Amphoteric Behaviour and Aluminium

OK, Aluminium ions are amphoteric. Which basically tells you that they will react with acids and bases. The question is though if that is true, how can this be proved using just one chemical (sodium hydroxide). Right lets think this through

Think of aluminium Nitrate. The aluminium ion is the amphoteric bit (not the nitrate).


So, if you start with aluminium nitrate it will turn into aluminium hydroxide (white ppt)

like this...

Al3+  +  3OH-  --> Al(OH)3(s)

Now at this point the aluminium hydroxide could do the obvious thing of behaving like a base and reacting with some acid to make salt and water

e.g.

Al(OH)3  +  HCl  --> AlCl3  +  3H2O

or (and this is the super important/exciting amphoteric bit)

it could react with NaOH and become Al(OH)6 3-

Al(OH)3  +  3OH-  --> Al(OH)6 3-

So adding more OH- proves it is amphoteric because it proves it can do both things.

Any old hydroxide salt can react with an acid but only magic amphoteric ones can react with bases as well

NOTE - Amphoteric compounds aren't magic