What is Lassaigne Test? – Complete Explanation and Procedure
The Lassaigne Test is an important topic in JEE Main Chemistry, helping students understand how to detect elements like nitrogen, sulphur, and halogens in organic compounds. To simplify your preparation, we provide a clear explanation that covers everything you need to know about this test. This includes detailed explanations, solved examples, and essential tips curated by subject matter experts, ensuring you are fully prepared for your exam. Download it now and study anytime, anywhere!
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Lassaigne Test for Nitrogen, Sulphur and Halogens
Lassaigne's TEST helps to detect Nitrogen, Sulphur, and Halogens in organic compounds. Here, sodium metal (Na) is heated in the presence of the organic compound inside a fusion tube. The primary objective of this test is that all the available elements are being converted by Sodium metal (Na) into the ionic state.
Lassaigne test reaction for sodium is given below
N + Na + C → NaCN
S +2Na → Na2S
X + Na → NaX
In the third experiment, X can be anything such as Cl, Br, or I.
Do You know about the Test for Halogen?
The newly converted ionic salts are taken out from the fused (bonded) mass by boiling it and applying distilled water. This entire procedure is known as sodium fusion extract.
Sodium halide Halogens compounds are created via the fusion of sodium metal along with the organic compound which exists inside that tube. The Sodium halide can be removed with water.
After we apply the acid, dilute HNO3, it can be recognized without any difficulty by putting silver nitrate solution.
There is a chance of the formation of a snowy curdy precipitate, which is soluble in ammonium hydroxide solution if any chlorine atom exists.
Cl + Na → NaCl
AgNO3+ NaCl → NaNO3+ AgCl
There is a chance of forming a yellowish-white precipitate partially soluble in ammonium hydroxide solution if bromine is present in that solution.
Br + Na → NaBr
AgNO3 + NaBr → NaNO3 + AgBr
There is a chance of getting a yellow precipitate, which is insoluble in the ammonium hydroxide solution if the iodine is present in that particular solution.
I + Na → NaI
AgNO3 + NaI → NaNO3 + AgI
Lassaigne Test for Nitrogen
The salt which is extracted is heated with FeSO4 and applied with the concentrated acid called H₂SO₄. The existence of nitrogen can be confirmed if we get a Prussian blue colour.
The reactions are taking place in this process are,
6CN- + Fe2+→ \[\left [ Fe\left ( CN \right ) _{6}\right ]^{4-}\]
H++ Fe2+ → e⁻ + Fe3+
4Fe3+ + \[\left [ Fe\left ( CN \right ) _{6}\right ]^{4-}\] → Fe4 \[\left [ Fe\left ( CN \right ) _{6}\right ]\].H2O
The ferrous ions oxidation is conducted by the acids, which are helpful to form ferric ions. The presence of nitrogen can be confirmed when there is the availability of the ferriferous cyanide in that solution.
The important fact about this test is that it is not prearranged by the complexes comprising N but not C atoms.
For illustration, NH2NH2 does not respond to this test regardless of having an N atom. This is the reason that both C and N are mandatory to produce CN– ions.
The test we are conducting is not satisfied by diazonium salts because they break down to generate nitrogen gas on boiling.
Lassaigne Test Procedure
Lassaigne's test, also known as the sodium fusion test, has the application in the elemental analysis for the subjective calculation of the existence of the foreign compounds, specifically; Halogens, Nitrogen, and Sulphur, in an organic compound.
This procedure was established by J. L. Lassaigne. This experiment involves boiling the sample roughly with the pure sodium metal, by fusing it with the solution.
There are numerous types of methods that have been defined. The water helps the “fused" sample to emerge within it, and the usual conditional trials are executed on the subsequent solution for the particular possible constituents.
What is the Lassaigne Test?
The type of bonding of the components such as halogens, nitrogen, and sulphur with the organic compounds are covalent.
For the identification, the components are required to be changed into their ionic states. The procedure is possible with the help of the fusion of the organic compound in the presence of sodium metal.
Discuss Lassaigne Test
There is a procedure to test the existence of a halogen, nitrogen, or sulphur in an organic solution. A model is excited with heat in a test tube with a bit of sodium. The boiling tube is released into the pure water and the wreckages thoroughly in an adhesive.
The halogen existence can be noticed by its precipitation with silver nitrate solution.
The Nitrogen existence can be exposed by the creation of a precipitate of Prussian blue on the warming portion of the solution with iron.
The presence of sulphur can be identified by the precipitation of Lead Ethanoate or Sodium Nitroprusside.
1. Test for Sulphur
The presence of sulphur can be indicated if there is a black precipitate of lead sulphide. This can be done when the clean fusion solution is added to the diluted Acetic acid, producing a lead acetate solution.
