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I mean in principle, in general.

How can we tell if some phenomena was a chemical reaction(s)? Change in physical properties cannot be since large-scale structural change can change physical properties as well, like heat treatment of steel. And what about changing diamond into graphite by heating. Hence, assuming there are only chemical and physical properties, only a change in chemical properties implies chemical reaction. But how do we know if chemical properties changed? Observing chemical reactions of the result(s). But how do we know what happens is a chemical reaction? A different instance of the same problem, which we started with.

How can anyone sure claim that a chemical reaction happened, or equivalently that some sample is a different substance and not just an allotrope or has larger "grains" for example?

Maybe mass or energy change can rule out those cases?

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    $\begingroup$ Welcome to chemistry.SE! If you had any questions about the policies of our community, please ‎visit the help center. || I know there was little actual experiments going on before Boyle, so the scope of this question might be somewhere between Boyle and Dalton, or maybe Faraday. IIRC, many believed that when you see a change in odor, color or luster, there was a chemical reaction going on. Since the elements' idea was not really established, it's hard to reckon whether they actually recognized the products. $\endgroup$ – M.A.R. Jul 25 '15 at 14:35
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    $\begingroup$ It's a difficult, quite possible impossible, question. First you have to say what, exactly, you mean by chemical properties and a chemical reaction. You indicate the problems, but to play devils advocate and take deliberately extreme examples I've seen A+BC(v=0)->A+BC(v=1) where v shows the vibrational quantum number called a reaction. Or does forming a clathrate count as a chemical reaction? Or an endo-fullerene compund? And how about the difference between chemi- and physisorbtion. And formation of charge transfer complexes, is that a reaction? It's all too tricky - I'm off too the pub! $\endgroup$ – Ian Bush Jul 25 '15 at 16:08
  • $\begingroup$ For your level it would suffix to say that you start off with some chemicals and after they react that you end up with different chemicals. // Chemical reactions do not create mass differences (I won't go much much deeper and confuse you...). So you start off with 100 grams of chemicals (reactants) and you end up with 100 grams of chemicals (products). $\endgroup$ – MaxW Feb 18 '17 at 19:12
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For some chemical reactions, it would be very difficult to prove without chemical analysis of the materials before and after the reaction. However, with modern analysis techniques, you could show whether a reaction took place. For example, if you tempered steel or converted diamond to graphite, analysis would show that it was a physical change or change in the crystal structure that occurred.

For a low-tech example, consider the combustion of some condensed-phase hydrocarbon in oxygen. If you ignite the hydrocarbon under oxygen in a container sealed except for an oxygen source and an exit to a condenser, you would see the hydrocarbon vanish and water would appear in the condenser. For this we'll just ignore carbon dioxide production. You then easily confirm that the water is not just another form of the hydrocarbon by measuring it's freezing and boiling temperatures. You could then repeat the experiment without ignition of the hydrocarbon and see that you get nothing. This would confirm beyond any reasonable doubt that you've had a chemical reaction that converted the hydrocarbon to water.

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