Chapter 4 · Question 10
Describe the combustion of carbon compounds. Why do saturated hydrocarbons generally burn with a clean blue flame while unsaturated hydrocarbons burn with a yellow sooty flame? Write balanced equations for the combustion of methane and ethanol.
Q10
Describe the combustion of carbon compounds. Why do saturated hydrocarbons generally burn with a clean blue flame while unsaturated hydrocarbons burn with a yellow sooty flame? Write balanced equations for the combustion of methane and ethanol.
Answer Revealed
Direct Answer:
Carbon compounds burn in oxygen to release carbon dioxide, water, heat, and light. Saturated hydrocarbons (alkanes) burn with a clean blue flame because the carbon-to-hydrogen ratio is low — complete combustion is achieved and all carbon oxidises to . Unsaturated hydrocarbons have a higher carbon proportion — they undergo incomplete combustion, producing unburnt carbon particles (soot) that glow yellow when heated. The blackened bottom of a cooking vessel indicates blocked air holes and incomplete combustion. Balanced equations: ; .
Simple Explanation
When carbon compounds burn, they react with oxygen to make , water, and lots of energy as heat and light. If the fuel has enough oxygen, it burns cleanly with a blue flame (like a well-adjusted gas stove). If oxygen is limited, the carbon does not fully burn — unburnt tiny carbon particles (soot) float in the flame and glow yellow, which is why a candle flame is yellow. Unsaturated compounds have more carbon per hydrogen, so they naturally make more soot and burn yellower.
Exam-Ready Structure
Combustion is one of the most important chemical properties of carbon compounds because most fuels are carbon-based: 1. Definition: Carbon, in all its allotropic forms, and its compounds burn in oxygen to give carbon dioxide along with the release of heat and light. 2. Combustion reactions: (a) Carbon: . (b) Methane: . (c) Ethanol: . 3. Flame colour difference: (a) Saturated hydrocarbons (alkanes) generally burn with a clean blue flame. This is because they undergo complete combustion — all the carbon is fully oxidised to , leaving no unburnt carbon particles. (b) Unsaturated carbon compounds (alkenes, alkynes, aromatic compounds) burn with a yellow, sooty (luminous) flame. The higher percentage of carbon and the tendency toward incomplete combustion produce fine particles of unburnt carbon (soot) that glow yellow when heated in the flame. The yellow colour of a candle flame is due to this same mechanism — glowing carbon particles. 4. Activity 4.4 (Bunsen burner): Adjusting the air hole changes the flame. A closed or partially closed air hole restricts oxygen → incomplete combustion → yellow sooty flame. An open air hole allows sufficient oxygen → complete combustion → clean blue flame. 5. Domestic application: Gas or kerosene stoves have air inlets to ensure a sufficiently oxygen-rich mixture for clean combustion. Blackened bottoms of cooking vessels indicate blocked air holes or incomplete combustion — this wastes fuel and produces soot. 6. Environmental note: Coal and petroleum contain nitrogen and sulphur; their combustion produces oxides of sulphur () and nitrogen (), major environmental pollutants.
Key Points
- Combustion: carbon compounds + → + + heat and light
- Methane: ; Ethanol:
- Saturated hydrocarbons → clean blue flame (complete combustion, high oxygen availability)
- Unsaturated hydrocarbons → yellow sooty flame (incomplete combustion, unburnt carbon particles glow)
- Blackened vessel bottom = blocked air holes = incomplete combustion = wasted fuel + soot
- Fossil fuel combustion also produces and pollutants
Related Questions
Q5
Distinguish between saturated and unsaturated carbon compounds with the help of examples — ethane (), ethene (), and ethyne (). Draw their electron dot structures. Which type is more reactive and why?
Q11