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Lassaigne Test

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Last updated date: 19th Jul 2024
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Explanation, Procedure and Important FAQs

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

  1. N + Na + C → NaCN

  2. S +2Na → Na2S

  3. X + Na → NaX


In the third experiment, X can be anything such as Cl, Br, or I.


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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,


  1. 6CN-  + Fe2+→ \[\left [ Fe\left ( CN \right ) _{6}\right ]^{4-}\]

  2. H++ Fe2+ → e⁻ + Fe3+

  3. 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 ↓


Importance of Lassaigne’s Test in IIT JEE:

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FAQs on Lassaigne Test

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. Is there any chance of getting killed By Phosphorus?

The phosphorus can cause some burning issues to flesh or skin that is exposed to it. It can also cause injuries and lead to death by scorching to the bone.

5. 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 in the form of One-word questions, fill in the blanks, a short description like Discuss the Chemistry of Lassaigne Test and Detection of Sulphur in sodium extract Lassaigne test Class 12 along with the verified answers and related questions by a talented team of teachers at the platform. Students can also write and upload their answers by just signing up at Vedantu, for free.

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7. Write at least 3 points in brief as to Why is the Lassaigne test useful?

The Following points describe the importance of the Lassaigne Test-

1) Lassaigne Test is useful in the detection of the elements. 

2) The Lassaigne Test is also used widely to prepare the extract of Lassaigne.

3) Lassaigne test is quite useful in conducting several tests like the Silver Nitrate Test, Sodium Nitroprusside test and the test of Lead Acetate.

4) Lassaigne Test is most commonly and most importantly used for the detection of Halogens, Sulphur and Nitrogen.

8. Which Classes include the topic of the Lassaigne Test as a part of the syllabus for the students of Science?

Science students of Class 12 will find this topic of Lassaigne Test in regards to Detection of Nitrogen as a part of Chemistry syllabus, details of which are provided by the experienced Vedantu teachers in this question-answer article here along with verified answers and related questions. Science Students of Class 11 will find the Lassaigne Test as a part of their syllabus for the detection of nitrogen and in the topic, Organic Chemistry, details of the same are provided herein.

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Students who want to excel in the Topic of Lassaigne Test and other related topics of Chemistry should start their basic learning right from school. Students are required to have a keen interest and passion for science especially chemistry and push their comfort zone by solving references like RD Sharma and RS Aggarwal. This extra mile walk will pay off in their exam. As a result, it makes students familiar with tricky questions and prepares them to face exam pressure with ease. Through this, students can make a strong foundation of learning. And a consistent study from the Vedantu foundation course with excellent videos and study material by expert teachers especially designed for Chemistry subjects will make your scoreboard go higher than ever. But that is not all. Even if students did not build a firm foundation in the beginning, students can start their success by beginning their crash course in Chemistry covering it all in just a few months. Though this will demand high-level dedication but with the passionate teachers at Vedantu, students can break the iceberg without fail.