Thermal decomposition of magnesium nitrate produces oxygen gas and nitric acid

Steps:

• Magnesium nitrate decomposes on heating: Mg(NO3)2 → MgO + 2NO2 + ½O2.
• NO2 gas dissolves in water in the trough, forming HNO3 and lowering pH to acidic levels (around 2).
• Oxygen gas is produced and can be tested with a glowing splint, causing it to relight.
• The solution's pH is measured as acidic, and the gas relights the splint.

Why B is correct:

• NO2 hydrolysis follows HNO2 + HNO3 formation in water, yielding acidic pH 2; oxygen relights splint per standard test for O2.

Why the others are wrong:

• A: pH 8 is basic, but solution is acidic from HNO3.
• C: pH 8 is incorrect; splint relights with oxygen, not extinguishes.
• D: pH 2 is correct, but splint relights, not extinguishes.

Final answer: B