# Magnesium oxide dissolving in distilled water [closed]

Magnesium oxide is a salt, and salts dissolve in distilled water, due to water being polarized. Therefore I would expect that you would find free ions, which is confirmed by the water conducting electricity. But most sources I find states that magnesium hydroxide is created through a reaction with the water. I would appreciate if someone could tell me what is correct and why the other one is false.

• The first step is to write up the equations. – Karl Jan 19 '20 at 10:23

All of that is true, these facts do not contradict each other.

The missed part is quantification. Salts and generally ion compounds are soluble, but solubility comes in range of many orders of magnitude.

Also, some compounds, even if largely ionic, react with water.

So magnesium oxide reacts with water forming magnesium hydroxide, both being almost insoluble in water.

Oxide anion as a very strong Lewis base immediatelly reacts with water $$\ce{O^2- + H2O -> 2 OH-}$$. So the only chance for metal oxides to stay being oxides is to be insoluble, with their lattice energy too high to be broken by water hydration of ions.

• One question, how come the distilled water with the magnesium hydroxide dissolved in it can conduct electricity? Does the magnesium hydroxide split up into free ions? (Considering dissolving something in distilled water and seeing if it conducts electricity often is a good way to see if it is a salt) – Melvin Jan 19 '20 at 13:36
• Yes, magnesium hydroxide dissociates. But in this case, ion concentration is very low. Sodium hydroxide and oxalic acid conduct electricity very well, but are not salts. – Poutnik Jan 19 '20 at 13:40

When Melvin says that "salts dissolve in distilled water, due to water being polarized", it is by far not a general law. Some salts are soluble and some are not. Nobody is really able to predict the solubility of a given salt. There are plenty of rules describing this subject, using polarity, lattice energy, or other parameters. They are valid for plenty of compounds. But there are always strange exceptions.

A good example is the solubilities of calcium halogenides. These compounds must have similar structures. $$\ce{CaCl2}$$, $$\ce{CaBr2}$$ and $$\ce{CaI2}$$ are all extremely soluble in water, being dissolved in less that their weight of water. But $$\ce{CaF2}$$ is extremely insoluble in water, as it is one of the most important minerals on Earth to produce fluorine compounds. If $$\ce{CaF2}$$ was a tiny little bit soluble in water, this mineral would have been already washed out by the rains, and drained to the seas. It is not the case, as $$\ce{CaF2}$$ is a common mineral that can be found in many different countries.

The general theory for predicting the solubility of any compound has yet to be discovered. From time to time articles are published in papers like the Journal of Chemical Education, for explaining the solubility with new parameters like the electronegativity, the polarizability, the atomic dimensions, the hydrogen bonds, etc. They are able to explain quite a lot of compounds. But there are always exceptions.