Fuels Around Us

Fuels are substances that burn in air to release heat and sometimes light. We use fuels for cooking, heating, lighting, running machines, and vehicles.

A. Fuels used at home:

• Cow dung – Used in rural areas for cooking on open fires or stoves. It burns slowly and produces heat.

• Wood – Common fuel for stoves, fireplaces, and cooking. Easy to ignite.

• Kerosene – Used in lamps or stoves. Burns with a bright flame.

• LPG (Liquefied Petroleum Gas) – Used in modern kitchens for cooking. Burns cleanly without smoke.

• Charcoal – Burns for a long time and gives good heat, used for cooking in some areas.
  
  

B. Fuels used in trade and industry:

• Coal – Major industrial fuel. Used in thermal power plants, factories, and steam engines. Burns with heat and produces smoke.

• Diesel – Used in engines, machines, and some heating devices.

• Petrol – Used in generators, some machinery, and vehicles.

• Natural Gas – Clean fuel for industries, used for heating, cooking, and generating electricity.
  
  

C. Fuels used for running automobiles:

• Petrol – Used in cars, motorbikes. Burns easily to produce energy for engines.

• Diesel – Used in trucks, buses, and some cars. Produces more energy than petrol.

• Compressed Natural Gas (CNG) – Used in some vehicles to reduce pollution. Burns cleanly.
  
  

Observation: The choice of fuel depends on purpose, availability, and efficiency.

Candle vs Coal Burning

• Candle burning:

  • Burns with a bright yellow flame.

  • Flame is visible and moves slightly with air.

  • Produces heat and light.
    
    

• Coal burning:

  • Usually burns with a red or orange glow, no visible flame.

  • Produces heat, less light.

  • Burns slowly over a longer period.
    
    

Key Difference:

• Flame = visible light during burning

• Glow = burning without a visible flame
  
  

Some materials burn with a flame (e.g., candle, kerosene), while others burn slowly and glow (e.g., coal, charcoal).

What is Combustion?

• Definition: Combustion is a chemical reaction between a substance and oxygen that produces heat, and sometimes light.

• Fuel/Combustible substance: The substance that burns is called a fuel.
  
  

• Fuels can be in different states:

  • Solid: Wood, coal, charcoal

  • Liquid: Petrol, kerosene

  • Gas: LPG, CNG
    
    

Examples of Combustion:

1. Magnesium burning:

   • Magnesium ribbon + air → magnesium oxide + heat + light
     
     

2. Charcoal burning:

   • Charcoal + air → carbon dioxide + heat + small light/glow
     
     

Interesting Fact:

• Food is also a fuel for the body, producing energy when digested.

Requirements for Combustion

Combustion can only take place if three things are present:

1. Fuel: Something that can burn.

2. Oxygen (air): Required for the chemical reaction.

3. Heat: To start the reaction and reach ignition temperature.
   
   

Observations with Candle Flame:

• If air enters freely → flame burns steadily.

• If air partially blocked → flame flickers, produces smoke.

• If air is blocked completely → flame goes off.
  
  

Conclusion: Fire needs oxygen. Without oxygen, it cannot burn.

Activity – Why Fire Stops

Experiment: Place burning wood or charcoal on a plate and cover it with a glass jar.

Observation: The fire stops after some time.

Reason: Oxygen inside the jar gets used up. Fire cannot continue without oxygen.

Real-life example:

• If a person’s clothes catch fire, wrapping a blanket stops the fire. 

• Reason: Blanket blocks air (oxygen), extinguishing the fire.

Ignition of Substances

• Different substances catch fire at different temperatures.

• Ignition temperature: Minimum temperature at which a substance catches fire.

Examples:

• A matchstick does not burn at room temperature.

• It starts burning when rubbed on the matchbox – friction produces heat, reaching ignition temperature.

• Wood or coal cannot catch fire with just a matchstick. Paper or kerosene is used to help start it.
  
