What Is Combustion — and Why Does It Matter?
Combustion is a chemical reaction between a substance and oxygen that produces heat and light. That’s the textbook definition, but here’s what actually matters: combustion is everywhere — from the LPG burning in your kitchen to the petrol igniting in a car engine to the matchstick you light during a power cut.
The substance that burns is called a fuel. The minimum temperature at which a fuel catches fire and starts burning on its own is called its ignition temperature (also written as kindling temperature). Below this temperature, no combustion happens — no matter how much oxygen is present.
For combustion to occur, three things must come together at the same time. Remove any one, and the fire goes out. This is the basis of every fire extinguisher ever made.
Three conditions for combustion:
- Fuel — the substance that burns
- Oxygen (or air) — supports the burning
- Heat — temperature must reach the ignition point
Remove any one → fire stops.
Key Terms and Definitions
Combustion — A chemical process in which a substance reacts with oxygen to give off heat and light. Example: coal burning in a furnace.
Fuel — Any material that burns to produce energy. Classified as solid (coal, wood), liquid (petrol, kerosene), or gaseous (LPG, CNG).
Ignition temperature — The lowest temperature at which a fuel starts burning spontaneously. Paper: ~233°C. Petrol: ~280°C. Coal: ~400°C.
Inflammable substances — Materials with very low ignition temperatures that catch fire easily. Petrol, alcohol, and LPG are inflammable. These need to be stored carefully — away from heat sources.
Students often confuse “inflammable” with “non-flammable”. They sound like opposites, but “inflammable” actually means easily catches fire — same as “flammable”. The prefix “in-” here comes from Latin in-flammare (to set on fire), not the English “not”.
Calorific value — The amount of heat energy produced when 1 kg of a fuel is completely burned. Measured in kJ/kg. Higher calorific value = better fuel for generating energy.
| Fuel | Calorific Value (kJ/kg) |
|---|---|
| Wood | 17,000–22,000 |
| Coal | 25,000–33,000 |
| LPG | 55,000 |
| CNG | 50,000 |
| Petrol | 45,000 |
| Hydrogen | 1,50,000 |
Flame — The visible, glowing part of a fire where combustion of gaseous substances occurs. Not all fuels produce a flame — charcoal and coal glow but do not produce a flame because they burn in solid state.
Types of Combustion
1. Rapid Combustion
Combustion that happens quickly and produces heat and light almost instantly. Lighting a gas stove is rapid combustion — the LPG burns as soon as you spark it.
2. Spontaneous Combustion
Combustion that occurs without any external heat source. The substance ignites on its own when its temperature reaches the ignition point due to some internal process.
The classic CBSE example: white phosphorus catches fire in air at room temperature without any spark or flame applied to it. This is why it’s stored underwater — water keeps it away from air.
Another real-world case: piles of coal dust or oily rags left in a closed space can spontaneously combust because heat builds up from slow oxidation.
3. Explosion
Sudden, extremely fast combustion that produces a large amount of gas and energy in a very short time. Firecrackers explode because the chemical reaction inside happens almost instantaneously.
CBSE Class 8 frequently asks you to classify a given example as rapid, spontaneous, or explosive combustion. For white phosphorus → spontaneous. For firecrackers → explosion. For a burning candle → rapid combustion.
Structure of a Flame
This is a favourite CBSE question. Light a candle and look carefully — the flame has three distinct zones.
Outer zone (blue): Complete combustion, highest temperature, gets enough oxygen.
Middle zone (yellow/bright luminous): Partial combustion, moderate temperature. This is the brightest part.
Inner zone (dark/black): Unburnt wax vapour, lowest temperature, no combustion here.
The outer zone is the hottest. Goldsmith uses this zone to melt gold — they position the gold at the tip of the blue flame for maximum heat.
A common CBSE 2-mark question: “A goldsmith uses the outermost part of the flame to melt gold. Why?” Answer: The outer zone has complete combustion and is therefore the hottest part of the flame.
Complete vs Incomplete Combustion
This distinction is critical and shows up in both CBSE and NEET context questions.
Complete combustion happens when there is sufficient oxygen. Products: carbon dioxide (CO₂) and water (H₂O). The flame burns blue. Example: LPG burning on a properly adjusted stove.
Incomplete combustion happens when oxygen supply is limited. Products: carbon monoxide (CO) + water + soot (unburnt carbon). The flame turns yellow/orange.
Carbon monoxide (CO) is colourless, odourless, and extremely toxic. It is the reason you should never run a generator or vehicle engine in a closed room — CO accumulates and causes suffocation.
When your gas stove burns yellow, it means incomplete combustion. The fix: clean the holes in the burner so more air enters. Yellow flame → less heat, more soot, dangerous CO.
