Match List-I with List-II.
List-I (Test / Reagents / Observation(s)) | List-II (Species detected)
A.

Q: Why do we need to prepare Lassaigne's extract? Can't we just test the organic compound directly?

Organic compounds have covalent bonds, which don't easily dissociate into ions in solution. Most qualitative tests rely on ionic reactions (like forming a precipitate). Fusing with sodium metal breaks these covalent bonds and forms ionic compounds like NaCN or Na2S, which readily react in our tests.

Q: Does the Silver Nitrate test detect all halogens?

It detects Chlorine, Bromine, and Iodine, which form AgCl (white), AgBr (pale yellow), and AgI (yellow) precipitates respectively. However, it does not detect Fluorine because Silver Fluoride (AgF) is soluble in water and won't form a precipitate.

Q: Why is acetic acid added before lead acetate in the sulphur test?

Acetic acid is added to acidify the solution and prevent the precipitation of other lead salts like lead carbonate or lead hydroxide, which could interfere with the detection of the black Lead sulfide (PbS) precipitate.

Q: Can the Copper(II) oxide test detect other elements besides Carbon and Hydrogen?

The primary purpose of the CuO test is to oxidize Carbon to CO2 (which turns lime water milky) and Hydrogen to H2O (which turns anhydrous copper sulfate blue). It is specifically designed for these two elements and isn't used for detecting N, S, or halogens.

Q: What if both Nitrogen and Sulphur are present in the compound during Lassaigne's test?

If both are present, they can fuse with sodium to form Sodium thiocyanate (NaSCN). In this case, adding Iron(III) ions will give a blood-red coloration due to the formation of the [Fe(SCN)]2+ complex, instead of the usual Prussian blue color for nitrogen.

Question

Match List-I with List-II.

$\begin{array}{ll} 	ext{List-I (Test / Reagents / Observation(s))} & 	ext{List-II (Species detected)} \ 	ext{A. Lassaigne's test} & 	ext{(i) Carbon} \ 	ext{B. Cu(II) oxide} & 	ext{(ii) Sulphur} \ 	ext{C. Silver nitrate} & 	ext{(iii) N, S, P, and halogen} \ 	ext{D. The sodium fusion extract gives black precipitate with acetic acid and lead acetate} & 	ext{(iv) Halogen specifically} \end{array}$

The correct match is

Options:

(A) A-(iii), B-(i), C-(ii), D-(iv)

(B) A-(i), B-(iv), C-(iii), D-(ii)

(C) A-(iii), B-(i), C-(iv), D-(ii)

(D) A-(i), B-(ii), C-(iv), D-(iii)

Sigma Percentile · Accepted Answer

### Unlocking the Secrets of Organic Compounds: A Journey Through Qualitative Analysis

Welcome to the fascinating world of qualitative analysis! When chemists synthesize or discover a new organic compound, the very first question they ask is, "What elements are hiding inside this molecule?" Since organic compounds are primarily made of carbon and hydrogen, we often need to hunt for "extra" elements like nitrogen, sulfur, phosphorus, and halogens.

This classic matching question tests your fundamental knowledge of the chemical tests used to detect these hidden elements. Let's break down each test logically and see the beautiful chemistry behind them.

### The Magic of Lassaigne's Extract

Organic compounds are held together by strong covalent bonds. If you just drop an organic compound into water and add a testing reagent, nothing will happen because the elements won't dissociate into ions.

To solve this, we perform **Lassaigne's Test**. We take the organic compound and fuse it with highly reactive sodium metal at a high temperature. This violent reaction shatters the covalent bonds and forces the elements to form ionic sodium salts.