  

Observation:

• Some fuels catch fire easily (low ignition temperature).

• Some fuels need high heat to burn (high ignition temperature).

Interesting Facts

• Ancient Egyptians used pinewood dipped in sulphur as early matches, more than 5000 years ago.

• Modern safety matches were developed about 200 years ago.

• The Sun produces heat and light through nuclear reactions, not combustion.

Forest Fires and Accidental Fires

• During the extreme heat of summer, dry grass, leaves, and twigs can catch fire spontaneously due to high temperatures and dryness.

• Once the fire starts, it spreads quickly from the grass to shrubs and then to trees, eventually turning into a forest fire.

• Forest fires are very difficult to control and can cause widespread destruction to trees, wildlife, and human property.

• Forest fires may occur naturally due to lightning strikes or extreme heat, but most are caused by human carelessness such as leaving campfires unattended, throwing away lit cigarettes, or burning waste improperly.

• To prevent accidental forest fires, it is very important to ensure that all campfires are completely extinguished before leaving the area, and littering in forests should be strictly avoided.

How Matches Work?

• In ancient times, matches contained a mixture of antimony trisulphide, potassium chlorate, white phosphorus, glue, and starch on the match head.

• When the match was struck against a rough surface, the friction produced heat, which ignited the white phosphorus and started the combustion of the matchstick.

• White phosphorus was extremely dangerous for both the workers who manufactured the matches and the users, making these matches unsafe.

• Modern safety matches have improved to make them safer by containing only antimony trisulphide and potassium chlorate in the match head.

• The rubbing surface of a safety match contains powdered glass and a small amount of red phosphorus. When the match is struck against this surface, some red phosphorus converts into white phosphorus.

• The white phosphorus reacts with potassium chlorate in the match head, producing enough heat to ignite antimony trisulphide and start the combustion of the matchstick safely.

Ignition Temperature

• Every combustible substance has a minimum temperature called the ignition temperature at which it can catch fire.

• Substances with lower ignition temperatures catch fire easily, while substances with higher ignition temperatures need more heat to ignite.

• For example, cooking oil can catch fire if it is heated for a long time, while kerosene oil can catch fire when slightly heated. Wood, on the other hand, requires a much higher temperature to catch fire.

• This shows that kerosene oil has a lower ignition temperature than wood, so extra care must be taken while storing and handling kerosene.

• In an experiment, if a paper cup contains water and is heated, the cup does not burn because the water absorbs the heat, preventing the paper from reaching its ignition temperature.

Inflammable Substances

• Inflammable substances are materials that catch fire easily and burn rapidly when exposed to a flame or heat.

• Examples of inflammable substances include petrol, alcohol, kerosene, and liquefied petroleum gas (LPG).

• Other inflammable materials are paint thinners, ether, spirit, and some solvents used in laboratories.

• These substances must always be stored carefully, away from heat, flames, or sparks, and in properly labeled containers to prevent accidents.

How Do We Control Fire?

• Fire requires three essential things to start and continue burning: fuel, oxygen, and heat. These three requirements are also known as the fire triangle.

• To control or extinguish fire, one can remove any one of these requirements.

• Removing fuel is not always possible, for example, in the case of a building fire where the building itself acts as fuel.

• Removing oxygen can be done by covering the fire with a blanket, sand, or using carbon dioxide, which prevents oxygen from reaching the flames.

• Removing heat can be done by cooling the fire with water or other suitable fire extinguishing agents.

• Water is commonly used to put out fires involving wood and paper because it cools the combustible material below its ignition temperature and surrounds it with water vapor, which cuts off oxygen.

• Water should not be used for oil fires, petrol fires, or electrical fires, as water conducts electricity and can make the situation more dangerous

• Carbon dioxide (CO₂) is used to extinguish fires involving oil or electrical equipment because it is heavier than air and forms a blanket over the fire, cutting off oxygen and also cooling the fuel. CO₂ is usually stored under high pressure in cylinders or can be produced by chemical reactions, such as using baking soda near a fire.

