Problem 18 - A Complete Resource Book in Chemistry for JEE Main

Question

Action of caustic soda on \(\mathrm{Al}(\mathrm{OH})_{3}\) gives a compound having formula (a) \(\mathrm{Al}_{2}(\mathrm{OH})_{4}\) (b) \(\mathrm{Na}_{3} \mathrm{AlO}_{3}\) (c) \(\mathrm{Na}_{2} \mathrm{Al}(\mathrm{OH})_{4}\) (d) \(\mathrm{NaAlO}_{2}\)

Step-by-Step Solution

Answer

The compound formed is (d) \( \mathrm{NaAlO}_{2} \).

Step 1: Understand the Reaction Context

Caustic soda is another term for sodium hydroxide, denoted as NaOH. When considering its interaction with aluminum hydroxide, Al(OH)_3, it is important to notice that their reaction will likely form a soluble salt. This is because Al(OH)_3 is amphoteric, meaning it can react with acids and bases, such as NaOH.

Step 2: Write the Chemical Reaction

Aluminum hydroxide reacts with sodium hydroxide to form sodium aluminate. The balanced chemical equation for the reaction is: \[ \mathrm{Al(OH)}_{3} + \mathrm{NaOH} \rightarrow \mathrm{NaAlO}_{2} + 2\mathrm{H}_{2}\mathrm{O} \] This shows the formation of the sodium aluminate compound, which is soluble in water.

Step 3: Identify the Resulting Compound

From the balanced chemical reaction, the product after the reaction of Al(OH)_3 with NaOH is NaAlO_2, which is sodium aluminate. This matches with option (d) provided in the exercise.

Key Concepts

Aluminum Hydroxide

Aluminum hydroxide, usually represented as \( \mathrm{Al(OH)}_{3} \), is classified as an amphoteric compound. This means it has the unique ability to react with both acids and bases. Amphoteric behavior is not very common and sets aluminum hydroxide apart from many other hydroxide compounds. This substance is often found in nature as a solid and appears in three main crystalline forms: gibbsite, boehmite, and diaspore. 


• **Gibbsite** is the most stable and common form.

• Found in hydroxide layers, **boehmite** and **diaspore** play roles in the mining and refining processes.

Aluminum hydroxide’s ability to interact with both acids and bases makes it especially useful in various chemical processes. It can neutralize gastric acid in the stomach and is also useful in water purification, where it acts as a coagulant to help remove impurities.

Sodium Aluminate

Sodium aluminate, represented by the formula \( \mathrm{NaAlO}_{2} \), is a product that arises when aluminum hydroxide reacts with sodium hydroxide (caustic soda). This reaction is noteworthy for its formation of a soluble compound that is frequently employed in water treatment and as an additive in the paper industry.The formula might appear complex, but it reflects a straightforward conversion where the aluminum hydroxide dissolves into a form that can interact with other substances in various industrial processes. As a water treatment agent, sodium aluminate helps with:

• **Improving coagulation and flocculation**: These processes help in removing particulates.

• **pH adjustment**: It helps in managing the alkalinity of water effectively.


Sodium aluminate's role in chemical reactions extends beyond water treatment, contributing to the preparation of zeolites and acting as a cement setting accelerator. Its double-action capability in both adjusting chemical conditions and promoting physical changes makes it quite valuable.

Chemical Reactions with Bases

Chemical reactions with bases often involve substances that can donate or accept protons. In the context of amphoteric compounds like aluminum hydroxide, reactions with bases such as sodium hydroxide ( **caustic soda**) open up fascinating possibilities.When aluminum hydroxide reacts with a base, it behaves differently than it would in the presence of an acid. For instance, the reaction between  \( \mathrm{Al(OH)}_{3}  \) and  \( \mathrm{NaOH}  \) produces sodium aluminate ( \( \mathrm{NaAlO}_{2}  \)) and water. Here are some key points about these reactions:

• **Solubility increase**: The reaction results in a soluble product, making it accessible for further reactions.

• **Hydration formation**: Water is normally released during the chemical transformation.

• **Industrial significance**: These reactions are pivotal in producing materials required for industrial applications. 

Reactions involving bases help in broadening the utility of compounds like aluminum hydroxide, enhancing their industrial and environmental applications. They showcase how adaptable these reactions are in achieving various chemical outcomes.