to fullerene (simple molecular lattice, sigma plus delocalised pi-bonds), which clearly shows some characteristics of a chemical transformation (the change in the bonding nature). Still, even such changes are formally "physical". The traditions do not go easily!

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