Combustion and Flame
Focused last-minute revision notes with clear concepts, exam traps, key comparisons and compact practice.
Chapter Snapshot
- Combustion is a chemical reaction in which a substance reacts with oxygen and releases heat, often light.
- Combustion needs fuel, oxygen and a temperature at or above the ignition temperature.
- Highly flammable substances ignite easily and require careful handling.
- Fire can be controlled by removing fuel, oxygen or heat.
- Combustion may be rapid, spontaneous or explosive.
- A candle flame has different zones with different temperatures and completeness of burning.
- The outer non-luminous zone is usually the hottest because combustion is most complete there.
- A good fuel should release useful heat, be safe and convenient, and produce minimal harmful pollution.
What You Will Learn
- Define combustion and ignition temperature
- Explain the fire triangle
- Describe methods of fire control
- Differentiate rapid, spontaneous and explosive combustion
- Identify flame zones
- Explain calorific value conceptually
- Relate fuel choice to pollution and efficiency
Last-Minute Chapter Map
A candle flame contains regions with different amounts of oxygen and different completeness of combustion.
1. Combustion
Combustion is a chemical reaction in which a substance reacts with oxygen and releases heat; light may also be produced. A substance that burns is called a combustible substance or fuel.
2. Ignition Temperature
A fuel does not burn merely because oxygen is present. It must reach a minimum temperature called the ignition temperature. Paper can be heated until it ignites; water in a paper cup can keep the paper below its ignition temperature for some time.
3. Fire Triangle
Three conditions are needed for continued fire: fuel, oxygen and sufficient heat. Remove any one and combustion stops or cannot continue. Water often removes heat; foam or carbon dioxide can reduce oxygen contact; shutting a fuel valve removes fuel.
4. Types of Combustion
Rapid combustion begins quickly when a flame or spark is supplied. Spontaneous combustion starts without an external flame when self-heating raises a material to ignition temperature. An explosion is extremely rapid combustion that releases heat, light, gases and pressure suddenly.
5. Structure of a Flame
In a candle flame, the innermost dark zone contains unburnt wax vapour, the luminous yellow zone has partial combustion and glowing carbon particles, and the outer non-luminous zone has more complete combustion and is generally hottest.
6. Fuels and Calorific Value
A fuel is useful when it produces sufficient heat safely and conveniently. Calorific value describes heat released by complete combustion of a unit mass of fuel and is commonly expressed in kJ/kg.
7. Environmental Effects
Incomplete combustion can produce carbon monoxide, a poisonous gas. Burning fossil fuels also releases carbon dioxide, particulate matter and other pollutants. Cleaner fuels and efficient combustion reduce harmful emissions.
Zones of a Candle Flame
| Zone | Main Feature | Relative Temperature |
|---|---|---|
| Innermost dark zone | Unburnt wax vapours | Lowest |
| Middle luminous zone | Partial combustion; glowing carbon | Moderate |
| Outer non-luminous zone | More complete combustion | Highest |
Exam Focus
Must Revise Before the Exam
- Combustion definition
- Ignition temperature
- Inflammable substances
- Fire triangle
- Fire control
- Rapid/spontaneous combustion/explosion
- Flame zones
- Calorific value
- Pollutants from fuels
Common Mistakes
Key Terms
Key Terms
Quick Revision
Quick Revision
- Combustion needs fuel + oxygen + ignition temperature.
- Fire can be controlled by removing heat, oxygen or fuel.
- Inflammable substances have low ignition temperatures.
- Rapid combustion needs an initiating flame/spark; spontaneous combustion can start through self-heating.
- Outer flame zone is hottest and shows more complete combustion.
- Calorific value compares heat released by fuels.
- Incomplete combustion may produce poisonous carbon monoxide.
Practice Questions
The minimum temperature at which a substance catches fire is:
A. melting point
B. ignition temperature
C. boiling point
D. flash colour
View Solution
Correct Answer: B. ignition temperature
Ignition temperature is the minimum temperature needed to begin combustion.
Which zone of a candle flame is generally hottest?
A. innermost dark
B. middle yellow
C. outer non-luminous
D. wick only
View Solution
Correct Answer: C. outer non-luminous
The outer zone receives more oxygen and combustion is most complete.
Fire can be controlled by removing:
A. only fuel
B. only oxygen
C. one element of the fire triangle
D. only ash
View Solution
Correct Answer: C. one element of the fire triangle
Removing fuel, heat or oxygen can stop combustion.
Which gas may form during incomplete combustion and is poisonous?
A. oxygen
B. nitrogen
C. carbon monoxide
D. helium
View Solution
Correct Answer: C. carbon monoxide
Carbon monoxide can form when fuel burns with insufficient oxygen.
Why does blowing on a small candle sometimes extinguish it?
View Solution
The moving air removes heat from the flame region and can lower the fuel vapour below ignition temperature, stopping combustion.
Why is the outer zone of a candle flame hottest?
View Solution
It receives the most oxygen, so combustion is more complete and releases heat efficiently.
Why should water not be poured on a burning pan of oil?
View Solution
Water can sink below the hot oil, boil violently and spread burning oil. The fire should be smothered with an appropriate lid or suitable extinguisher instead.
Chapter Summary
Chapter Summary
- Combustion needs fuel + oxygen + ignition temperature.
- Fire can be controlled by removing heat, oxygen or fuel.
- Inflammable substances have low ignition temperatures.
- Rapid combustion needs an initiating flame/spark; spontaneous combustion can start through self-heating.
- Outer flame zone is hottest and shows more complete combustion.
- Calorific value compares heat released by fuels.
- Incomplete combustion may produce poisonous carbon monoxide.