I have a pure hunk of magnesium metal submerged in water. After several hours/days, the metal becomes coated with something.
Question 1: What is the coating?
I've been told it's magnesium oxide. This would make sense to me if it was exposed to air, but it's submerged in water. Is it still MgO? I'll assume there is enough free oxygen in the water to react slowly.
If I add an acid to the mix, the coating gradually disappears. I use some malic acid crystals. Where does the MgO go?
Question 2: What is in the water when the coating disappears?
As the coating disappears to expose the metal, the magnesium reacts more and more vigorously offgassing hydrogen, which I understand would leave magnesium malate in the water. But what about the MgO? Does the acid react with MgO? And either way, where does it go? Is the MgO just dissolved invisibly in the water or is it reacting to form something else?