Sample Questions on the Lassaigne Test
Following are some questions based on the Lassaigne Test:
Question 1: A student upon performing the Lassaigne test on a sample, observed the formation of a black precipitate when lead acetate solution was added to the sodium fusion extract. What does this observation indicate?
Answer:
This indicates the presence of sulphur in the organic compound tested as formation of black precipitate is due to formation of lead sulphide upon reaction with lead acetate.
Question 2: How is the Lassaigne test be used to distinguish between different halogens present in the organic compounds?
Answer:
In Lassaigne Test, silver nitrate is added to the sodium fusion extract to detect presence of halogens in the organic compound. Silver nitrate forms different halides upon reaction with sodium fusion extract. Thus, different colored precipitates are formed which indicate presence of different halogens listed as follows:
- White precipitate: Chlorine (Cl)
- Cream precipitate: Bromine (Br)
- Yellow precipitate: Iodine (I)
Question 3: What are some sources of error in the Lassaigne Test? How can these be avoided?
Solution:
Sources of error in Lassaigne Test are contamination, incomplete fusion, and improper acidification. These can be avoided by ensuring cleanliness, proper mixing during fusion and precise acidification using appropriate acids.
Lassaigne Test
Lassaigne Test is a set of procedures used to detect the presence of nitrogen, sulphur and halogens in an organic compound. In the theoretical approach, we use different symbols in the molecular formula of a compound to show the presence of a particular element in it but in the practical approach, we use a variety of tests to identify the presence of specific elements through observable reactions or characteristic behaviours when brought in contact with a specific reagent. For instance, when heated, certain metals exhibit certain characteristic flame colours, indicating their presence.
In this article, we will learn about the Lassaigne Test, its principle, the preparation of the Lassaigne Extract, the procedure for the Lassaigne Test, its applications, and some problems based on it.
Table of Content
- What is Lassaigne Test?
- Procedure for the Lassaigne Test
- Examples of Lassaigne Test
- Applications of Lassaigne Test