On burning hydrogen in air, the colour of flame is — Hydrogen Chemistry Question

💡 Solution & Explanation

# Burning of Hydrogen in Air **Chemical Reaction:** $$2H_2 + O_2 \xrightarrow{\text{ignition}} 2H_2O$$ **Why the flame is pale blue:** 1. **Hydrogen burns with a pale blue flame** because the combustion reaction releases significant energy (~286 kJ/mol per mole of $H_2$), which excites electrons in the product water molecules and surrounding gases. 2. **Emission spectrum:** The pale blue color arises from: - Energy emission in the visible blue region (~400-450 nm) - The flame is relatively cool compared to other fuel combustion (only ~2000-2100 K) - Water vapor and trace amounts of excited species emit blue light 3. **Why it's "pale":** Unlike flames from carbon/hydrocarbons (which burn yellow/orange due to incandescent soot particles), hydrogen produces **no soot or solid combustion products** — only gaseous water, making the emission pale rather than bright. **Answer: B (Pale blue)** The pale blue color is the diagnostic test for hydrogen gas and distinguishes it from other fuel combustions.