Types of Combustion

• Rapid combustion occurs when a substance burns quickly, producing heat and light in a short time. An example of rapid combustion is the burning of LPG in a gas stove or kerosene in a lamp.

• Spontaneous combustion occurs when a material bursts into flames suddenly without any apparent external cause. Examples include phosphorus burning at room temperature, coal dust catching fire in coal mines, or forest fires caused by heat of the sun.

• Explosion is a type of sudden combustion where heat, light, sound, and gases are released very quickly. Examples include the ignition of firecrackers or fireworks. Explosions can also happen if pressure is applied to certain substances.

Flame

• A flame is the visible part of a fire where the fuel reacts with oxygen and produces heat and light.

• Flames can have different colours depending on the type of fuel being burnt and the temperature.

• For example, the flame of LPG is blue, a candle flame is yellow, a kerosene lamp flame is orange, and a Bunsen burner flame is blue with a lighter inner cone and darker outer cone.

• Not all combustible substances burn with a visible flame. Some, like charcoal, burn slowly and only produce heat and glow without a flame.

Observation Table – Materials forming flame:

Structure of a Flame

• When a candle is lit, it produces a flame with different zones of temperature and combustion.

• In an activity, if a glass tube is held in the dark zone (inner part) of a candle flame and a matchstick is brought near the other end of the tube, a flame appears. This happens because vapours of wax or fuel rising from the candle catch fire outside the tube.

• The candle wax near the wick melts quickly, moves up through the wick, and vapourizes. These vapours are what actually burn and form the flame.

• Materials like kerosene and molten wax produce flames because they vapourize easily, whereas substances like charcoal do not produce a flame because they do not vaporize.

• If a clean glass plate or slide is held in the luminous (yellow) zone of the flame for some seconds, a blackish ring forms on the glass. This black deposit consists of unburnt carbon particles, showing that combustion in this zone is incomplete.

• A thin copper wire held in the non-luminous zone (blue outer zone) of the flame becomes red-hot. This indicates that the non-luminous zone is the hottest part of the flame, where complete combustion occurs.

• Zones of a candle flame:

  • Innermost zone: Black, contains unburnt wax vapours, least hot.

  • Middle (luminous) zone: Yellow, partially burnt, moderately hot.

  • Outer (non-luminous) zone: Blue, complete combustion, hottest

• Goldsmiths use the outermost (blue) zone of a flame to melt metals like gold and silver because it is the hottest part of the flame.

What is a Fuel?

• Substances that provide heat energy for domestic or industrial use are called fuels.

• Common fuels include wood, charcoal, coal, kerosene, petrol, diesel, LPG, and natural gas.

• A good fuel should be:

  • Readily available.

  • Cheap.

  • Able to burn easily in air at a moderate rate.

  • Able to produce a large amount of heat.

  • Produce minimal undesirable substances.

• There is no perfect fuel, so we select fuels based on their suitability for a particular purpose.

• Fuels can be grouped based on their state:

  • Solid fuels: Coal, wood, cow dung.

  • Liquid fuels: Kerosene, petrol, diesel.

  • Gaseous fuels: CNG, natural gas, biogas, LPG.

Fuel Efficiency

• Different fuels produce different amounts of heat energy. This can be tested, for example, by boiling water using cow dung, coal, or LPG.

• The fuel that produces more heat quickly is considered more efficient. LPG is more efficient than coal or cow dung because it burns faster and releases more heat per kilogram.

• The amount of heat released on complete combustion of 1 kg of a fuel is called its calorific value, measured in kilojoules per kilogram (kJ/kg).

• Calorific values of some common fuels:

Harmful Effects of Burning Fuels

Increasing fuel consumption can harm the environment in many ways.

1. Release of unburnt carbon particles:

• Burning carbon fuels like wood, coal, and petroleum releases fine carbon particles.

