Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

A deliquescent white crystalline hydroxide \(X\) reacts with a nitrate \(\mathrm{Y}\) to form another hydroxide which decomposes to give a insoluble brown layer of its oxide. \(X\) is a powerful cautery and breaks down then proteins of skin flesh to a pasty mass. \(\mathrm{X}\) and \(\mathrm{Y}\) are (a) \(\mathrm{NaOH}, \mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\mathrm{NaOH}, \mathrm{AgNO}_{3}\) (c) \(\mathrm{Ca}(\mathrm{OH})_{2} \cdot \mathrm{HgNO}_{3}\) (d) \(\mathrm{NaOH}, \mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\)

Step-by-Step Solution

Answer

The compounds are NaOH and AgNO3 (option b).

Step 1: Identify Properties of Compound X

Compound X is described as a deliquescent (absorbent of moisture) white crystalline hydroxide that is a powerful cautery. It breaks down proteins of skin flesh. Sodium hydroxide (NaOH) is a well-known substance with these properties: it absorbs moisture readily, is white and crystalline, and is a strong base that can break down organic tissues.

Step 2: Determine the Reaction with Nitrate

Compound X (NaOH) reacts with a nitrate (Y) to form another hydroxide, which when decomposed, produces an insoluble brown oxide layer. The nitrate that pairs with NaOH in this manner is silver nitrate (AgNO3), where the formed AgOH decomposes to Ag2O, a brown insoluble oxide.

Step 3: Analyze Decomposition of Hydroxide

The product hydroxide formed from NaOH reacting with AgNO3 is AgOH. Silver hydroxide (AgOH) is unstable and decomposes to form silver oxide (Ag2O), which is insoluble and brown in color.

Step 4: Match Properties with Options

Review the provided options to match the compounds to the descriptions and steps identified. Option (b) NaOH, AgNO3 meets all criteria: NaOH as X and AgNO3 as Y match the given properties and reaction outcome.

Key Concepts

Deliquescent compounds

Deliquescent compounds have a unique ability to absorb moisture from their environment until they dissolve into an aqueous solution. Sodium hydroxide (NaOH) is an excellent example of such a compound. It is known for its white crystalline appearance and its hygroscopic nature, meaning it has a strong tendency to absorb water.
This property often requires these compounds to be stored in airtight containers to prevent them from dissolving in humid conditions. One practical application of deliquescent compounds is their use in moisture absorption packs, which help keep products dry during shipping and storage.

Silver Nitrate Reactions

Silver nitrate (AgNO3) is a versatile compound widely used in chemical reactions due to its unique properties. When it reacts with sodium hydroxide (NaOH), it forms silver hydroxide (AgOH).
This hydroxide is unstable and readily decomposes under certain conditions. This reactivity makes silver nitrate particularly useful in laboratory settings and industrial applications, such as in the manufacturing of mirrors and inks. Silver nitrate is also used in photography and as an antiseptic due to its ability to destroy harmful organisms.

Decomposition of Silver Hydroxide

Silver hydroxide (AgOH) is a compound that tends to decompose into silver oxide (Ag2O) and water. This decomposition occurs because AgOH is thermodynamically unstable.
The resulting silver oxide is insoluble in water and forms as a brown, solid layer, which distinguishes it from other products that may form under similar conditions.

• AgOH, when subjected to heat or simply on standing, converts to Ag2O.
• Ag2O is insoluble and forms a permanent lacquering over the surface where it forms.

Understanding this decomposition process is crucial, particularly in applications where silver oxide's properties are desired or play a role in further reactions.