Question

why in aluminium an amphoteric substances.

why adding excess reagent causes some precipitates to dissolve - acidic or basic nature

My lab teacher does reviewed more lab report and she added two questions can you help with this

Answer 1

amphoteric: a substance that may be either a base or an adic

Aluminium is amphoteric since it may form H+ in solution or OH- depending on the reaction

i.e.

Al+3 + 3H2O <--> Al(OH)3(s) + 3H+ (1)

Al(OH)₃(s)+ 4OH- → [Al(OH)₄](aq) (2)

The second question has a lot to do with reactions (1) and (2)

if we add a stoichiometric amount of base, we form Al(OH)3(s) (from reaction 1) which is SOLID

if we still add more base, i.e. NaOH of KOH

then we dissolve the solid formed into a complex ion (aquous solutin) therefore dissolved