eli5 since diamonds are compressed carbon is it combustible? I don’t think so but why?
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Diamonds are not chemically stable at the surface of the earth in the presence of oxygen, and SHOULD react (burn; combine with oxygen to make carbon dioxide). Your question is a good one.
The bonds among the carbons in diamond are very strong, so it takes a lot of energy to break them. Once broken, the reaction of the released carbon with nearby oxygen will happen very fast. You can drive the reaction (cause diamond to burn) if you provide enough heat (enough energy to cause carbon in the diamond to break its bonds), but that much heat is pretty rare up here at the surface of the earth, so it doesn’t happen except when humans make it happen on purpose.
The basic problem is that the energy gained when a carbon combines with oxygen is not enough to make a carbon in the diamond break apart from the matrix, so even if you do manage to get a reaction started, it will not continue. The reaction stops because it does not release enough energy to keep itself going. The fire puts itself out, in a way, sort of like how trying to burn wet wood also tends to fail. Energy needed to keep the reaction going is lost in trying to heat up the fuel, unless the fuel (the diamond) is already really hot.
So, while diamond “should” react with air and make carbon dioxide, it simply doesn’t. It is what is called “metastable”, which means it is not the most stable form for the condition, but it can’t get from what it is now to that other form where it would be “happier”. It would, if it could. A big energy hill has to be crossed though, and there isn’t nearly enough energy to climb that hill at normal earth surface temperatures.
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