• These particles are pollutants and can cause respiratory diseases such as asthma.

• Wood, although cheap and available, produces a lot of smoke and harmful gases when burned.

2. Carbon monoxide production:

• Incomplete combustion of fuels like coal and kerosene produces carbon monoxide, a poisonous gas.

• Burning coal in a closed room is extremely dangerous because carbon monoxide can kill sleeping persons.

• This is why we are always advised never to sleep in a room with burning coal or smouldering fire.
  
  

3. Carbon dioxide and global warming:

• Combustion of most fuels releases carbon dioxide (CO₂).

• An increased concentration of CO₂ in the atmosphere leads to global warming, which is the rise in the Earth's average temperature.

• Global warming causes the melting of polar ice, rising sea levels, and flooding of low-lying coastal areas.
  
  

4. Acid rain from other gases:

• Burning coal and diesel produces sulphur dioxide (SO₂), a corrosive and suffocating gas.

• Petrol engines release nitrogen oxides (NOₓ).

• These gases dissolve in rainwater to form acid rain, which harms crops, soil, and buildings.
  
  

5. Cleaner fuels like CNG:

• Compressed Natural Gas (CNG) is used in automobiles because it produces fewer harmful products than petrol or diesel.

• CNG is considered a cleaner fuel that is safer for the environment.

Keywords with Pronunciation 

1. Acid Rain (AS-id rayn) – Rain that contains harmful acids formed when oxides of sulphur and nitrogen dissolve in water. It can damage crops, buildings, and soil.

2. Calorific Value (ka-luh-RIF-ik VAL-yoo) – The amount of heat energy released when 1 kilogram of a fuel is completely burnt. Measured in kilojoules per kilogram (kJ/kg).

3. Combustion (kuhm-BUS-chun) – A chemical process in which a substance reacts with oxygen and releases heat and light.

4. Deforestation (dee-FORE-uh-STAY-shun) – Cutting down of trees for fuel, farming, or construction, which harms the environment.

5. Explosion (ik-SPLO-zhun) – A sudden, violent reaction that releases heat, light, sound, and gas rapidly. Examples include fireworks or crackers.

6. Flame (flaym) – The visible, gaseous part of a fire where the fuel reacts with oxygen. Flames have different zones with different temperatures.

7. Fire Extinguisher (fy-er ik-STING-gwish-er) – A device used to put out fire by removing heat, oxygen, or both. Types include water, carbon dioxide (CO₂), and dry chemical extinguishers.

8. Fuel (fyoo-el) – Any substance that burns in air to release heat energy for domestic, industrial, or transportation purposes.

9. Fuel Efficiency (fyoo-el i-FISH-en-see) – A measure of how much heat energy a fuel releases on complete combustion. Fuels with higher calorific value are more efficient.

10. Global Warming (GLOH-bul WAR-ming) – Gradual increase in Earth’s average temperature caused by higher levels of greenhouse gases like carbon dioxide.

11. Ideal Fuel (eye-DEE-ul fyoo-el) – A fuel that is cheap, easily available, burns easily, produces a lot of heat, and does not release harmful gases or residues.

12. Ignition Temperature (ig-NISH-un TEM-per-uh-chur) – The minimum temperature at which a combustible substance catches fire.

13. Inflammable Substances (in-FLAM-uh-bul SUB-stun-siz) – Substances that catch fire very easily because they have a low ignition temperature.

WHAT YOU HAVE LEARNT

1. Substances that burn in air are called combustible.

2. Oxygen from the air is essential for combustion.

3. Combustion always releases heat and light.

4. Ignition temperature is the lowest temperature at which a substance can catch fire.

5. Inflammable substances catch fire easily because they have a very low ignition temperature.

6. Fire can be controlled by removing one or more essential requirements for fire: fuel, heat, or oxygen.

7. Water is the most common agent used to control fires, but it cannot be used for fires involving electrical equipment or oil.

