🧪 Chemistry · Topic 6 of 10
Start with the study notes on Combustion, Flame & Fuels (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
Combustion is a chemical reaction in which a substance reacts with oxygen and gives out heat (and often light).
Combustion needs all three of these at the same time:
Example: A candle covered with a glass jar goes out once the oxygen inside is used up.
Remember: Remove any one of the three and the fire stops. This is the idea behind all fire fighting.
The ignition temperature is the lowest temperature at which a substance catches fire.
Example: Striking a match: friction produces enough heat to raise the chemicals on its head to their ignition temperature.
Example: Respiration in our cells is often described as slow combustion: food is oxidised slowly, releasing energy without a flame.
A flame forms only when a substance vaporises as it burns. Wax, kerosene and LPG give flames; charcoal only glows.
Example: Goldsmiths blow the outer zone of a flame onto gold to melt it.
A fuel is a substance burnt to produce heat. Fuels can be solid (wood, coal), liquid (kerosene, petrol, diesel) or gas (LPG, CNG).
Calorific value is the heat produced by completely burning 1 kg of a fuel. It is measured in kilojoules per kilogram (kJ/kg).
Example: Hydrogen has the highest calorific value of all fuels, about 150,000 kJ/kg, and burning it produces only water.
An ideal fuel is cheap, easily available, easy to store and transport, has a high calorific value, burns at a steady rate and causes little pollution.
Example: CNG is cleaner than petrol or diesel because it burns almost completely, with very little smoke.
Example: A blue flame on a gas stove means complete combustion; a yellow, sooty flame means not enough air.
A fire is put out by removing fuel, oxygen or heat.
Example: Firefighters clear a strip of forest ahead of a forest fire, so that there is no fuel for it to spread to.
| Misconception | Correct understanding |
|---|---|
| Anything will burn if it is hot enough and has a flame. | It also needs oxygen; and some substances, like stone, don't burn at all. |
| The yellow part of a flame is the hottest. | The outer blue zone is the hottest; the yellow zone is only partly burning. |
| Water puts out every fire. | Never use water on electrical or oil fires; use carbon dioxide or sand. |
| Carbon monoxide can be smelt. | It has no colour and no smell, which makes it so dangerous. |
| Charcoal burns with a big flame. | Charcoal does not vaporise, so it only glows. |
What three things are needed for combustion?
A fuel, oxygen and heat to reach the ignition temperature.
Why does a candle under a glass jar go out?
The oxygen inside gets used up.
Which zone of a candle flame is hottest?
The outer, blue (non-luminous) zone.
What is calorific value measured in?
Kilojoules per kilogram (kJ/kg).
Which fuel has the highest calorific value?
Hydrogen.
Why is it dangerous to burn coal in a closed room?
Incomplete burning makes poisonous carbon monoxide.
Why shouldn't water be used on an electrical fire?
Water conducts electricity, so it can give a shock.
Why is white phosphorus stored under water?
It catches fire on its own in air (spontaneous combustion).
Key takeaway: Burning needs fuel, oxygen and heat. Take any one away and the fire goes out; choose clean fuels to keep the air safe.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
A chemical process in which a substance reacts with oxygen to give off heat is called:
Answer: A — Combustion
Combustion is the reaction of a substance with oxygen, releasing heat and often light. Burning of wood, coal and LPG are all combustion reactions.
Q2. easy
A substance that burns in air producing heat and light is called:
Answer: A — A combustible substance
Combustible substances such as paper, wood, LPG and kerosene burn in air. Non-combustible substances like stone, glass and iron nails do not.
Q3. medium
Which three conditions are necessary for combustion?
Answer: A — A combustible substance, a supporter of combustion (oxygen), and the ignition temperature
All three must be present at once. Removing any one of them - fuel, oxygen or heat - stops the burning, which is the basis of all fire fighting.
Q4. medium
The lowest temperature at which a substance catches fire is called its:
Answer: A — Ignition temperature
A substance will not burn until it reaches its ignition temperature, no matter how much oxygen is available. Kerosene oil has a low ignition temperature; coal has a high one.
Q7. medium
Which gas supports combustion?
Answer: A — Oxygen
Oxygen is the supporter of combustion. A burning candle goes out when covered by a glass because the oxygen inside is used up.
Q8. medium
A burning candle covered with an inverted glass goes out because:
Answer: A — The supply of oxygen is cut off
Once the limited oxygen inside the glass is consumed, combustion cannot continue and the flame dies. This shows oxygen is essential for burning.
Q11. medium
Why is white phosphorus stored under water?
Answer: A — It undergoes spontaneous combustion in air at room temperature
White phosphorus has an extremely low ignition temperature and catches fire on its own in air. Storing it under water keeps it away from oxygen.
