Problem 143 - A Complete Resource Book in Chemistry for JEE Main

Question

A colourless water soluble salt \(X\) on heating gives brown gas and leaves a metallic residue. Solution of \(X\) gives brick red precipitate with solution of \(\mathrm{K}_{2} \mathrm{CrO}_{4}\). An ammonical solution of \(X\) gives a silver mirror with formic acid. Salt \(X\) is used in making indelible election ink. Identify the salt \(X\) here? (a) \(\mathrm{AgCl}\) (b) \(\mathrm{HgNO}_{3}\) (c) \(\mathrm{AgNO}_{3}\) (d) \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

The salt \(X\) is \(\mathrm{AgNO}_{3}\).

Step 1: Analyze the Brown Gas

When the salt \( X \) is heated and gives off a brown gas, it suggests the presence of nitrogen dioxide \( \text{NO}_2 \), which is common for nitrates. Thus, \( X \) is likely a nitrate salt.

Step 2: Identify the Brick Red Precipitate

The salt \( X \) gives a brick red precipitate with \( \text{K}_2\text{CrO}_4 \), indicating the presence of silver ions \( \text{Ag}^+ \), as silver chromate \( \text{Ag}_2\text{CrO}_4 \) forms a characteristic brick red precipitate. This implies \( X \) contains silver, so \( X \) could be \( \text{AgNO}_3 \).

Step 3: Examine the Silver Mirror Test

An ammoniacal solution of \( X \) gives a silver mirror with formic acid. This is a classic Tollens' test, confirming the presence of \( \text{Ag}^+ \), as only \( \text{AgNO}_3 \) would react as such to form a silver mirror.

Step 4: Usage in Indelible Ink

Silver nitrate \( \text{AgNO}_3 \) is well-known for its use in indelible inks, particularly in marking election papers. This corroborates the identification of \( X \).

Key Concepts

Chemical Identification

Chemical identification involves determining the specific chemicals present in a substance. In the exercise, we are tasked to identify a salt, denoted as \( X \), based on its reactions and properties. Upon heating, \( X \) releases a brown gas known to be nitrogen dioxide \( \text{NO}_2 \). This indicates that \( X \) contains nitrate \( \text{NO}_3^- \) ions, as nitrates decompose to release \( \text{NO}_2 \).
Furthermore, reacting \( X \) with potassium chromate \( \text{K}_2\text{CrO}_4 \) results in a brick red precipitate. This is characteristic of the presence of silver ions \( \text{Ag}^+ \), forming silver chromate \( \text{Ag}_2\text{CrO}_4 \). Such observations assist in narrowing down the identity of the unknown salt.

Nitrate Compounds

Nitrate compounds are salts or esters of nitric acid \( \text{HNO}_3 \), with the composition \( \text{NO}_3^- \) as their pivotal component. These substances often release nitrogen dioxide \( \text{NO}_2 \) when heated, which manifests as a brown gas.
Nitrates are known for their diverse applications, such as in fertilizers, food preservation, and pyrotechnics. In the context of the exercise, the nitrate group of \( X \) helps in confirming that the salt could be silver nitrate \( \text{AgNO}_3 \), as it follows the decomposition pattern of releasing \( \text{NO}_2 \).
This characteristic release of gas becomes a key identifying property in chemical analysis.

Tollens' Test

Tollens' test is an essential chemical test to identify aldehydes. It uses a reagent known as Tollens' reagent, which is basically an ammoniacal silver nitrate solution. When Tollens' reagent is added to an aldehyde, it oxidizes the aldehyde to a carboxylic acid. This reaction results in the reduction of \( \text{Ag}^+ \) ions to metallic silver, forming a silver mirror on the test tube.
In the exercise, \( X \) forms a silver mirror upon reaction with formic acid, confirming its identity as silver nitrate \( \text{AgNO}_3 \), since it contains \( \text{Ag}^+ \) ions, necessary for Tollens' test.
This silver mirror formation is a distinct confirmation of the presence of certain metal ions, serving as a crucial step in validating chemical compositions.

Silver Nitrate Applications

Silver nitrate \( \text{AgNO}_3 \) is a highly versatile chemical with several remarkable applications. It is used in making indelible ink because of its unique capacity to stain organic material permanently, a feature useful in creating enduring marks.
Such ink is often employed during elections to prevent double voting, where a voter's finger is marked after casting their vote. Other applications of silver nitrate include its use as a cauterizing agent in medical procedures, due to its property of promoting tissue healing. Additionally, it plays a role in photography and mirror production.
These applications underscore the practical, multifaceted uses of silver nitrate and solidify its reputation as a valuable chemical in various industries.