Question

Aluminum hydroxide, $Al{{(OH)}_{3}}$ is insoluble in water but dissolves readily in both acidic and basic solutions. Such behavior is characteristic of:
(a)- polyprotic behavior
(b)- amphoteric behavior
(c)- a buffer
(d)- hydrophilic

Answer 1

Aluminum hydroxide is a naturally occurring compound as a mineral gibbsite. It can act as both Bronsted-Lowry base, by accepting protons from an acidic solution, and as a Lewis acid, by accepting an electron pair from hydroxide ions in a basic solution.

Complete step by step solution:

Aluminum hydroxide can react with an acid. It exhibits its basic nature. For example, when added to dilute hydrochloric acid, it forms a colorless solution of aluminum chloride.
$Al{{(OH)}_{3}}+3HCl\to AlC{{l}_{3}}+3{{H}_{2}}O$
Similarly, it can also react with base. This exhibits its acidic nature. For example, when added to a sodium hydroxide solution, it again gives a colorless solution of sodium tetrahydroxoaluminate.
$Al{{(OH)}_{3}}+NaOH\to NaAl{{(OH)}_{4}}$
From the above two reactions, we can conclude that aluminum hydroxide can act both as an acid and a base. Therefore, it is amphoteric in nature.
So, the correct option is (b).
Let us see why the other options are incorrect:
(a)- Substances that can donate or accept more than one proton per molecule are known as polyprotic acids and bases. But, $Al{{(OH)}_{3}}$ is tribasic in nature.
(c)- A buffer is a solution containing an acid and a base, or a salt, that tends to maintain a constant hydrogen ion concentration.
(d)- Hydrophilic means "water-loving." But, $Al{{(OH)}_{3}}$ is insoluble in water.

Note: Generally, metal hydroxides with a highly-charged central metal atom can be amphoteric. Oxides or hydroxides of metals such as zinc, tin, lead, and beryllium can also exhibit amphoteric behavior. Their behavior as acids or bases depends on the surrounding solution’s pH.