Hint: Lassaigne’s test is used in case of mainly in the detection of the first element of group 15 and 2nd and 3rd period of group 16 and group 17 elements, i.e. to test their presence in the hydrocarbons. It is done with the help of alkali metals. So, we can explain the test with the help of reactions.
Complete answer: Now, let us discuss the Lassaigne’s test. > As mentioned it is used to test the presence of halogens, nitrogen, and sulphur in the hydrocarbons, or the organic compounds. We can say that this test is done by the use of sodium metal. > Suppose, we have a fusion tube containing the sodium metal in consideration with the organic compound. Then, that small piece of sodium will be heated, and there will be conversion of all the metals into the ionic form. Now, if we look at the reactions, then it can be written as: Na + C + N $\rightarrow$ NaCN 2Na + S $\rightarrow$ Na$_2$S Na + X $\rightarrow$ NaX (X = Cl, Br, or I) > Now, after seeing the reactions, we can predict that there is formation of ionic salts. > This process is also named as the sodium fusion extract because the salts attained will be extracted further by boiling with the distilled water. > In the last, we can conclude that the Lassaigne’s test is the presence detector test of halogens in the hydrocarbons as discussed.
Note: Don’t get confused with the nitrogen compounds. Suppose we have the diamine compound then it will not perform this test as according to the reactions mentioned there should be presence of carbon with the nitrogen atom. As the formation of cyanide ion is required.
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