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In my book it says that aluminium can react with water as a base when it's in the form $\ce{[Al(OH)2(OH2)4]^+}$.

And this way as an acid: $\ce{[Al(OH2)_6]^3+}$.

So ... does it depend on with which ion it's accompanied by? Let's say, $\ce{AlCl_3}$ will react as an acid?

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    $\begingroup$ Note that these species are in equilibrium. If you meant whether aqueous $\ce{AlCl3}$ is acidic, then sure: the aluminium ion has a large charge density, hence it polarises the water molecules. A base then deprotonates the water ligand. The first deprotonation equilibrium is shifted to the right (pH $< 7$). Welcome to Chem.SE and see here for help on Mathjax/mhchem. $\endgroup$ – Linear Christmas Jan 24 '17 at 16:13
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Yes, you are right. The reaction depends on with which ion it is accompanied with. Note that these species are in equilibrium and present in the solution and that particular reaction proceeds according to which ion is present in the solution in concentrated form.

  1. Aluminum hydroxide is formed when soluble salt of $\ce{Al^3+}$ i.e $\ce{[Al(H2O)6]^3+}$ when it is reacted with base.

    $$\ce{[Al(H2O)6]^3+(aq) + 3 OH-(aq) → Al(H2O)3(OH)3(s) + 3 H2O(l)}$$

    This insoluble metal hydroxide can act as a base, since it can be redissolved by reacting it with an acid.

  2. As Brønsted base

    $$\ce{Al(H2O)3(OH)3(s) + 3 H3O+(aq) → [Al(H2O)6]^3+(aq) + 3 H2O(l)}$$

    Alternatively, the metal hydroxide can act as an acid, since it can react with a base.

  3. Lewis acid

    $$\ce{Al(H2O)3(OH)3(s) + OH-(aq) → [Al(H2O)2(OH)4]-(aq) + H2O(l)}$$

Thus, all aluminium compounds are said to be amphoteric.

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