If we add 2-3 drops of a freshly prepared dilute solution of sodium pentacyanonitrosyl ferrate Na2[Fe(CN)5NO] to the solution, then a purple pattern designates Sulphur.
S2- + \[\left [ Fe\left ( CN \right )_{5}NO \right ]^{2-}\rightarrow \left [ Fe\left ( CN \right )_{5} NOS\right ]^{4-}\]
2. Test for Halogens
The solution is added with an acid such as HNO3 and then applied with AgNO3.
The presence of Cl is recognized if a white precipitate soluble in NH4OH is found. For the presence of Br, it is yellowish, and for the presence of I, it is yellow. X can be anything such as Cl, Br, or I.
NaX + AgNO3 → NaNO3 +AgX ↓
Significance of Lassaigne Test for Nitrogen, Sulphur and Halogens in JEE main
The Lassaigne Test appears moderately frequently in JEE Main, with around 1-2 questions related to this topic in the organic chemistry section each year.
These questions often focus on reaction mechanisms, identification processes, and the interpretation of results for nitrogen, sulphur, or halogens in compounds.
Understanding this test is essential for solving organic analysis problems effectively in the exam.
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Conclusion
The Lassaigne's test is used to detect the presence of elements like nitrogen, sulphur, halogens (chlorine, bromine, iodine), and phosphorus in organic compounds. The test involves converting these elements into water-soluble inorganic forms by heating the organic compound with sodium metal, followed by specific chemical reactions to confirm their presence. Students should understand and learn the important topics of the IIT entrance exam like the Lassaigne Test by understanding each concept from the best online platform Vedantu with the best teaching and learning material available at an affordable price.
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FAQs on Lassaigne Test for JEE Main Chemistry 2025
1. Describe the Fact about the usage of Sodium Metal in the Lassaigne Test?
The reason behind the application of sodium metal is that this metal responds with the elements currently available in the organic compounds to create the inorganic compounds.
2. Can you State the Fact that the Preservation of Metallic Sodium should be Kept in Kerosene?
As we know, Potassium, as well as Sodium, are extremely sensitive metals. They interact so energetically with the carbon dioxide, oxygen, and moisture available in the air that it may even be the reason for the generation of fire. The safest way is to store sodium in kerosene as it does not make any kind of activity with the metal.
3. Can you explain the Reason Behind the Preservation of Phosphorus Inside the Water?
As we know, the white phosphorus is extremely volatile, and impulsively explodes near about 30°C in humid or wet air. This is why it is frequently kept underwater, to avert its coverage to the air. Also, it is enormously toxic even in tiny amounts.
4. How long will it take for students to understand and learn about the Lassaigne Test in their preparation for the entrance exam of IIT JEE?
Students who want to study the Lassaigne Test can understand the topic well from the given article in approximately two hours. For further detailed study about the Lassaigne test, students can also refer to the Question-Answer articles on this topic provided by Vedantu teachers.
5. What is Lassaigne Test?
The Lassaigne Test is a method used to detect the presence of elements like nitrogen, sulphur, and halogens in organic compounds by converting them into ionic forms.
6. How is Lassaigne Test for nitrogen performed?
Lassaigne Test for nitrogen involves fusing the compound with sodium to form sodium cyanide, which reacts with ferrous sulphate to form Prussian blue, confirming nitrogen.
7. Which compound does not give Lassaigne Test?
Compounds like azides and nitrates do not give the Lassaigne Test as they decompose during the fusion process without forming detectable products.
8. Why is blood red colour observed in Lassaigne Test?
Blood red colour in Lassaigne Test is due to the formation of ferric thiocyanate when both nitrogen and sulphur are present in the compound, lassaigne test blood red colour is the observation of the experiment.
9. What is the Lassaigne Test reaction for halogens?
In Lassaigne Test, halogens react with sodium to form sodium halides, which give precipitates with silver nitrate: white for chloride, yellow for iodide, and pale yellow for bromide.
10. Where can I download the Lassaigne Test PDF for JEE Main 2025?
You can download the Lassaigne Test PDF for JEE Main 2025 from Vedantu. It includes detailed concepts, solved examples, and practice questions for better preparation.
11. How does Lassaigne Test diagram help in understanding the process?
Lassaigne Test diagrams simplify the explanation of chemical reactions, showing how nitrogen, sulphur, and halogens form ionic compounds detectable through the test.
12. Why is Lassaigne Test significant for JEE and NEET exams?
Lassaigne Test is significant for JEE and NEET as it tests foundational concepts in organic chemistry, frequently appearing in exam questions.
13. What challenges can arise in Lassaigne Test for nitrogen?
Challenges in Lassaigne Test for nitrogen include false negatives if the compound does not produce sodium cyanide during fusion, leading to incorrect results.
