# Reaction of aluminium trichloride with water

Since $\ce{Al^3+}$ is an acidic cation, shouldn't it react with water like this? \begin{align} \ce{AlCl3 + 6H2O &<=> [Al(H2O)6]^3+ + 3Cl^-} \tag1\\ \ce{[Al(H2O)6]^3+ +H2O &<=> [Al(H2O)5(OH)]^2+ + H3O^+} \tag2\\ \ce{[Al(H2O)5(OH)]^2+ + H2O &<=> [Al(H2O)4(OH)2]^+ + H3O^+} \tag3\\ \ce{[Al(H2O)4(OH)2]^+ +H2O &<=> [Al(H2O)3(OH)3](s) + H3O^+} \tag4\\ \end{align}

The 3rd step of the equilibrium rarely happens in water. But if we add a base, strong or not ($\ce{NaOH}$, or $\ce{Na2S}$) the equilibrium weighs to the right and $\ce{Al(OH)3}$ is formed.

But I've seen people write it as this. $$\ce{AlCl3 + 3H2O -> Al(OH)3 + 3HCl}$$ Is this correct? I need a certain answer.

• It's not incorrect, but certainly as it stands there, it is missing a few key details. The aluminium water complexes are very sensitive to pH change, it really depends on all conditions and on the context whether this can be seen as correct or not. – Martin - マーチン Jul 25 '17 at 11:31
• In 25 degrees celcius when the pH in water is 7, if we add anhydrous AlCl3 into the water. Can we see any precipitation? And if we heat the system all equlibrium constants move right? And then will Al(OH)3 be formed? We have very limited lab recources here. I have no way of knowing it practically. Thats why im asking for your help. Thanks so much for everyone who contributed thus far. More help is appreciated. – Nuwan yasantha Jul 25 '17 at 14:07
• I doubt you'll see precipitation. Aluminium is a very delicate system and you need to get the pH just right to get it. Also keep in mind that the equilibria are necessarily linked and happening at the same time. If I have the time I'll add some authoritative resource. Just give it a little time, it's a good question and I'm positive you'll find a good answer. – Martin - マーチン Jul 25 '17 at 14:13

Assuming you dont have any buffers or bases in solution when you add $AlCl_3$, the reaction will lower the pH. At a pH lower than 4-ish, $Al^{3+}$ is the most stable cation. Thus, the reaction $$\ce{AlCl3 + 3H2O -> Al(OH)3 + 3HCl}$$ is pretty accurate. Aluminum hydroxides are decreasingly stable at low pH, as you can see below. Since HCl is formed upon dissolution of $AlCl_3$ you can safely assume that $Al^{3+}$ is the only Al-species present.