The gases produced when Cu(NO3)2(s) was heated are

Analyze the question: We are asked which gases are given off when solid copper(II) nitrate is heated (thermal decomposition).

Concept: Thermal decomposition of nitrates depends on how reactive the metal is:

• K, Na nitrates give the nitrite + O₂
• Nitrates of metals from Mg to Cu (less reactive) give the metal oxide + NO₂ + O₂
• Nitrates of very unreactive metals (Ag, Au) give the metal + NO₂ + O₂

Copper is in the middle group, so its nitrate decomposes to the oxide, with brown nitrogen dioxide and oxygen released.

Solution:

2Cu(NO₃)₂(s) → 2CuO(s) + 4NO₂(g) + O₂(g)

• Blue Cu(NO₃)₂ crystals turn into black CuO residue.
• Brown fumes of NO₂ are seen.
• Oxygen is also released (it relights a glowing splint, though it is diluted by the NO₂).

So the gases produced are O₂ and NO₂.

Checking the options:

• A. NO and NO₂: wrong, NO is not formed.
• B. O₂ and NO: wrong, NO is not formed.
• C. NO₂ and N₂O4: N₂O₄ is just the dimer of NO₂ in equilibrium, not a separate product, and O₂ is missing.
• D. O₂ and NO₂: correct.

Tip: Remember "Middle metals give Oxide + NO₂ + O₂." Brown fumes always mean NO₂. Only Group 1 nitrates (K, Na) give just O₂ with a nitrite.