Combustion and Flame
Combustion is the oxygen-driven reaction that releases heat and often light, and this chapter explains its conditions, its types, the zones of a flame, how fuels are judged by calorific value, and how fires are controlled.
Prelims regularly tests basic Science & Technology from here: types of combustion, ignition temperature, the hottest zone of a flame, the right fire extinguisher (water vs CO2), safety-match chemistry, and comparative calorific values of fuels. For GS-III it underpins environmental pollution (particulate matter, carbon monoxide, carbon dioxide), climate change/global warming, the shift to cleaner fuels (CNG, LPG, biogas, hydrogen), and fire-related disaster management. It also feeds energy-security and air-quality debates.
Understand the chapter
What is Combustion?
Combustion is a chemical reaction in which a substance reacts with oxygen to give off heat; sometimes light is also produced, either as a flame or as a glow. The substance that burns is the combustible, also called the fuel, and it may be solid, liquid or gas. A classic distinction: the Sun's heat and light come from nuclear reactions, not from combustion.
- Combustible/fuel burns (wood, charcoal, magnesium, kerosene); non-combustible does not (iron nails, stone, glass).
- Respiration is slow combustion - food (fuel) is oxidised in the body to release heat.
- Sun is NOT combustion: its energy is from nuclear reactions.
Conditions Necessary for Combustion
Three things must be present together to burn: fuel, air (to supply oxygen) and heat to raise the fuel to its ignition temperature. The ignition temperature is the lowest temperature at which a substance catches fire, and a substance cannot burn until it reaches this temperature. The glass-chimney experiment shows that without a fresh air (oxygen) supply the flame flickers, smokes and finally dies.
- Ignition temperature: lowest temperature at which a substance catches fire - differs for each substance.
- Inflammable substances: very low ignition temperature, catch fire easily with a flame - petrol, alcohol, LPG.
- Paper cup with water does not burn: heat is conducted to the water, so paper never reaches its ignition temperature.
- Kerosene's ignition temperature is lower than wood's - so kerosene needs careful storage.
How Do We Control Fire?
Fire needs the 'fire triangle' - fuel, oxygen and heat - so removing any one extinguishes it. Water is the commonest extinguisher: it cools the fuel below its ignition temperature and its vapour blankets out air, but it works only for solids like wood and paper. Water is unsafe for electrical fires (it conducts electricity) and for oil/petrol fires (water sinks while oil floats and keeps burning).
- Fire triangle = fuel + oxygen (air) + heat; remove one to stop fire.
- CO2 is best for electrical and inflammable-liquid fires: heavier than oxygen, it blankets the fuel, cools on expansion, and spares equipment.
- Dry powders of sodium bicarbonate / potassium bicarbonate release CO2 near a fire.
- A blanket on burning clothes works by cutting off the air (oxygen) supply.
Types of Combustion
Combustion is classified by speed and cause. Rapid combustion gives heat and light quickly (gas burning at a stove); spontaneous combustion occurs without any apparent external cause (white phosphorus burns at room temperature; coal-dust and some forest fires). An explosion is a sudden reaction releasing heat, light, sound and a large volume of gas, as when a cracker is ignited or pressure is applied.
- Rapid combustion: fast burning with heat and light (gas stove).
- Spontaneous combustion: bursts into flame with no apparent cause (phosphorus, coal-dust fires in mines).
- Explosion: sudden reaction evolving heat, light, sound and large gas volume; can be triggered by pressure.
- Most forest fires are caused by human carelessness, not nature.
The Flame and its Structure
Only substances that vaporise on heating burn with a flame - molten wax and kerosene rise up the wick, vaporise and burn, whereas charcoal does not vaporise and merely glows. A candle flame has three zones. The outermost blue zone is complete combustion and the hottest; the middle yellow zone is partial combustion and luminous, depositing unburnt carbon (soot); the innermost black zone is unburnt wax vapour and the least hot.
- Vaporising fuels (wax, kerosene) give flames; non-vaporising fuels (charcoal) glow without a flame.
- Outer zone (blue): complete combustion, non-luminous, hottest - goldsmiths use it via a blow-pipe to melt gold/silver.
- Middle zone (yellow): partial combustion, luminous, leaves a black ring of unburnt carbon on a slide.
- Inner zone (black): unburnt wax vapours, least hot.
Fuels, Calorific Value and Fuel Efficiency
A good fuel is cheap, readily available, burns easily in air at a moderate rate, gives out a lot of heat and leaves no undesirable residue - though no fuel is ideal. Fuel efficiency is measured by calorific value: the heat energy produced on complete combustion of 1 kg of fuel, expressed in kilojoules per kilogram (kJ/kg). Comparing values guides fuel choice - hydrogen is by far the highest.
- Calorific value: heat from complete combustion of 1 kg of fuel, unit kJ/kg.
- Ascending order: cow-dung < wood < coal < biogas < petrol/kerosene/diesel (45000) < CNG/methane (50000) < LPG (55000) < hydrogen (150000).
- Good fuel: cheap, available, moderate burning rate, high heat, no residue - no ideal fuel exists.
- Fuels grouped as solid (coal), liquid (kerosene) and gaseous (natural gas).
Burning of Fuels and the Environment
Burning carbon fuels harms the environment in three main ways. They release unburnt carbon particles (particulate matter) that cause respiratory diseases like asthma; incomplete combustion produces carbon monoxide, a poison that can kill people sleeping in a closed room; and complete combustion releases carbon dioxide, whose rising concentration is believed to cause global warming. Wood burning adds harmful smoke and drives deforestation.
- Particulate matter (unburnt carbon) -> respiratory illness such as asthma.
