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Q14 (CDS-I/2019) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Organic chemistry basics Answer Verified

Which one of the following elements cannot be detected by "Lassaigne's test"?

Result

Your answer: — · Correct: D

Explanation

Lassaigne's test, also known as the sodium fusion test, is a qualitative analytical technique used to detect nitrogen, sulfur, and halogens (chlorine, bromine, and iodine) in organic compounds. During the test, the organic compound is fused with metallic sodium to convert covalent elements into water-soluble sodium salts like NaCN, Na2S, and NaX. While chlorine, bromine, and iodine are detected by reacting the sodium extract with silver nitrate to form characteristic precipitates (AgCl, AgBr, and AgI), fluorine cannot be detected using this method. This is because silver fluoride (AgF) is highly soluble in water, unlike other silver halides, and therefore does not form a visible precipitate to confirm its presence. Consequently, fluorine is the element among the options that cannot be detected by Lassaigne's test.

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Detailed Concept Breakdown

8 concepts, approximately 16 minutes to master.

1. Introduction to Heteroatoms in Organic Chemistry (basic)

Welcome to the fascinating world of organic chemistry! To understand this subject, we must first look at its protagonist: Carbon. Due to its unique properties of catenation (the ability to form long chains) and tetravalency (the ability to form four bonds), carbon creates a staggering variety of molecules Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.63. While the simplest organic compounds, known as hydrocarbons, consist only of carbon and hydrogen, the real chemical "magic" happens when other elements join the structure.

In a hydrocarbon chain, one or more hydrogen atoms can be replaced by other elements such as Oxygen (O), Nitrogen (N), Sulfur (S), or Halogens (like Chlorine, Bromine, and Iodine). In organic chemistry, any element in the molecule that is not Carbon or Hydrogen is referred to as a heteroatom Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66. The prefix "hetero-" comes from the Greek word for "different," signifying that these are "different" atoms entering the carbon-hydrogen framework.

Heteroatoms are not just passive passengers; they are the engines of chemical reactivity. They often exist within specific clusters called functional groups, which dictate how a molecule behaves, regardless of how long the carbon chain is Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66. For example, replacing a hydrogen atom with an -OH group (where Oxygen is the heteroatom) transforms a simple hydrocarbon into an alcohol, giving it entirely new properties Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.68.

Feature	Hydrocarbons	Heteroatom-containing Compounds
Composition	Only Carbon and Hydrogen	Carbon, Hydrogen, and at least one other element (O, N, S, etc.)
Chemical Activity	Generally less reactive	Highly reactive at the site of the heteroatom

Key Takeaway A heteroatom is any atom other than carbon or hydrogen that replaces hydrogen in an organic molecule, conferring specific chemical properties to the compound.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.62-63; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.68

2. Covalent vs. Ionic Bonding in Analytical Chemistry (basic)

In chemistry, the way atoms hold onto each other dictates how a substance behaves in a lab. Ionic bonding occurs when there is a complete transfer of electrons from one atom to another, resulting in the formation of ions (charged particles). These ions are held together by powerful electrostatic forces of attraction. Because of this, ionic compounds are typically hard solids with very high melting and boiling points Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49. In analytical chemistry, ionic compounds are often easier to identify because they are generally soluble in water and dissociate into ions that can conduct electricity or react quickly with other reagents Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.58.

On the other hand, most organic compounds are built using covalent bonding. Here, atoms don't give away electrons; they share them. This sharing results in the formation of molecules rather than a lattice of ions. Because the forces of attraction between these molecules (intermolecular forces) are relatively weak, covalent compounds usually have much lower melting and boiling points compared to ionic ones Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. A critical distinction for analysis is that these compounds are non-conductors of electricity, indicating that their bonding does not naturally give rise to free-moving ions Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59.

When we perform analytical tests on organic molecules to find "heteroatoms" like Nitrogen, Sulfur, or Halogens Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66, we face a challenge: these atoms are locked inside stable covalent structures. To detect them using standard inorganic reagents (which look for ions), we must first find a way to break those covalent bonds and convert the elements into an ionic form. This transition from covalent to ionic is the "secret sauce" of qualitative organic analysis.

Feature	Ionic Compounds	Covalent (Organic) Compounds
Bond Formation	Transfer of electrons (forming ions)	Sharing of electrons (forming molecules)
Melting/Boiling Points	High (strong ionic attraction)	Low (weak intermolecular forces)
Electrical Conductivity	Good (in molten state or solution)	Poor (no free ions)
Solubility in Water	Generally Soluble	Generally Insoluble

Key Takeaway Ionic compounds are characterized by strong electrostatic forces and ion formation, whereas covalent organic compounds consist of neutral molecules with weaker attractions, making them poor conductors.