Q13. medium
Which of these processes is a SLOW combustion happening in our bodies?
Answer: A — Respiration
During respiration, food is oxidised slowly inside our cells, releasing energy without a flame. It is often described as a form of slow combustion.
Q14. medium
The region where combustion of gases takes place, giving off light, is called a:
Answer: A — Flame
A flame is produced only by substances that vaporise on heating, such as wax, kerosene and LPG. Substances like charcoal glow without a flame.
Q17. medium
Which zone of a candle flame is the HOTTEST?
Answer: A — The outermost (non-luminous, blue) zone
Complete combustion occurs in the outer zone where oxygen is plentiful, so it is the hottest. Goldsmiths direct this part of the flame onto gold to melt it.
Q18. medium
Which zone of a candle flame is the LEAST hot?
Answer: A — The innermost dark zone (unburnt wax vapour)
The dark innermost zone contains unburnt wax vapour and no combustion, so it is the coolest part of the flame.
Q20. medium
Goldsmiths blow the outermost zone of a flame onto gold because that zone:
Answer: A — Is the hottest and can melt gold
The outer non-luminous zone has the most complete combustion and the highest temperature, so it can melt gold and silver for making ornaments.
Q22. medium
Which of these is a GASEOUS fuel?
Answer: A — CNG
CNG (compressed natural gas) and LPG are gaseous fuels. Coal and wood are solid, and kerosene, petrol and diesel are liquid fuels.
Q24. medium
Calorific value is measured in:
Answer: A — Kilojoules per kilogram (kJ/kg)
Since calorific value is the heat produced per unit mass of fuel, its unit is kJ/kg.
Q26. medium
Which of these is a characteristic of an IDEAL fuel?
Answer: A — High calorific value, low cost, easy availability and little pollution
An ideal fuel should be cheap, readily available, easy to store and transport, burn at a moderate rate with a high calorific value, and produce no harmful residues.
Q28. medium
Incomplete combustion of a fuel produces:
Answer: A — Poisonous carbon monoxide and unburnt carbon particles (soot)
When oxygen is insufficient, carbon burns only partly, producing carbon monoxide and soot. Carbon monoxide is a very poisonous gas.
Q30. medium
Carbon monoxide is dangerous because it:
Answer: A — Combines with haemoglobin and prevents the blood from carrying oxygen
Carbon monoxide binds to haemoglobin far more strongly than oxygen does. It is colourless and odourless, which makes it especially dangerous.
Q32. medium
The trapping of heat by gases like carbon dioxide in the atmosphere is called:
Answer: A — The greenhouse effect
Greenhouse gases let sunlight in but trap the heat radiated back by the Earth, keeping the planet warm. Excess of these gases causes harmful global warming.
Q34. medium
The unburnt carbon particles released during incomplete combustion cause:
Answer: A — Respiratory diseases like asthma and bronchitis
Fine soot particles enter the lungs and cause or worsen respiratory illnesses such as asthma, bronchitis and even cancer over long exposure.
Q37. medium
Why should water NOT be used on an oil or petrol fire?
Answer: A — Oil is lighter than water, so it floats and continues to burn while spreading
Water sinks below the oil, which keeps floating and burning on top and can spread the fire. Foam, sand or carbon dioxide is used for oil fires.
Q39. medium
When CO2 is released from a cylinder it also helps put out fire because it:
Answer: A — Expands and cools, lowering the temperature below the ignition point
As compressed CO2 escapes it expands rapidly and cools sharply. This drop in temperature, along with the oxygen cut-off, extinguishes the fire.
Q41. medium
In a soda-acid fire extinguisher, the CO2 is produced by the reaction of:
Answer: A — Sulphuric acid with sodium hydrogen carbonate solution
Turning the extinguisher lets the acid mix with the sodium bicarbonate solution, generating carbon dioxide that forces the liquid out and smothers the fire.
Q43. medium
Which of these fuels causes the LEAST air pollution?
Answer: A — CNG
CNG burns cleanly with very little smoke, soot or sulphur emission. Coal, wood and dung cakes all produce large amounts of smoke and particulate matter.
Q45. medium
Which is a good way to conserve fuel?
Answer: A — Use public transport, cycle or walk for short distances
Sharing transport, cycling and switching off the engine at red lights all reduce fuel consumption, saving resources and cutting pollution.
Q47. medium
Which of these is a FOSSIL fuel?
Answer: A — Coal
Coal, petroleum and natural gas are fossil fuels formed from dead plants and animals buried millions of years ago. Wood, dung and biogas are not fossil fuels.
Q49. medium
Which of these would NOT catch fire easily?
Answer: A — An iron nail
Iron has a very high ignition temperature and is essentially non-combustible in everyday conditions. Petrol, alcohol and LPG are inflammable.
