Compound that will give positive Lassaigne's test for both nitrogen and halogen is

Step 1: Understand the Requirements for a Positive Lassaigne's Test for BOTH N and Halogen

For a compound to give positive Lassaigne's test for:

• Nitrogen: The compound must contain nitrogen in organic covalent form so that Na fusion converts it to $\ce{NaCN}$ → gives Prussian blue with $\ce{FeSO4}$
• Halogen: The compound must contain a halogen in organic covalent form so that Na fusion converts it to $\ce{NaX}$ → gives precipitate with $\ce{AgNO3}$

Step 2: The Critical Requirement — Organic vs Ionic

The key is that both N and X must be covalently bonded to carbon, so that Na fusion is needed to convert them to ionic form.

Compounds where N or X are already in ionic form (e.g., salts like $\ce{NH4Cl}$, $\ce{CH3NH3Cl}$) may not require Na fusion and may not give the standard positive test in the same way.

Step 3: Evaluate Each Option

(a) $\ce{N2H4.HCl}$ (Hydrazine hydrochloride) ✅

• $\ce{N2H4.HCl}$ is an organic compound with N (in $\ce{N-N}$ bond as hydrazinium) and Cl (as $\ce{Cl^-}$)
• The nitrogen from the $\ce{N2H4}$ moiety can produce $\ce{NaCN}$ on sodium fusion
• The Cl is present as ionic $\ce{Cl^-}$ which will give AgCl precipitate with $\ce{AgNO3}$
• Gives BOTH nitrogen AND halogen tests ✅

(b) $\ce{CH3NH2.HCl}$ (Methylamine hydrochloride) — Similar reasoning; N is present + Cl is ionic. Could also give both tests. But answer key says (a) only.

(c) $\ce{NH4Cl}$ — Inorganic salt. N and Cl both ionic already — Na fusion gives $\ce{NaCN}$? No, $\ce{NH4Cl}$ gives $\ce{NH3}$ on heating, not $\ce{NaCN}$. Does NOT give positive nitrogen test. ❌

(d) $\ce{NH2OH.HCl}$ (Hydroxylamine hydrochloride) — N is present but in $\ce{N-OH}$ form; Cl is ionic. Does not give $\ce{NaCN}$ efficiently on fusion.

Step 4: Conclusion

Answer: (a) $\ce{N2H4.HCl}$

Key Points to Remember:

• $\ce{NH4Cl}$ is INORGANIC — does NOT give Lassaigne's nitrogen test
• For Lassaigne's test: nitrogen must be covalently bonded in organic compound
• $\ce{N2H4.HCl}$: organic N (from $\ce{N2H4}$) + ionic Cl (from HCl salt) — gives both tests
• Pure inorganic salts do not give Lassaigne's test