Fuel Efficiency and Calorific Value
A good fuel should:
- Have high calorific value (more energy per kg)
- Be easily available and affordable
- Produce low pollution on burning
- Have an appropriate ignition temperature — not too low (safety risk) and not too high (hard to ignite)
Hydrogen has the highest calorific value (1,50,000 kJ/kg) — much higher than petrol. This is why scientists consider hydrogen a fuel of the future. It burns to give only water, so zero pollution. The current challenge is storing it safely.
Control of Fire — How Extinguishers Work
Fire extinguishers work by removing one or more sides of the fire triangle.
Water extinguisher: Water absorbs heat → lowers temperature below ignition point. Works on paper and wood fires.
Never use water on electrical fires or oil/petrol fires. Water conducts electricity (electrical fires) and can splash burning oil (oil fires), spreading the fire. Use CO₂ or dry powder extinguishers instead.
CO₂ extinguisher: CO₂ is heavier than air — it settles around the fire and cuts off oxygen supply. Also cools the fuel. Best for electrical fires and oil fires.
Sand: Poured over small fires. Covers the fuel, blocks oxygen. Traditional method still used for small kitchen fires.
Solved Examples
Example 1 — CBSE Level
Q: Why does a matchstick ignite when struck against a rough surface?
The rough surface of a matchbox contains red phosphorus. When we strike the match, friction generates heat. The match head contains antimony sulphide and potassium chlorate. The heat from friction ignites the antimony sulphide, which in turn provides enough heat to reach the ignition temperature of the matchstick head.
Key point: The ignition temperature is reached through friction — no external flame needed.
Example 2 — CBSE Level
Q: Classify the following as inflammable or non-inflammable: kerosene, water, alcohol, stone, petrol, iron.
| Substance | Classification | Reason |
|---|---|---|
| Kerosene | Inflammable | Low ignition temperature |
| Water | Non-inflammable | Does not burn |
| Alcohol | Inflammable | Catches fire easily |
| Stone | Non-inflammable | Does not burn |
| Petrol | Inflammable | Very low ignition temperature |
| Iron | Non-inflammable | Does not burn under normal conditions |
Example 3 — Application Level (HOTS)
Q: A factory worker closes all windows of a room where coal is stored. After a few days, a fire breaks out with no one having lit a flame. Explain.
This is spontaneous combustion. Coal undergoes slow oxidation when it reacts with air — this releases heat gradually. In a closed room, this heat cannot escape. Over time, the temperature rises until it reaches coal’s ignition temperature (~400°C). Once that threshold is crossed, the coal ignites on its own.
Prevention: Large coal stores are kept in well-ventilated areas and are broken up regularly so heat cannot accumulate.
Exam-Specific Tips
CBSE Class 8 Marking Scheme:
- “Define combustion” → 1 mark. Write: chemical reaction + substance + oxygen + heat and light produced.
- “Draw and label zones of flame” → 2–3 marks. Draw three concentric zones, name them, state temperatures relative to each other.
- “Explain fire triangle” → 2 marks. Name all three sides + state that removing any one stops combustion.
- Differences (complete vs incomplete, inflammable vs non-inflammable) → 3 marks if asked in table format.
High-weightage subtopics in Class 8 board exams:
- Structure of flame (almost always in exam)
- Spontaneous combustion with white phosphorus example
- Calorific value comparison questions
- Fire extinguisher — which to use when
Common Mistakes to Avoid
Mistake 1: Saying “flame” and “fire” are the same. Flame = the luminous, gaseous part of a fire. Charcoal burns without a flame — it glows red but has no visible flame. Only gaseous substances burn with a flame.
Mistake 2: Confusing ignition temperature with burning temperature. Ignition temperature is the minimum temperature to start burning. Once started, the fire can sustain itself (because it generates its own heat). These are different concepts.
Mistake 3: Thinking all combustion produces a flame. Burning of coal, charcoal: no flame (solid burns → glows). Burning of LPG, candle wax vapour, wood gas: produces a flame (gas burns → flame).
Mistake 4: Using water on oil fires. Water and burning oil are dangerous together. Water, heavier than oil, sinks below the oil and instantly vaporises from the heat. This steam explosion scatters burning oil in all directions — the fire explodes outward.
Mistake 5: Thinking CO₂ is produced in all combustion. In incomplete combustion, CO (carbon monoxide) is produced, not CO₂. This is both a chemistry error and a safety knowledge error. CO is toxic; CO₂ is not (in normal amounts). Know which is produced based on oxygen availability.
Practice Questions
Q1. Name the three zones of a candle flame and state which is the hottest.
The three zones are: (1) Inner dark zone — unburnt wax vapour, lowest temperature. (2) Middle luminous zone — partial combustion, bright yellow. (3) Outer blue zone — complete combustion, highest temperature. The outer zone is hottest.
Q2. Why is white phosphorus stored under water?
White phosphorus has a very low ignition temperature — it catches fire spontaneously in air at room temperature. Storing it under water keeps it away from air (oxygen), preventing spontaneous combustion.
