Decomposition of calcium nitrate produces oxygen gas

Steps:

• Anhydrous calcium nitrate decomposes on heating: 2Ca(NO3)2 → 2CaO + 4NO2 + O2.
• The reaction produces oxygen gas (O2) along with brown NO2, but the test focuses on the relighting effect of O2.
• A glowing splint relights in oxygen due to its oxidizing property.
• The gas mixture appears colourless overall in this context, as NO2 is minimal or dissipated.

Why B is correct:

• Oxygen is colourless and relights a glowing splint, matching the standard laboratory test for O2.

Why the others are wrong:

• A: Brown gas (NO2) would not relight the splint; it extinguishes flames.
• C: Brown gas (NO2) is toxic and does not support combustion to relight the splint.
• D: Colourless gas that extinguishes a splint indicates CO2 or nitrogen, not oxygen.

Final answer: B