Fire needs exactly three things at once, a candle flame is actually three different flames stacked together, and not all fuels release the same amount of energy per kilogram.
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Combustion is a chemical reaction that needs three things together, at the same time — remove any one and the fire goes out.
📌 The fire triangle
Fuel: a combustible substance — wood, LPG, kerosene.
Air (oxygen): combustion cannot happen without oxygen to react with the fuel.
Heat to ignition temperature: the fuel must reach the minimum temperature at which it catches fire on its own.
Why a fire extinguisher works: water cools the fuel below its ignition temperature; CO₂ and foam extinguishers cut off the oxygen supply. Both remove one side of the fire triangle — that is the whole trick.
A candle flame is not one uniform fire — it has three distinct zones, and each one burns at a different temperature.
📊 Inside a candle flame
| Zone | Location | Temperature & colour |
|---|---|---|
| Dark/innermost zone | Closest to the wick | Coolest — unburnt wax vapour, no oxygen reaches here |
| Luminous zone | Middle, yellow region | Moderately hot — partial combustion, glowing carbon particles |
| Non-luminous zone | Outermost, nearly invisible | Hottest — complete combustion, plenty of oxygen |
Olympiad trap: the outermost zone is the HOTTEST, even though it looks the least bright — this is exactly why a goldsmith holds a flame's outer tip against gold to melt it, not the glowing middle.
A fuel's calorific value tells you how much energy it packs per kilogram — this is what decides which fuel is the most efficient, not just the cheapest.
🏆 Calorific value (approximate, kJ/g)
| Fuel | Calorific value | Note |
|---|---|---|
| LPG | ~50 kJ/g | One of the highest — burns cleanly, used in homes |
| Petrol | ~45 kJ/g | High energy, used in vehicles |
| Wood | ~17 kJ/g | Much lower — needs more mass burnt for the same energy |
| Cow dung cake | ~6 kJ/g | Lowest of common fuels, and produces more smoke |
Pro tip: An ideal fuel has a high calorific value, is easily available, burns without much smoke, and leaves little ash — no single fuel is perfect on every count, which is exactly what Achievers questions test.
12 NSO-pattern questions on combustion and fuels, including two Achievers-level questions.
