Question

Aluminium hydroxide is soluble in excess of $NaOH$ forming the ion?
A.$Al{O^{3 - }}_2$
B.$Al{O_2}^{3 + }$
C.$Al{O_2}^ -$
D.$Al{O_3}^ -$

Answer 1

Aluminium is a group $13$ element hence is amphoteric in nature. Aluminium hydroxide is a Lewis acid and can willingly take up electrons. So it acts both as acid and base, it combines with a strong base as sodium hydroxide.

Complete step by step answer:
Aluminium hydroxide$Al {(OH) _3}$ is insoluble in water. When aluminium hydroxide reacts with sodium hydroxide $NaOH$ it produces a compound known as sodium aluminate $Na [Al {(OH) _4}]$.
$Al {(OH) _3} + NaOH \to Na [Al {(OH) _4}]$
Sodium aluminate is soluble in water. It is an inorganic chemical. It is used in industrial work. Pure sodium aluminate is white crystalline. It is odourless and insoluble in alcohol. It has an orthorhombic structure. The temperature during this reaction is ${1000^ \circ} c$. Sodium aluminate is used as a water softener. It is also used on construction sites and paper industries. Sodium aluminate acts as an intermediate in the process of making zeolites. The reaction of aluminium hydroxide with sodium hydroxide is highly exothermic and is accompanied with evolution of hydrogen.
$2Al + 2NaOH + 2{H_2} O \to 2NaAl {O_2} + 3{H_2}$
The answer is C i.e $Al {O_2} ^ -$

Additional information:
Aluminium hydroxide is found as a mineral in nature known as gibbsite. It is soluble in acidic and alkaline. It is an anta-acid. It is used in pharmaceutical industries, patients with heartburn, upset or sour stomach or indigestion are treated with the medicines that have aluminium hydroxide as its salt.

Note: Aluminium belongs to the group $13$. The by-product of this reaction is ${H_2} O$. This reaction is also identified as an acid and base reaction. Since \ [Al {(OH) _3}] has empty or vacant orbitals so it can accommodate a lone pair of electrons that is donated by the base$NaOH$. But remember that $Al {(OH) _3}$ is not completely acidic; it has a nature to act as a base as well.