What is meant by the term amphoteric? and give an example of a reaction using NaOH that could be used to distinguish between a solution of amphoteric AlCl3 and non-amphoteric MgCl2.

The phrase amphoteric describes a compound that reacts with both acids and bases. 

NaOH can react with both AlCl₃ and MgCl₂ to give a white precipitate but the addition of excess NaOH causes the amphoteric Al(OH)₃ to dissove, whereas the magnesium precipitate does not dissolve with excess NaOH.

Al³⁺(aq) + 3OH^-(aq) -> Al(OH)₃(s)

Al(OH)₃(s) + OH^-(aq) -> [Al(OH)4]^- (aq)