Question

Which below best describe(s) the behavior of an amphoteric hydroxide in water?

a) with concentrated aqueous NaOH, its suspension dissolves.
b) with concentrated aqueous HCl, its suspension dissolves.
c) with concentrated aqueous NaOH, its clear solution forms a precipitate.
d) with concentrated aqueous HCl, its clear solution forms a precipitate.
e) with both concentrated aqueous NaOH and conc, aq. HCl, its suspension dissolves.

Answer 1

An amphoteric hydroxide can act as both an acid and a base. It forms a precipitate when reacting with concentrated aqueous NaOH and dissolves when reacting with both concentrated aqueous NaOH and concentrated aqueous HCl.

Step-by-step explanation:

An amphoteric hydroxide is one that can act as both an acid and a base. When an amphoteric hydroxide reacts with concentrated aqueous NaOH, it forms a precipitate. This indicates that the amphoteric hydroxide is behaving as an acid, reacting with the strong base to form a solid compound. For example, when an amphoteric hydroxide like aluminum hydroxide (Al(OH)3) reacts with concentrated NaOH, it forms a precipitate of aluminum hydroxide:

Al(OH)3 + NaOH → NaAl(OH)4

On the other hand, when the same amphoteric hydroxide reacts with both concentrated aqueous NaOH and concentrated aqueous HCl, it dissolves. This suggests that the amphoteric hydroxide is behaving as a base, neutralizing the acid and forming a soluble compound. For example, when aluminum hydroxide reacts with both NaOH and HCl, it first forms a precipitate of Al(OH)3 with NaOH, but then dissolves when HCl is added:

Al(OH)3 + NaOH → NaAl(OH)4

NaAl(OH)4 + HCl → NaCl + AlCl3 + H2O