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MCQEasyJEE 2025Detection of Elements in Organics

JEE Chemistry 2025 Question with Solution

Given below are two statements:

Statement I: In Lassaigne's test, the covalent organic molecules are transformed into ionic compounds.

Statement II: The sodium fusion extract of an organic compound having N and S gives prussian blue colour with FeSO4FeSO_4 and Na4[Fe(CN)6]Na_4[Fe(CN)_6].

In the light of the above statements, choose the correct answer from the options given below.

  • A

    Both Statement I and Statement II are true

  • B

    Both Statement I and Statement II are false

  • C

    Statement I is false but Statement II is true

  • D

    Statement I is true but Statement II is false

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: Two statements about Lassaigne's test are to be checked.

Find: Which statement is true or false, and hence the correct option.

In Lassaigne's test, covalently bonded elements in an organic compound are converted into water-soluble ionic sodium salts on fusion with sodium. Thus nitrogen, sulfur, and halogens become detectable in ionic form.

Examples mentioned in the solution are:

NaCN,Na2S,NaCl,NaBr,NaINaCN, \quad Na_2S, \quad NaCl, \quad NaBr, \quad NaI

So Statement I is true.

Now examine Statement II. If an organic compound contains both N and S, sodium fusion produces thiocyanate:

NaSCNNaSCN

This does not give Prussian blue colour as stated. Instead, such a case is associated with blood-red colour formation with iron in acidic medium.

Prussian blue is characteristic of the nitrogen test, not the combined N and S case described here.

Therefore, Statement I is true but Statement II is false. Hence, the correct option is D.

Detailed Statement Analysis

Given:

  • Statement I concerns conversion of covalent organic compounds into ionic compounds in Lassaigne's test.
  • Statement II concerns the colour obtained for an organic compound containing N and S.

Find: The correct truth-value combination.

Analysis of Statement I: Lassaigne's test involves fusion of the organic compound with sodium metal. During this process, covalently bonded elements are converted into ionic sodium salts that are soluble in water and can be tested.

For example:

N→NaCNN \rightarrow NaCN S→Na2SS \rightarrow Na_2S X→NaXX \rightarrow NaX

where XX represents a halogen.

Hence, Statement I is true.

Analysis of Statement II: When both nitrogen and sulfur are present in the organic compound, fusion with sodium leads to formation of thiocyanate:

NaSCNNaSCN

This does not produce Prussian blue colour in the manner stated.

The solution text states that the combined presence of N and S gives a blood-red colour with iron in acidic medium, whereas Prussian blue is associated with the nitrogen test.

Therefore, Statement II is false.

Conclusion: Statement I is true but Statement II is false. So the correct option is D.

Common mistakes

  • Assuming that any positive result involving nitrogen must give Prussian blue is incorrect. Prussian blue is associated with the nitrogen test, whereas the simultaneous presence of N and S leads to thiocyanate formation and a different colour test. First identify whether nitrogen is present alone or together with sulfur.

  • Thinking that Lassaigne's test only breaks the compound and does not change bonding nature is incorrect. The purpose of sodium fusion is to convert covalently bonded elements into water-soluble ionic sodium salts. Always focus on this covalent-to-ionic conversion.

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