# Why is a solution of PCl3 more acidic than MgCl2?

Why is a solution of $\ce{PCl3}$ more acidic than $\ce{MgCl2}$? I thought that both $\ce{MgCl2}$ and $\ce{PCl3}$ would react with water to form $\ce{HCl}$ so both solutions should have a very low pH. But does $\ce{MgCl2}$ actually form $\ce{HCl}$ in water or it simply dissolves and forms $\ce{[Mg(H2O)6]^2+}$, but if it only forms this, how is it slightly acidic?

• I thought $\ce{MgCl2}$ would form a pretty neutral solution. The only reason I can think of for the formation of an acidic solution is the low solubility of magnesium hydroxide could result in precipitation of some of the hydroxide, so the solution could become slightly acidic. Compared to the situation with $\ce{PCl3}$, however, I'd say a solution of $\ce{MgCl2}$ should be pretty neutral. – airhuff Apr 22 '17 at 5:11
• Hydrolysis does not have to result in precipitation of hydroxide. – Ivan Neretin Apr 22 '17 at 6:08

There is no such thing as a water solution of $\ce{PCl3}$. It reacts with water, and quite vigorously. Once the reaction is over, there is no more $\ce{PCl3}$ in the solution. Arguably, there is no $\ce{MgCl2}$ in the other solution either, but it would form (as a hexahydrate) if you evaporate the solution; $\ce{PCl3}$ would not.
The reason for this is that $\ce{Mg}$ is a metal and $\ce{P}$ is a nonmetal. They are different elements, about as different as two elements can be. They have nothing in common, except for a few sparse random traits, like being able to burn brightly in air when ignited. No wonder they are different in this regard as well. $\ce{MgCl2}$ hydrolyzes only very slightly (say, one part in a million). $\ce{PCl3}$ hydrolyzes completely.