Which of the following is an example of leaching?

Al2O3 + 2NaOH + 3H2O -----> 2Na[Al(OH)4]

The answer is 1. \(Al_2O_3 + 2NaOH + 3H_2O \longrightarrow 2Na[Al(OH)_4]\).

Leaching is a process in which a soluble compound is extracted from a solid by a solvent. In the reaction shown in option 1, the soluble compound Al2O3 is extracted from the solid bauxite by the solvent NaOH. The other reactions are not examples of leaching because they do not involve the extraction of a soluble compound from a solid.

Here is a brief explanation of the reaction in option 1:

\(Al_2O_3\) is an oxide of aluminum. It is a solid at room temperature.

\(NaOH\) is a base. It is a soluble compound in water.

\(H_2O\) is water. It is a solvent.

When \(Al_2O_3\), \(NaOH\), and \(H_2O\) are mixed, the \(NaOH\) reacts with the \(Al_2O_3\) to form a soluble compound called sodium aluminate. The sodium aluminate is then dissolved in the water.

The leaching of bauxite is an important process in the extraction of aluminum. The bauxite is first crushed and then mixed with a solution of \(NaOH\). The \(NaOH\) dissolves the \(Al_2O_3\) in the bauxite, leaving behind a residue of impurities. The solution of sodium aluminate is then filtered to remove the impurities. The sodium aluminate is then concentrated and electrolyzed to produce aluminum.