Remember Ionic = Ions (Strong & High); Covalent = Co-operation/Sharing (Weak & Low).

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.58-59; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66

3. Foundations of Qualitative Analysis (basic)

Welcome back! Now that we understand the unique nature of carbon—specifically its tetravalency and catenation properties Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.63—we need to learn how chemists actually "identify" what is hidden inside an organic molecule. While organic compounds are primarily made of carbon and hydrogen, they often play host to other elements like Nitrogen (N), Sulfur (S), and Halogens (Cl, Br, I) Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.65. The challenge? These elements are locked away in covalent bonds, meaning they don't just pop out in water for us to test easily.

To solve this, we use the Lassaigne’s Test (also known as the Sodium Fusion Test). The logic is simple but brilliant: we force these covalent elements to become ionic so they can dissolve in water. We do this by melting a tiny piece of the organic compound with a pellet of metallic sodium. The sodium is a powerful reducing agent that "rips" the Nitrogen, Sulfur, or Halogens out of their organic structure and converts them into water-soluble sodium salts. This resulting liquid is called the Sodium Fusion Extract or Lassaigne's Extract.

Element Present	Reaction during Fusion	Sodium Salt Formed
Nitrogen (N)	Na + C + N → NaCN	Sodium Cyanide
Sulfur (S)	2Na + S → Na₂S	Sodium Sulfide
Halogens (X = Cl, Br, I)	Na + X → NaX	Sodium Halide

Once we have these salts in the extract, we can run simple chemical tests. For example, to detect Halogens, we add Silver Nitrate (AgNO₃). If chlorine is present, it forms a white precipitate of AgCl; bromine gives a pale yellow precipitate of AgBr; and iodine gives a distinct yellow AgI. However, there is a catch: Fluorine cannot be detected this way. Even if sodium fluoride (NaF) is formed, when you add silver nitrate, the resulting Silver Fluoride (AgF) is highly soluble in water. Since no precipitate forms, we cannot "see" the result, making the test useless for Fluorine.

Key Takeaway Lassaigne’s test converts covalent elements (N, S, Halogens) into water-soluble ionic salts using sodium fusion, but it fails to detect Fluorine because Silver Fluoride does not form a precipitate.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.63; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.65

4. Purification Techniques for Organic Compounds (intermediate)

In organic chemistry, obtaining a substance in its pure form is the first step toward understanding its properties. Because most organic compounds coexist with impurities in nature or during synthesis, we rely on physical property differences to isolate them. Purification is the process of removing these unwanted substances, while qualitative analysis (like Lassaigne's test) verifies which elements are present in the purified sample.

Two of the most fundamental physical techniques are sublimation and distillation. Sublimation occurs when a solid transitions directly into a gas phase without becoming a liquid Physical Geography by PMF IAS, Hydrological Cycle, p.329. This is an excellent way to purify solids like camphor or naphthalene if the impurities do not sublime. On the other hand, distillation is used for liquids. It relies on the fact that different hydrocarbons or organic fractions have distinct boiling points. In a mixture like crude petroleum, we use heat to split the oil into "groups" or "fractions"; each group is extracted as it reaches its specific boiling point Certificate Physical and Human Geography, Fuel and Power, p.269.

Technique	Physical Principle	Common Use Case
Sublimation	Direct Solid to Gas transition	Purifying volatile solids (e.g., Iodine, Camphor)
Distillation	Difference in Boiling Points	Separating liquid mixtures like crude oil
Lassaigne's Test	Conversion of covalent to ionic bonds	Detecting Nitrogen, Sulfur, and Halogens

Once a compound is purified, we use chemical tests to identify its constituent elements. A gold-standard method is Lassaigne's Test (sodium fusion test). In this process, the organic compound is fused with metallic sodium, converting covalent bonds (like C-N or C-X) into water-soluble ionic salts like NaCN, Na₂S, or NaX (where X is a halogen). These salts are then reacted with specific reagents to produce a visible change, much like how breathing into lime water causes it to turn milky to detect CO₂ Science Class X (NCERT), Life Processes, p.87. However, a crucial exception exists: Fluorine cannot be detected this way because the resulting AgF (Silver Fluoride) is highly soluble in water and does not form the characteristic precipitate needed for a positive identification.