For example, if nitrogen and carbon are present, they form sodium cyanide:
$$ 	ext{Na} + 	ext{C} + 	ext{N} \xrightarrow{\Delta} 	ext{NaCN} $$

If sulfur is present, it forms sodium sulfide:
$$ 2	ext{Na} + 	ext{S} \xrightarrow{\Delta} 	ext{Na}_2	ext{S} $$

If halogens are present, they form sodium halides:
$$ 	ext{Na} + 	ext{X} \xrightarrow{\Delta} 	ext{NaX} $$

Because this single extract can be used to detect Nitrogen, Sulfur, Phosphorus, and Halogens, **Lassaigne's test (A)** perfectly matches with **(iii) N, S, P, and halogen**.

### Hunting for Carbon and Hydrogen

How do we prove that a compound is actually organic? We look for carbon and hydrogen. We do this by heating the compound strongly with Copper(II) oxide ($	ext{CuO}$).

Copper(II) oxide acts as a powerful oxidizing agent. It strips the carbon and hydrogen from the organic molecule and oxidizes them. Carbon is oxidized to carbon dioxide gas, which turns lime water milky. Hydrogen is oxidized to water vapor, which turns anhydrous copper sulfate from white to blue.

$$ 	ext{C} + 2	ext{CuO} \xrightarrow{\Delta} 2	ext{Cu} + 	ext{CO}_2 $$
$$ 2	ext{H} + 	ext{CuO} \xrightarrow{\Delta} 	ext{Cu} + 	ext{H}_2	ext{O} $$

Therefore, the **Cu(II) oxide test (B)** is specifically used to detect **(i) Carbon** (and Hydrogen).

### The Silver Nitrate Precipitation

Once we have our ionic Lassaigne's extract, detecting halogens becomes a classic inorganic precipitation reaction. We add Silver Nitrate ($	ext{AgNO}_3$) to the acidified extract.

Silver ions ($	ext{Ag}^+$) have a strong affinity for halide ions ($	ext{X}^-$), instantly forming insoluble silver halide precipitates.

$$ 	ext{Ag}^+_{(aq)} + 	ext{X}^-_{(aq)} ightarrow 	ext{AgX}_{(s)} \downarrow $$

Depending on the halogen, you get different colors: $	ext{AgCl}$ is white, $	ext{AgBr}$ is pale yellow, and $	ext{AgI}$ is yellow. Thus, the **Silver nitrate test (C)** matches with **(iv) Halogen specifically**.

### The Dark Secret of Sulfur

If our organic compound contained sulfur, our Lassaigne's extract now contains sodium sulfide ($	ext{Na}_2	ext{S}$). To confirm this, we acidify the extract with acetic acid and add lead acetate.

The lead ions ($	ext{Pb}^{2+}$) react with the sulfide ions ($	ext{S}^{2-}$) to form Lead(II) sulfide, which crashes out of the solution as a highly distinct, heavy black precipitate.

$$ 	ext{Na}_2	ext{S} + (	ext{CH}_3	ext{COO})_2	ext{Pb} ightarrow 	ext{PbS} \downarrow + 2	ext{CH}_3	ext{COONa} $$

This unmistakable black precipitate confirms that **The sodium fusion extract with lead acetate (D)** detects **(ii) Sulphur**.

### The Final Verdict

By connecting the dots, we get our final sequence:
- **A $ightarrow$ (iii)**
- **B $ightarrow$ (i)**
- **C $ightarrow$ (iv)**
- **D $ightarrow$ (ii)**

This perfectly aligns with option **(c)**. Mastering these qualitative tests not only secures easy marks in exams but also builds the foundational intuition required for advanced quantitative analysis techniques like Kjeldahl's and Carius methods!

Select Answer:

Visualized Solution

$Qualitative Analysis$

$Lassaigne’s Test$

$Cu(II) Oxide Test$

$Silver Nitrate Test$

$Lead Acetate Test$

$Final Match$

$The Way Forward$

The Sigma Insight: Nomenclature and Characterisation

Unlocking the Secrets of Organic Compounds

The Magic of Lassaigne's Extract

Hunting for Carbon and Hydrogen

The Silver Nitrate Precipitation

The Dark Secret of Sulfur

The Final Verdict

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