To detect foreign elements present in a given organic compounds (N, S and X) | NEB Grade XII Chemistry Practical | Nepali Educate

Experiment No.: 10

Name of the Experiment: Detecting Foreign Elements Present in Given Organic Compound

Apparatus Required:

• Ignition tube
• Tong
• Mortar
• Pestle
• Funnel
• Test tube
• Beaker
• Porcelain basin
• Burner
• Funnel stand

Chemicals Required:

• Sodium metal
• Organic compound
• Silver nitrate (AgNO₃) solution
• Iron(II) sulfate (FeSO₄) solution
• Iron(III) chloride (FeCl₃) solution
• Sodium hydroxide (NaOH)

Theory:

Foreign elements in organic compounds are covalently bonded with carbon and do not ionize in aqueous solution, making direct detection difficult. To convert them into ionic compounds, they are fused with sodium metal, which then ionizes in aqueous solution.

1. If nitrogen is present, it forms sodium cyanide: Na + C + N → NaCN → Na⁺ + CN^-
2. If sulfur is present, it forms sodium sulfide: 2Na + S → Na₂S → 2Na⁺ + S^--
3. If halogens are present, sodium halides are produced: Na + X → NaX → Na⁺ + X^-
4. If both nitrogen and sulfur are present, sodium thiocyanate is formed: Na + C + N + S → NaCNS → Na⁺ + CNS^-

Procedure:

Preparing Sodium Extract (Lassaigne's Solution):

1. Take a freshly cut pea-sized sodium metal from kerosene oil and dry it with filter paper.
2. Place it in a fusion tube and heat it to its melting point.
3. Add a pinch of organic compound to the tube containing sodium metal.
4. Heat the fusion tube slowly at first and then strongly until it becomes red-hot.
5. Plunge the red-hot tube into distilled water in a mortar and grind it with a pestle.
6. Filter the solution. The filtrate obtained is called sodium extract (Lassaigne's solution).

Specific Tests:

Experiment	Observation	Inference
Detecting Nitrogen	2ml of sodium extract in a test tube + 2 drops NaOH + 1ml of freshly prepared FeSO4 solution + boiling + cooling + few drops of dil. H2SO4	A deep green color was observed, confirming the presence of Nitrogen.
Detecting Sulphur	2ml of sodium extract + few drops of acetic acid + lead acetate solution.	No black precipitate was formed, indicating the absence of Sulphur.
Detecting Nitrogen & Sulphur Together	2ml of sodium extract + 2ml of HCl + 1-2 drops of ferric chloride	No blood red color was obtained, confirming the absence of both Sulphur and Nitrogen together.
Detecting Halogen	2ml of sodium extract + 1ml of dil. HNO3 + boil + cool + few drops of AgNO3 solution.	No precipitate was observed, indicating the absence of Halogen.

Result:

Hence, Nitrogen was detected as a foreign element in the given organic compound.

Conclusion:

Foreign elements can be detected using Lassaigne's test.

Precautions:

• Ensure sodium metal is dried properly.
• Apply gentle heat during the experiment.
• Use only a small amount of the organic compound.