What chemical compounds might be present in drinking water?

4.1 Aqueous chemistry of aluminium

Aluminium is a metal forming an aqueous cation, unlike the non-metal boron found above it in Group 13. For example, hydrates of aluminium sulfate, Al2(SO4)3, can be made by dissolving bauxite, Al2O3.3H2O, in sulfuric acid and evaporating the solution. Dissolving such sulfates in water forms Al3+(aq) (Figure 16):

Al2(SO4)3(s) = 2Al3+(aq) + 3SO4 2−(aq)
Figure 16 The hexaaquaaluminium(III) complex ion.
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What is the coordination geometry of aluminium in Figure 16?

Answer

Al3+(aq) is shorthand for the octahedral coordination complex
Al(H2O)6 3+.

Careful addition of aqueous sodium hydroxide to this solution will first precipitate insoluble aluminium hydroxide:

Al3+(aq) + 3OH−(aq) = Al(OH)3(s)

The oxide is produced by filtering and heating the hydroxide:

2Al(OH)3(s) = Al2O3(s) + 3H2O(g)

Both oxide and hydroxide are unusual in being amphoteric . Hence they will first dissolve in and neutralise acids:

Al(OH)3(s) + 3H+(aq) = Al3+(aq) + 3H2O(l)

Watch Video 5 to determine how else an amphoteric hydroxide (or oxide) behaves.

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Video 5 The reaction of aluminium(III) ions with hydroxide ions.
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Answer

It will dissolve in an alkali and will neutralise it.

As excess sodium hydroxide is added to the precipitate that is initially formed in Equation 37, the precipitate dissolves to form the tetrahydroxyaluminate ion:

Al(OH)3(s) + OH−(aq) = [Al(OH)4]−(aq)

This shows a close resemblance of Al(OH)3 to Be(OH)2, which is also amphoteric. Note that the bases ammonia and sodium carbonate are not strong enough to cause this dissolution.

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