- Carbon monoxide (CO): from incomplete combustion; highly poisonous - never burn coal in a closed room.
- Carbon dioxide (CO2): greenhouse gas; rising levels -> global warming -> melting glaciers, sea-level rise, coastal floods.
- Wood as fuel -> smoke, respiratory problems, deforestation.
Key terms
- Combustion
- A chemical reaction in which a substance reacts with oxygen to release heat (and often light).
- Combustible / Fuel
- A substance that undergoes combustion; may be solid, liquid or gas.
- Ignition temperature
- The lowest temperature at which a substance catches fire.
- Inflammable substances
- Substances with very low ignition temperature that catch fire easily with a flame (petrol, alcohol, LPG).
- Rapid combustion
- Fast burning that releases heat and light quickly, e.g. gas at a stove.
- Spontaneous combustion
- Sudden bursting into flame without any apparent external cause (white phosphorus, coal dust).
- Explosion
- A sudden reaction evolving large amounts of heat, light, sound and gas, often under pressure.
- Calorific value
- Heat energy produced on complete combustion of 1 kg of fuel, measured in kJ/kg.
- Carbon monoxide (CO)
- A poisonous gas from incomplete combustion of carbon fuels that can kill in a closed room.
Must-know facts exam-ready
- Combustion = substance + oxygen -> heat (and sometimes light as flame or glow).
- Fire triangle: fuel + air/oxygen + heat to reach ignition temperature; remove one to extinguish.
- Ignition temperature = lowest temperature at which a substance catches fire.
- Inflammable substances (very low ignition temperature): petrol, alcohol, LPG.
- Hottest part of a candle flame = outermost blue zone (complete combustion); goldsmiths use it to melt gold/silver.
- Flame zones: blue (outer, complete, hottest), yellow (middle, partial, luminous, soot), black (inner, unburnt vapour, coolest).
- CO2 is best for electrical and oil/petrol fires: heavier than oxygen, blankets the fire, and cools on expansion; spares equipment.
- Calorific value unit = kJ/kg; Hydrogen 150000 (highest), LPG 55000, CNG/Methane 50000, Petrol/Kerosene/Diesel 45000, cow-dung 6000-8000 (lowest).
- Incomplete combustion -> carbon monoxide (CO), poisonous; never burn coal in a closed room.
- Vaporising fuels (wax, kerosene) burn with a flame; charcoal does not vaporise, so it only glows.
- Modern safety match: head has antimony trisulphide + potassium chlorate; striking surface has powdered glass + red phosphorus.
- Sun's heat and light come from nuclear reactions, not combustion.
Memory tricks remember it for good
Traps to avoid
- Combustible vs inflammable: 'inflammable' means easily catches fire (very LOW ignition temperature) - it does NOT mean non-flammable.
- The hottest zone of a flame is the outer blue (non-luminous) zone, NOT the bright yellow middle that merely looks hottest.
- The Sun is NOT an example of combustion - its energy is from nuclear reactions, not burning.
- Calorific value is per 1 kg of fuel (kJ/kg) on COMPLETE combustion - do not confuse it with ignition temperature.
- Water must NOT be used on electrical or oil/petrol fires; use CO2 (water conducts electricity and sinks below oil).
- Charcoal and coal burn with a glow, NOT a flame, because they do not vaporise - only vaporising fuels (wax, kerosene) give a flame.
Exam focus
🧠 Prelims angles
- Matching types of combustion (rapid, spontaneous, explosion) to examples (gas stove, phosphorus/coal dust, crackers).
- Flame structure: linking zones (blue/yellow/black) to combustion type and temperature; identifying the hottest zone.
- Comparative calorific values and the unit kJ/kg - ordering fuels; hydrogen highest, cow-dung lowest.
- Choosing the correct fire extinguisher (water vs CO2) and how CO2 / baking soda act.
- Safety-match chemistry: head (antimony trisulphide + potassium chlorate) and striking surface (powdered glass + red phosphorus).
- Pollutants from burning fuels: particulate matter (asthma), carbon monoxide (toxic), carbon dioxide (global warming).
✍️ Mains angles GS-III
- Burning of fossil and biomass fuels is a major driver of air pollution and climate change - examine.Link particulate matter -> respiratory disease, CO -> toxicity, CO2 -> global warming and sea-level rise; argue for cleaner fuels (CNG, LPG, biogas, hydrogen).
- India's transition to cleaner cooking and transport fuels.Use calorific value plus emissions to justify the shift from wood/cow-dung/coal to LPG/CNG/biogas; connect to public health and deforestation.
- Fire safety and disaster management in urban and forest contexts.Apply the fire triangle - prevention by removing fuel/air/heat; note most forest fires are man-made; stress correct extinguisher choice.
Last-minute revision tick as you recall
- Combustion = fuel + oxygen -> heat (and sometimes light).
- Fire triangle: fuel, air, heat to reach ignition temperature.
- Ignition temperature = lowest temp to catch fire; inflammables (petrol, alcohol, LPG) have very low ones.
- Flame zones: blue (outer, hottest, complete) > yellow (partial, soot) > black (inner, coolest).
- Goldsmith uses the outer blue zone because it is hottest.
- CO2 best for electrical/oil fires; water only for wood/paper.
- Calorific value in kJ/kg: hydrogen highest (150000), LPG 55000, cow-dung lowest.
- Vaporising fuels flame (wax/kerosene); charcoal only glows.
- Fuel burning -> particulate matter (asthma), CO (poison), CO2 (global warming).
Distilled from NCERT Class 8 · Science (Class 8) for UPSC. Always cross-check facts with the original NCERT.