8. Combustion can occur in various ways: rapid combustion, spontaneous combustion, and explosion

9. A flame has three zones – the dark (inner) zone, the luminous (middle) zone, and the non-luminous (outer) zone.

10. An ideal fuel is cheap, readily available, easy to transport, burns easily, has a high calorific value, and does not pollute the environment.

11. Fuels differ in efficiency and cost, so the choice of fuel depends on the purpose and availability.

12. Fuel efficiency is measured in terms of calorific value, expressed in kilojoules per kilogram (kJ/kg).

13. Unburnt carbon particles released in air during combustion are harmful and can cause respiratory problems.

14. Incomplete combustion of fuels produces poisonous carbon monoxide, which can be deadly in enclosed spaces.

15. Higher levels of carbon dioxide in the air are linked to global warming, causing climate change and rising sea levels.

16. Oxides of sulphur and nitrogen produced by burning coal, petrol, and diesel dissolve in rainwater to form acid rain, which harms crops, soil, and buildings.
    
    

Textbook Exercise with Answers

1. List conditions under which combustion can take place.

Combustion can take place only when the following three conditions are fulfilled:

1. Fuel – A combustible substance must be present.

2. Oxygen (air) – Oxygen is necessary for the chemical reaction.

3. Heat – The fuel must be heated to its ignition temperature to start burning.

2. Fill in the blanks.

(a) Burning of wood and coal causes smoke and release of carbon dioxide.
(b) A liquid fuel, used in homes is kerosene.
(c) Fuel must be heated to its ignition temperature before it starts burning.
(d) Fire produced by oil cannot be controlled by removing water or cutting off the supply of air alone; water is ineffective for oil fires.

3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

CNG (Compressed Natural Gas) is a cleaner fuel compared to petrol and diesel.

• It produces less smoke, less carbon monoxide (CO), and fewer oxides of nitrogen and sulphur.

• This reduces air pollution, respiratory diseases, and the risk of acid rain in cities.

• Hence, using CNG in vehicles makes the environment healthier and cleaner.

4. Compare LPG and wood as fuels.

5. Give reasons.

(a) Water is not used to control fires involving electrical equipment because water conducts electricity, which can cause electric shock to people trying to extinguish the fire.

(b) LPG is a better domestic fuel than wood because it burns cleanly, produces more heat, and is easier to store and use without smoke.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not because the aluminium pipe conducts heat away from the paper, preventing it from reaching its ignition temperature.

6. Make a labelled diagram of a candle flame.

(You can draw a candle with these three zones in your notebook.)

7. Name the unit in which the calorific value of a fuel is expressed.

The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).

8. Explain how CO₂ is able to control fires.

• CO₂ is heavier than oxygen, so it covers the fire like a blanket.

• This cuts off the supply of oxygen required for combustion.

• CO₂ also cools down the fuel because it expands and absorbs heat when released from a cylinder.

• Therefore, CO₂ effectively extinguishes fires, especially for electrical equipment and oil fires.

9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

• Green leaves contain water, which absorbs heat and prevents the temperature of the leaves from reaching the ignition temperature.

• Dry leaves have little or no water, so they heat up quickly and catch fire easily.

10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

• Goldsmiths use the outermost (non-luminous, blue) zone of the flame because it is the hottest part of the flame and produces complete combustion.

• This high temperature is required to melt metals efficiently.

11. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.

Solution:

12. Can the process of rusting be called combustion? Discuss.

• No, rusting is not combustion.

• Rusting is a slow chemical reaction of iron with oxygen and water to form iron oxide, without producing heat or light.

• Combustion always involves rapid reaction with oxygen and release of heat and light, which rusting does not.

13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?

• Ramesh’s water will get heated faster because the outermost (blue) zone of the flame is the hottest part.

• The yellow zone is cooler and does not transfer heat as efficiently as the outer zone.

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Combustion and Flame 

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Cell - Structure and Functions