Q51. medium
Why is LPG stored in strong steel cylinders under pressure?
Answer: A — The gas is compressed into liquid form so a large amount fits in a small cylinder
Under pressure, LPG liquefies and occupies far less volume, making storage and transport practical. The cylinder must be strong to hold that pressure safely.
Q53. medium
If you smell gas leaking in the kitchen, you should FIRST:
Answer: A — Turn off the regulator, open windows, and avoid switching any electrical switch
A spark from a switch or match can ignite the gas. Shut the supply off, ventilate the room by opening doors and windows, and leave electrical switches untouched.
Q55. medium
Combustion of a fuel containing carbon and hydrogen in plenty of air produces mainly:
CH4 + 2 O2 → CO2 + 2 H2O + heatAnswer: A — Carbon dioxide and water vapour
Complete combustion of a hydrocarbon gives carbon dioxide and water, with the maximum release of heat. Limited oxygen instead gives carbon monoxide and soot.
Q57. medium
Which overall statement about combustion and fuels is CORRECT?
Answer: A — Combustion needs fuel, oxygen and ignition temperature; good fuels have high calorific value and cause little pollution
This sums up the topic: all three conditions must be met for burning; fires are fought by removing one of them; and the best fuels are cheap, high in calorific value, easy to handle and clean-burning.
Q58. medium
Why does a candle flame always point upwards, whichever way the candle is tilted?
Answer: A — Hot gases are lighter and rise, while cooler air moves in from below
The hot gases produced by burning are less dense than the surrounding air, so they rise. Cool air rushes in at the bottom to replace them, giving the flame its familiar upward teardrop shape.
Q60. medium
A wet cloth or blanket thrown over a small fire puts it out mainly because it:
Answer: A — Cuts off the oxygen supply and also cools the burning material
The blanket smothers the fire by keeping air away, and the water in it absorbs heat, lowering the temperature below the ignition point - two of the three fire-fighting methods at once.
Q5. hard
Substances with a VERY LOW ignition temperature that catch fire easily are called:
Answer: A — Inflammable substances
Petrol, alcohol, LPG and spirit have very low ignition temperatures and catch fire easily, so they are called inflammable (or flammable) substances and must be handled with care.
Q6. hard
Why does a piece of paper burn easily but an iron nail does not, even in the same flame?
Answer: A — Paper has a low ignition temperature while iron's is very high
Every substance has its own ignition temperature. Paper reaches its ignition temperature quickly in a small flame, while iron would need a far higher temperature.
Q9. hard
Rapid combustion is combustion in which:
Answer: A — A substance burns quickly producing heat and light, as when LPG burns
In rapid combustion, the reaction proceeds fast, giving out large amounts of heat and light in a short time - as in a gas stove or a burning match.
Q10. hard
Spontaneous combustion is combustion that:
Answer: A — Occurs on its own without any external heat being applied
Some substances, like white phosphorus, burst into flame at room temperature simply on exposure to air. Forest fires can also start spontaneously from heat build-up.
Q12. hard
An explosion is a type of combustion in which:
Answer: A — A large amount of heat, light, sound and gas is produced suddenly
In an explosion, such as a firecracker going off, gases are produced very rapidly and expand violently, producing a loud sound along with heat and light.
Q15. hard
Which of these burns WITHOUT producing a flame?
Answer: A — Charcoal
Charcoal does not vaporise, so it glows red hot instead of producing a flame. A flame forms only when a substance vaporises during burning.
Q16. hard
How many distinct zones does a candle flame have?
Answer: A — Three - innermost dark, middle luminous, outer non-luminous
The dark innermost zone contains unburnt wax vapour, the middle yellow zone burns partially, and the outer blue zone burns completely and is the hottest.
Q19. hard
The middle yellow zone of a candle flame is luminous because:
Answer: A — Partial combustion produces unburnt carbon particles that glow
Insufficient oxygen in this zone leads to incomplete combustion. The hot, unburnt carbon particles glow yellow and can be collected as soot on a cold glass slide.
Q21. hard
A substance that is burnt to produce heat energy is called a:
Answer: A — Fuel
Fuels such as wood, coal, kerosene, LPG and CNG release heat on burning. They may be solid, liquid or gaseous.
Q23. hard
The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its:
Answer: A — Calorific value
Calorific value is expressed in kilojoules per kilogram (kJ/kg). The higher the calorific value, the better the fuel.
Q25. hard
Which fuel has the HIGHEST calorific value?
Answer: A — Hydrogen
Hydrogen has a calorific value of about 150,000 kJ/kg, far above any other fuel, and it produces only water on burning. Storing and transporting it safely is the challenge.
Q27. hard
Why is CNG considered a cleaner fuel than petrol or diesel?
