# Why is Mg(OH)2 sparingly soluble in water but soluble in NH4Cl

• Is $$\ce{Mg(OH)2}$$ sparingly soluble in water because it's hydration enthalpy is not greater than its lattice enthalpy?
• Also, is it soluble in $$\ce{NH4Cl}$$ because it is a stronger base than $$\ce{NH4(OH)}$$ hence it displaces Ammonia and since it is a gas the equilibrium reaction goes forward as the gas is removed?
• Ammonia is basically infinitely soluble in water, and magnesium hydroxide is more soluble in an acidic medium. – Zhe Jan 26 at 13:25

Probably it won't (fully) answer your question, but I spotted a mistake: NH4OH is the common way you use to describe an aqueous solution of ammonia. Therefore, when you write your equations, you consider is as ammonia, not as ammonium hydroxide: it acts as a base. On the other hand, NH4Cl is ammonium chloride, therefore it acts as an acid.

This explains why Mg(OH)2 is more soluble in NH4Cl than NH4OH: in the first case, you mix a (weak) base and a (weak) acid, in the second one you mix two (weak) bases.

With respect to your first question, keep in mind that when the hydration (solvation) energy overcomes the lattice energy the compound is soluble in water (solvent). If they are about the same, then it is mildly soluble (not super sure about it, though). I hope this helps!

The ammonia does not come off as a gas. The reaction lis driven forward, dissolving more magnesium hydroxide, even though the ammonia remaibsin solution.

Magnesium hydroxide comes off a bit strange when we try to classify bases as "strong" or "weak". It looks like a weak base because it is only sparingly soluble in water, but the portion that does dissolve acts like a dilute solution of a strong base (see, for instance, the reaction of magnesium hydroxide with a typical transition metal ion discussed here). So, like other strong bases, the magnesium hydroxide will tend to react with the ammonium ion according to the reaction we already know:

$$\ce{NH4+ + OH- <=> NH3 + H2O}$$

It is this removal of hydroxide ions, not any evolution of ammonia gas, that pulls the dissolution of magnesium hydroxide along.

Because magnesium hydroxide is sparingly soluble, the reaction proceeds only until the solubility equilibrium is re-established. You will typically end up with a mixture of ammonia, ammonium salt and magnesium salt in a slightly alkaline solution. But there will be a lot more magnesium ions in solution with this reaction than with adding magnesium hydroxide to pure water.