Q3. A gas stove is producing a yellow flame instead of blue. What does this indicate, and how would you fix it?
Yellow flame indicates incomplete combustion — the burner is not getting enough oxygen/air. Incomplete combustion produces less heat, more soot, and toxic carbon monoxide (CO). Fix: clean the air holes in the burner to allow more air to mix with the gas. A properly burning stove should always show a blue flame.
Q4. The calorific value of LPG is 55,000 kJ/kg and that of wood is 17,000 kJ/kg. Which is a better fuel? Give two reasons.
LPG is a better fuel because: (1) Higher calorific value — produces more heat per kg burned. (2) Cleaner combustion — LPG burns completely and produces far less smoke and soot compared to wood. (3) More convenient to store and use.
Q5. Why should CO₂ extinguishers be used for electrical fires instead of water?
Water is a good conductor of electricity. If poured on an electrical fire, current can flow through the water stream back to the person holding the extinguisher, causing electric shock. CO₂ is a non-conductor of electricity, so it extinguishes the fire safely by cutting off oxygen supply without any risk of electrocution.
Q6. Differentiate between spontaneous combustion and rapid combustion with one example each.
Rapid combustion: Requires an external source of heat to initiate. Happens quickly. Example: LPG burning on a stove after you light it with a spark.
Spontaneous combustion: Occurs without any external ignition — the substance ignites on its own when internal heat buildup reaches its ignition temperature. Example: white phosphorus catching fire in air at room temperature.
Q7. Give two reasons why hydrogen is considered an ideal fuel for the future.
(1) Extremely high calorific value — 1,50,000 kJ/kg, much higher than petrol, LPG, or coal. (2) Zero pollution — burns to produce only water (H₂O), no CO₂, no CO, no soot. This makes it completely environment-friendly. (Challenge: safe storage is currently expensive and technically difficult.)
Q8. Can combustion occur in a closed jar? What happens if you place a burning candle inside a closed glass jar?
Initially yes, combustion continues using the oxygen already present in the jar. But as oxygen gets consumed and CO₂ accumulates, combustion slows down and eventually stops — the candle goes out. This demonstrates that oxygen is necessary for combustion. No oxygen supply → fire cannot sustain itself.
FAQs
Why does a candle’s flame have a dark inner zone?
The dark inner zone contains unburnt wax vapour that has just evaporated from the wick. It hasn’t reached the outer zones where it comes into contact with oxygen. No oxygen → no combustion → no light or heat generated there. It looks dark because it’s the coolest part with zero combustion happening.
What is the difference between burning and rusting? Both involve oxygen.
Both are oxidation reactions, but at very different speeds. Rusting is slow oxidation — iron reacts with oxygen over days and weeks, releasing heat so slowly you cannot feel it. Combustion is rapid oxidation — releasing heat and light in seconds. The key difference: combustion produces a visible flame or glow; rusting does not.
Why do we blow on burning coal to make it burn faster, but blow out a candle flame to extinguish it?
Blowing on coal increases oxygen supply to the solid fuel, which speeds up combustion. Blowing out a candle works differently — the sharp blast of air cools the wax vapour below its ignition temperature, stopping combustion instantly. For candles, the cooling effect dominates; for coal, the oxygen supply effect dominates (coal’s ignition temperature is high enough that a gentle blow doesn’t cool it below threshold).
Is LPG dangerous? Why do we smell it when there’s a leak?
LPG itself is odourless. Gas companies deliberately add a chemical called ethyl mercaptan (a sulphur compound) to LPG. This chemical has a very strong, unpleasant smell — detectable even in tiny amounts. When you smell “gas”, you’re actually smelling ethyl mercaptan, which warns you of a leak before the concentration of LPG reaches dangerous levels.
Why does a fire need oxygen but water (H₂O) contains oxygen and doesn’t support burning?
Good question — this confuses many students. The oxygen in water is already chemically bonded to hydrogen. For combustion, we need free molecular oxygen (O₂) that can react with the fuel. The oxygen in water is not available for this reaction — it’s already been “used up” in bonding with hydrogen. That’s why water actually extinguishes fire rather than supporting it.
Why do mountaineers and people in high-altitude areas find it difficult to light fires?
At high altitudes, air is thinner — the concentration of oxygen per unit volume is lower. Lower oxygen availability means combustion is harder to initiate and sustain. Fuels may not reach ignition conditions as easily, and even if they catch, the fire burns less intensely. Cooking at high altitudes is genuinely harder for this reason.
What makes a fuel “good”? Can you have a perfect fuel?
An ideal fuel would have high calorific value, be widely available, cheap, easy to store and transport, burn with zero pollution, and have an appropriate ignition temperature. No current fuel meets all criteria. Hydrogen comes closest on energy density and pollution (zero emissions), but storage is the unsolved problem. This is an active area of research in energy science.