Answer: A — It burns more completely and produces far less smoke and harmful gases
CNG burns almost completely, releasing very little unburnt carbon or sulphur compounds. Delhi switched public transport to CNG largely to reduce air pollution.
Q29. hard
Why is it dangerous to burn coal in a closed room?
Answer: A — Incomplete combustion produces carbon monoxide, which can kill a sleeping person
In a closed room, limited oxygen leads to incomplete combustion and the release of carbon monoxide. This gas binds to blood haemoglobin, preventing oxygen transport, and can be fatal.
Q31. hard
Which gas released by burning fuels is the main cause of GLOBAL WARMING?
Answer: A — Carbon dioxide
Carbon dioxide traps heat radiated by the Earth, creating an enhanced greenhouse effect that raises global temperatures - global warming.
Q33. hard
Burning of coal and diesel releases sulphur dioxide and nitrogen oxides, which cause:
Answer: A — Acid rain
These oxides dissolve in rainwater to form sulphuric and nitric acids, producing acid rain that damages crops, buildings, soil and aquatic life.
Q35. hard
The most common fire extinguisher for ordinary fires (wood, paper, cloth) is:
Answer: A — Water
Water cools the burning material below its ignition temperature and also blocks the air supply as steam. It works only for solid fuel fires.
Q36. hard
Why should water NOT be used on an electrical fire?
Answer: A — Water conducts electricity and the person may get an electric shock
Ordinary water conducts electricity, so pouring it on live electrical equipment risks a severe shock. Carbon dioxide extinguishers are used instead.
Q38. hard
Carbon dioxide is the BEST extinguisher for electrical and oil fires because it:
Answer: A — Is heavier than air, covers the fire and cuts off oxygen without damaging equipment
CO2 is heavier than air, so it forms a blanket over the fire and cuts off the oxygen supply. It leaves no residue and does not conduct electricity, so equipment is safe.
Q40. hard
The three ways a fire extinguisher can work are by removing:
Answer: A — The fuel, the oxygen supply, or the heat (bringing it below ignition temperature)
Since combustion needs fuel, oxygen and ignition temperature together, removing any one of them stops the fire. Extinguishers most often cut off oxygen and lower the temperature.
Q42. hard
Why do firemen sometimes create a 'fire break' by clearing vegetation in a forest fire?
Answer: A — To remove the fuel so the fire cannot spread further
Removing trees and dry material in a strip ahead of the fire takes away the fuel. Without fuel, the fire cannot cross the gap and eventually dies out.
Q44. hard
Burning of fossil fuels should be reduced mainly because:
Answer: A — They are limited in supply and cause serious air pollution and global warming
Fossil fuels took millions of years to form and cannot be replaced in our lifetime. Burning them releases CO2, SO2, NOx and soot, causing global warming and pollution.
Q46. hard
The temperature of the human body is about 37 °C. This heat comes from:
Answer: A — Slow combustion (oxidation) of food during respiration
Food is oxidised slowly inside cells during respiration, releasing energy that maintains body temperature and powers all life processes.
Q48. hard
Kerosene is preferred over wood for cooking because it:
Answer: A — Has a higher calorific value and produces less smoke
Kerosene gives far more heat per kilogram than wood and burns with less smoke and no ash, making it a more efficient and cleaner household fuel.
Q50. hard
Why does a matchstick light when struck on the side of the matchbox?
Answer: A — Friction produces heat that raises the chemicals to their ignition temperature
Rubbing generates heat by friction. The red phosphorus on the striking surface reacts with the chemicals on the match head, which then reach their ignition temperature and burn.
Q52. hard
A strong-smelling substance (ethyl mercaptan) is added to LPG so that:
Answer: A — Any leakage can be detected quickly by its smell
LPG itself is odourless. The added odorant lets people notice a leak immediately and act before the gas builds up to explosive levels.
Q54. hard
Which of these produces the LEAST residue after burning?
Answer: A — LPG
Gaseous fuels like LPG burn almost completely, leaving virtually no ash or residue. Solid fuels leave behind ash and unburnt carbon.
Q56. hard
A blue flame on a gas stove indicates:
Answer: A — Complete combustion with enough oxygen - the most efficient burning
A clean blue flame means the air supply is adequate and combustion is complete, giving maximum heat. A yellow, sooty flame means insufficient air and wasted fuel.
Q59. hard
Blowing on a small flame usually puts it out, but blowing air into a coal fire makes it burn better. Why?
Answer: A — A gentle blow cools the small flame below its ignition temperature, while a steady air supply gives the coal more oxygen
For a small flame, the moving air carries heat away faster than it supplies oxygen, so the temperature falls below the ignition point. A large coal fire is already very hot, so the extra oxygen simply speeds up combustion.