Problem 153 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following $$ \begin{array}{ll} \hline \text { List-I } & \text { List-II } \\ \hline \text { a. Chromyl chloride test } & \text { (p) } \mathrm{CH}_{3} \mathrm{COO}^{-} \\ \text {b. Ring test } & \text { (q) } \mathrm{S}^{2-} \\ \text { c. Smell of vinegar } & \text { (r) } \mathrm{Cl}^{-} \\ \text {d. Smell of rotten eggs } & \text { (s) } \mathrm{NO}_{3}^{-} \end{array} $$

Step-by-Step Solution

Answer

a - r; b - s; c - p; d - q.

Step 1: Understanding Chromyl Chloride Test

The chromyl chloride test is used to detect the presence of chloride ions \( \mathrm{Cl}^{-} \). When a chloride-containing compound reacts with potassium dichromate and concentrated sulfuric acid, deep red fumes of chromyl chloride \( \mathrm{CrO}_{2}\mathrm{Cl}_{2} \) are produced. Therefore, match (a) with (r).

Step 2: Analyzing the Ring Test

The ring test is used to detect nitrate ions \( \mathrm{NO}_{3}^{-} \). In this test, a brown ring at the interface is observed when ferrous sulfate is added to a solution containing nitrate ions, followed by the addition of concentrated sulfuric acid. Hence, match (b) with (s).

Step 3: Recognizing the Smell of Vinegar

The smell of vinegar is associated with the presence of acetate ions \( \mathrm{CH}_{3}\mathrm{COO}^{-} \). Acetic acid, which has a distinct vinegar-like smell, contains acetate ions. Thus, match (c) with (p).

Step 4: Identifying the Smell of Rotten Eggs

The smell of rotten eggs is due to the presence of hydrogen sulfide \( \mathrm{H}_{2}\mathrm{S} \), which indicates the presence of sulfide ions \( \mathrm{S}^{2-} \). Therefore, match (d) with (q).

Key Concepts

Chromyl Chloride Test

The chromyl chloride test is a classic qualitative analysis technique used to detect the presence of chloride ions \(\mathrm{Cl}^{-}\) in a sample. This test is primarily utilized in inorganic chemistry.
When a compound containing chloride ions is treated with potassium dichromate and concentrated sulfuric acid, an interesting thing happens. The reaction produces deep red fumes of chromyl chloride, which is \(\mathrm{CrO}_{2}\mathrm{Cl}_{2}\). This fume production is a clear indication of chloride ions, making the test quite distinctive.

• The presence of red fumes confirms the presence of \(\mathrm{Cl}^{-}\).
• It's crucial to carry out this test in a well-ventilated area or under a fume hood due to the release of toxic gases.

Ring Test

The ring test is a famous method used to detect nitrate ions \(\mathrm{NO}_{3}^{-}\). In this interesting chemical test, you mix the suspected solution with ferrous sulfate, followed by careful addition of concentrated sulfuric acid.
The remarkable and tell-tale feature of this test is the formation of a brown ring at the junction of the two layers. This brown ring is indicative of nitrate ions being present. It's a fascinating pathway to peel back layers of chemical reactions.

• The key event is the appearance of the brown ring, which is formed at the interface between the liquids.
• Ensure accurate layering of the acids to avoid mixing and losing the distinct ring evidence.

Acetate Ion Detection

Detecting acetate ions \( \mathrm{CH}_{3}\mathrm{COO}^{-} \) is synonymous with identifying the smell of vinegar. This is because acetic acid, which carries a distinctive vinegar-like aroma, contains acetate ions.
When handling a sample, sniffing for a vinegar smell can be a straightforward initial test. If the aroma is present, it can strongly suggest the presence of acetate ions in the specimen.

• A vinegar-like smell is a direct indication of \(\mathrm{CH}_{3}\mathrm{COO}^{-}\).
• Simple yet effective, this method leverages our natural sense of smell for ionic detection.

Sulfide Ion Detection

The detection of sulfide ions \(\mathrm{S}^{2-}\) can be uniquely conducted by identifying the smell of rotten eggs. This unpleasant odor is produced by hydrogen sulfide \(\mathrm{H}_{2}\mathrm{S}\), a gas indicative of sulfide ions.
This detection method is straightforward yet effective—employ your sense of smell to determine the presence of sulfides based on the characteristic odor. While this sounds simple, it necessitates caution as hydrogen sulfide is a toxic gas.

• The smell of rotten eggs points to \(\mathrm{S}^{2-}\) ions.
• Exercise care when performing this test due to the toxicity of \(\mathrm{H}_{2}\mathrm{S}\).