Remember: Lassaigne's "fuses" the elements into salts. If it stays dissolved (like AgF), you can't "see" it!

Key Takeaway Purification relies on physical phase changes (like boiling or subliming), while detection techniques like Lassaigne's Test rely on converting covalent elements into measurable ionic salts.

Sources: Physical Geography by PMF IAS, Hydrological Cycle, p.329; Certificate Physical and Human Geography, Fuel and Power, p.269; Science Class X (NCERT), Life Processes, p.87

5. Quantitative Estimation: Kjeldahl and Dumas Methods (intermediate)

In our previous steps, we focused on detecting the presence of elements (qualitative analysis). Now, we move to Quantitative Estimation—calculating the exact percentage of an element within a compound. Since nitrogen is an essential constituent of proteins and a basic building block of living tissue (Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19), accurately measuring it is vital for agriculture, food science, and medicine.

Because elemental nitrogen is relatively inert (Physical Geography by PMF IAS, Earths Atmosphere, p.272), we cannot measure it through simple reactions. Instead, we use two rigorous standard methods: the Dumas Method and the Kjeldahl Method. Each relies on breaking down the organic molecule to release nitrogen in a measurable form, such as N₂ gas or NH₃ (Ammonia).

Feature	Dumas Method	Kjeldahl Method
Core Principle	High-temperature combustion with Copper Oxide (CuO).	Digestion with concentrated sulfuric acid (H₂SO₄).
End Product	Nitrogen gas (N₂).	Ammonia gas (NH₃) which is then titrated.
Applicability	Works for all organic nitrogen compounds.	Does not work for nitro, azo groups, or nitrogen in rings (e.g., pyridine).

The Dumas Method is often called the 'absolute' method. The organic compound is heated with CuO in a CO₂ atmosphere. The nitrogen is converted to N₂ gas, which is collected over a solution of Potassium Hydroxide (KOH). The KOH absorbs the CO₂, leaving only pure nitrogen to be measured by volume. In contrast, the Kjeldahl Method is more popular for food and soil analysis. The nitrogen is converted into ammonium sulfate, which is then decomposed by an alkali to release ammonia. By calculating how much acid the ammonia neutralizes, we can work backward to find the nitrogen percentage.

Remember: Dumas is for Directly measuring N₂ gas; Kjeldahl is for Konverting to Ammonia (NH₃).

Key Takeaway While the Dumas method is universally applicable to all nitrogen compounds, the Kjeldahl method is the industrial standard for proteins, despite its inability to measure nitrogen in nitro or heterocyclic structures.

Sources: Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19; Physical Geography by PMF IAS, Earths Atmosphere, p.272

6. The Chemistry of the Sodium Fusion Test (Lassaigne's Test) (exam-level)

In organic chemistry, most elements like Nitrogen, Sulfur, and Halogens are bound to Carbon through covalent bonds. These bonds are strong and non-polar, meaning they don't easily break apart into ions in water. However, most qualitative analytical tests—like the ones we use to identify ions in a lab—require the elements to be in an ionic, water-soluble form. This is where the Sodium Fusion Test (also known as Lassaigne’s Test) comes in. It is a process designed to "brute-force" these covalent elements into inorganic, ionic salts.

The process involves heating a small amount of the organic compound with a pellet of metallic Sodium in a fusion tube. Sodium is chosen because it is highly reactive and readily transfers electrons to form ionic compounds Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49. Because Sodium reacts vigorously even with moisture Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44, this fusion must be done carefully. During the reaction, the Sodium "rips" the elements out of the organic structure: Carbon and Nitrogen combine with Sodium to form Sodium Cyanide (NaCN), Sulfur forms Sodium Sulfide (Na₂S), and Halogens (X) form Sodium Halides (NaX). These are entirely new substances with properties different from the original elements Science, Class VIII (NCERT 2025 ed.), Nature of Matter, p.128.

Once the fusion is complete, the red-hot tube is plunged into distilled water, breaking the glass and dissolving the newly formed salts into what we call the Lassaigne’s Extract. We then perform specific chemical tests on this extract. For example, to detect halogens, we add Silver Nitrate (AgNO₃). If Chlorine, Bromine, or Iodine are present, they form distinct precipitates: AgCl (white), AgBr (pale yellow), and AgI (yellow). However, Fluorine is a unique exception. Even though Sodium Fluoride (NaF) is formed, when Silver Nitrate is added, Silver Fluoride (AgF) does not precipitate because it is highly soluble in water. Therefore, Fluorine cannot be detected using this standard method.

