[darkprince304]

Hi Im new here

Why is Mg(OH)2 sparingly soluble in water but highly soluble in ammonium
chloride solution.
Can I know the reason?

Thanks

[UG]

Because NH₄⁺ is an acid and Mg(OH)₂ is a base, the two can react. Does that answer your question?

[darkprince304]

Um, thanks but I still don't get it. If they both can react how is it soluble in NH4Cl and what is the relation of solubility product with it?

[UG]

If they both can react how is it soluble in NH4Cl and what is the relation of solubility product with it?

You can think of the dissolution as an equilibrium Mg(OH)₂ (s)  Mg²⁺ + 2OH^-
Now if the hydroxide ions react with the NH₄⁺ what will happen to the equilibrium?

[darkprince304]

It will go backward?

[UG]

It will go backward?

Eh, not quite. The reaction with NH₄⁺ reduces the amount of OH^- ions, ie, they are 'consumed' from the equilibrium with magnesium hydroxide solid. So essentially, you are removing OH^- ions from the expression Mg(OH)₂ (s)  Mg²⁺ + 2OH^-, this will cause the solid to dissolve more in order to produce more OH^- ions to react with NH₄⁺. The result of this is that the solid has become more soluble in the NH₄Cl solution.