In my textbook called General Chemistry, I could find the solubility of certain substances in water but corundum was not one of them. I've also read some web pages claiming that corundum is insoluble in water. I also read in my textbook Nelson Chemistry 11 that nothing is totally insoluble in water and I believe what Nelson Chemistry 11 said is right. Since corundum is not totally insoluble in water, I want to know what its solubility is at SATP.

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    $\begingroup$ Fine, not perfectly insoluble - you are right, entropy overcomes enthalpy at some point. Just really, really, really not very soluble. As in, unmeasurably small... $\endgroup$ – Jon Custer Aug 19 '17 at 16:26
  • $\begingroup$ Surely, it's solubility can be predicted by quantum mechanical theory even if it's too small to be measured. Predicting it from entropy sometimes gives a really accurate approximation but not everything can be correctly predicted from entropy because according to my answer at physics.stackexchange.com/questions/10690/…, the second law of thermodynamics probably doesn't always hold. $\endgroup$ – Timothy Aug 19 '17 at 16:41
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    $\begingroup$ Of course it can be predicted from thermodynamics - just crank through the Gibbs free energies and see where equilibrium would be. And, given the 10's of kJ of difference in free energies, it will be really really really really small. As in, unmeasurably small, regardless of your feelings of the second law. I'll continue to believe in Gibbs free energies, regardless of some random answers on SE. $\endgroup$ – Jon Custer Aug 19 '17 at 16:48

Whoa there, bucky! What do you mean by "soluble"? As a thought experiment, imagine an enormous space filled with a liquid (of course, doesn't have to be a liquid; a solid or a gas would work as well...). This should, for simplicity, be a single chemical compound, perhaps water if you like. Now imagine in this that I inject a molecule of .... of anything. My question is: is this molecule dissolved or not? Is it soluble? Or imagine a slab of some solid and a ultra-high pressure (hypersonic) jet of water impinging on it (see https://en.wikipedia.org/wiki/Water_jet_cutter#Waterjet). And say I find traces of the solid in the water I collect. Is that proof that I've dissolved some of the solid? When you get down to the details, then it turns out that the details matter. Someone worried about contaminating his or her analytical results by using an alumina crucible has a different concern than someone studying the effects of water percolation on geological processes over millions of years. You're not going to get a single method to measure solubility to satisfy all parties. I'm a bit worried about your statement that you believe your textbook. Well, Reagan said "Trust, but verify." It is undoubtedly true that a student (whether of Religion or Science) has to rely on Authority to learn - there just ain't enough hours in the day to do otherwise (nor days in your life). The difference between Science and the rest of human knowledge is that we claim that we can back-up our claims by evidence, and not only that but that if you want to verify our claims, then you can (given the time and resources) verify it for yourself. (here, I should insert something about "objectivity", but I'll ignore that.) Anyway, if your text book didn't provide a citation to that claim, and if your textbook did not derive that conclusion based on well accepted physical/chemical laws and the rules of mathematics and logic, then while you may believe it, or not it is just a statement of faith. I hope I've shown you that Nelson's statement is at best misleading (if taken out of its context) and at worst just plain wrong. Ask yourself this: how would he know? Do you think he, or anyone else for that matter, has measured the solubility of all known compounds in water? How about a compound that hasn't been made yet? Is it even possible to measure the solubility of a compound (polymer) with a Molecular Mass of, say, 1E40 grams? (calculate that out in terms of volume, why don't you)....Actually, I agree with him. For all practical purposes, if I mix into water a compound X, as a liquid, gas, or finely divided solid and let it reach equilibrium and then remove the water, separating the physically distinct phases (gas, liquid or solid) and analyze the water, and if my detection techniques are sensitive enough, then I'll virtually always detect some X (as long as we're talking about Xs with fairly low MWs). As far as whether that X has dissolved or been eroded from the surface, well, that isn't easily established. (And of course, you should know that the surface of a solid generally has a different chemical structure and/or composition than the bulk (as does a liquid). So we go down another rabbit hole....

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  • $\begingroup$ The movement of water and corundum atoms is chaotic. Very rarely, a surface corundum atom will be moving fast enough to be ejected from the surface. I'm not sure whether it's more common for a single atom to get ejected or for An aluminum and an adjacent oxygen atom to get ejected in pairs and then pull a hydrogen atom from a water molecule making it dissolve as aluminum hydroxide but its stable solution concentration occurs the concentration is so low that atoms are attaching to the surface at the same rate as they're getting ejected from it. $\endgroup$ – Timothy Aug 20 '17 at 0:48

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