Key Takeaway Lassaigne's test converts covalent Nitrogen, Sulfur, and Halogens into ionic sodium salts to make them detectable via inorganic qualitative analysis.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44, 49; Science, Class VIII (NCERT 2025 ed.), Nature of Matter, p.128

7. Solubility Paradox of Silver Halides (exam-level)

In the world of qualitative analysis, the silver halides (compounds of silver with fluorine, chlorine, bromine, and iodine) usually behave like a well-coordinated team—except for Silver Fluoride (AgF). While AgCl, AgBr, and AgI are famous for being insoluble in water and forming distinct colored precipitates, AgF is highly soluble. This "solubility paradox" is a critical concept in chemistry because it explains why some elements are harder to detect than others in a laboratory setting.

According to Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.137, solubility is defined as the maximum amount of solute that can dissolve in a fixed quantity of solvent. The reason AgF dissolves so easily while its siblings do not lies in the balance between Lattice Energy (the energy holding the crystal together) and Hydration Energy (the energy released when ions interact with water). Because the fluoride ion (F⁻) is exceptionally small, it has a very high hydration energy. When AgF is placed in water, the energy released by the water molecules "grabbing" the F⁻ ions is enough to overcome the lattice structure, pulling the salt into solution. As we move down the group to Iodine, the ions become larger and more polarizable, increasing the covalent character of the bond (as per Fajans' Rules), which makes them resist dissolving in polar water.

Silver Halide	Appearance	Solubility in Water
AgF	Colorless/White	Highly Soluble
AgCl	White Precipitate	Insoluble
AgBr	Pale Yellow Precipitate	Insoluble
AgI	Yellow Precipitate	Insoluble

This paradox has a major impact on the Lassaigne’s Test (sodium fusion test). In this test, we look for precipitates to confirm the presence of halogens. When we add silver nitrate (AgNO₃) to a solution containing chloride, bromide, or iodide, we see a visible solid form immediately. However, because AgF is soluble, no precipitate forms even if fluorine is present. This is why the standard silver nitrate test fails to detect fluorine, making it the "hidden" halogen in organic analysis.

Key Takeaway Silver fluoride (AgF) is the only silver halide soluble in water due to its high hydration energy and high ionic character, which prevents it from forming a precipitate in standard qualitative tests.

Sources: Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.137

8. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of qualitative analysis, you can see how Lassaigne's test acts as a bridge between organic covalent bonding and inorganic ionic reactions. The core principle you learned is sodium fusion: we treat an organic compound with metallic sodium to convert elements like nitrogen, sulfur, and halogens into water-soluble sodium salts (NaCN, Na2S, and NaX). This transformation is essential because it allows us to use standard inorganic reagents to identify the presence of these elements through visible precipitates or color changes.

To arrive at the correct answer, (D) F, you must apply the specific logic of the silver nitrate test used for halogens. While chlorine, bromine, and iodine form insoluble precipitates (AgCl, AgBr, and AgI) when reacted with silver nitrate, fluorine behaves differently. Because silver fluoride (AgF) is highly soluble in water, it does not form a precipitate. Therefore, even if fluorine is successfully converted to sodium fluoride during the fusion process, there is no visible physical change to confirm its presence. This chemical exception is exactly the type of "anomaly" that UPSC loves to test.

The other options—Iodine (I), Chlorine (Cl), and Sulfur (S)—are common traps because they are the standard elements this test is designed to find. Sulfur is easily detected via the lead acetate test or the sodium nitroprusside reaction, while Chlorine and Iodine provide distinct white and yellow precipitates, respectively. When you see a list of halogens in a UPSC question, always look for the one that breaks the periodic trend; in this case, the unique solubility of AgF makes fluorine the only element here that remains "invisible" under these test conditions.

Sources:

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Question map

Which one of the following elements cannot be detected by "Lassaigne's test"?

Explanation

Detailed Concept Breakdown

1. Introduction to Heteroatoms in Organic Chemistry (basic)

2. Covalent vs. Ionic Bonding in Analytical Chemistry (basic)

3. Foundations of Qualitative Analysis (basic)

4. Purification Techniques for Organic Compounds (intermediate)

5. Quantitative Estimation: Kjeldahl and Dumas Methods (intermediate)

6. The Chemistry of the Sodium Fusion Test (Lassaigne's Test) (exam-level)

7. Solubility Paradox of Silver Halides (exam-level)

8. Solving the Original PYQ (exam-level)

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