Chapter 4: Combustion and Flame -Class 8

1. What is combustion?

A chemical process where a substance reacts with oxygen to give heat (+ often light).(Ek chemical process jisme padarth oxygen se react karke garmi (+ prakash) deta hai).

2. What is a fuel?

A substance that undergoes combustion.(Woh padarth jo jalta hai).

3. Give an example of a non-combustible substance.

Stone, Glass, Water. (Patthar, Kanch, Paani).

4. Name the three essential requirements for combustion.

Fuel, Air (Oxygen), Heat (Ignition Temperature).(Indhan, Hawa (Oxygen), Garmi (Jvalan Taap)).

5. What is ignition temperature?

Lowest temperature at which a substance catches fire.(Sabse kam taapmaan jis par koi cheez aag pakadti hai).

6. Give an example of an inflammable substance.

Petrol, LPG, Alcohol.

7. Name a gas commonly used in fire extinguishers.

Carbon Dioxide (CO₂).

8. Name a type of combustion where a substance burns suddenly without external heat.

Spontaneous Combustion.(Swatah Dahan).

9. Give an example of an explosion.

Bursting of firecrackers.(Patakhon ka fatna).

10. Do all burning substances produce a flame?

No, only those that vaporise during burning.(Nahi, sirf woh jo jalte samay bhaap bante hain).

11. Which zone of a candle flame is the hottest?

Outermost zone (Blue, Non-luminous).(Sabse bahari zone).

12. Which zone of a candle flame is luminous?

Middle zone (Yellow).(Madhya zone (Peela)).

13. What is the unit of calorific value?

Kilojoule per kg (kJ/kg).

14. Name a poisonous gas produced by incomplete combustion.

Carbon Monoxide (CO).

15. Name the phenomenon caused by excess CO₂ leading to increased Earth temperature.

Global Warming.(Vaishvik Ushnan).

1. What are combustible and non-combustible substances? Give two examples of each.

• Combustible: Substances that burn. Ex: Wood, Paper.(Jalne wale padarth. Udaharan: Lakdi, Kagaz).
• Non-combustible: Substances that do not burn. Ex: Stone, Glass.(Na jalne wale padarth. Udaharan: Patthar, Kanch).

2. Why does a matchstick start burning on rubbing it on the side of the matchbox?

• The matchstick head has chemicals (like potassium chlorate, antimony trisulphide).(Maachis ke sir par chemical hote hain).
• The rubbing surface has glass powder and red phosphorus.(Ragadne wali satah par kanch powder aur laal phosphorus hota hai).
• Rubbing creates friction, which generates heat.🔥 (Ragadne se gharshan hota hai, jisse garmi paida hoti hai).
• This heat raises the temperature of the chemicals to their ignition temperature, causing the matchstick to catch fire.(Yeh garmi chemicals ko unke jvalan taap tak pahuncha deti hai, jisse maachis jal uthti hai).

3. List the three essential conditions for combustion.

• Presence of a Fuel (combustible substance).(Indhan ki upasthiti).
• Presence of Air (or Oxygen).(Hawa ya Oxygen ki upasthiti).
• Attainment of Ignition Temperature (Heat).(Jvalan Taap tak pahunchna).

4. How does water help in controlling fire?

• Water cools the combustible material below its ignition temperature.💧➡️🌡️⬇️ (Paani jalne wali cheez ko uske jvalan taap se neeche thanda karta hai).
• When water turns into vapour, the vapour surrounds the fire and cuts off the supply of air (oxygen).💨❌🌬️ (Jab paani bhaap banta hai, toh woh aag ko gher leta hai aur hawa ki supply rok deta hai).
• This breaks two essential conditions for burning (Heat & Air).
• Note: Not suitable for electrical or oil fires.⚠️

5. Explain how CO₂ is able to control fires.

• Carbon Dioxide (CO₂) is heavier than oxygen. (CO₂ oxygen se bhaari hai).
• It forms a blanket ⬜ around the burning material, displacing oxygen. (Yeh jal rahi cheez ke charon taraf ek kambal banakar oxygen ko hata deta hai).
• This cuts off the contact between the fuel and oxygen, extinguishing the fire. 🔥➡️❌
• It also often cools down the fuel. 🥶
• Effective for electrical fires ⚡ and inflammable liquids like petrol.

6. Differentiate between Rapid and Spontaneous Combustion.

• Rapid Combustion: Burns quickly with external trigger (like matchstick), produces heat & light. Ex: LPG burning. (Bahari trigger se tezi se jalta hai, garmi/prakash deta hai. Udaharan: LPG ka jalna).
• Spontaneous Combustion: Material bursts into flames suddenly *without* external heat. Happens due to self-heating to ignition temp. Ex: Phosphorus, coal dust fires. (Bina bahari garmi ke achaanak jal uthta hai. Khud hi garam hokar jvalan taap tak pahunchne se. Udaharan: Phosphorus, koyle ki khan mein aag).

7. Why is the yellow part of a candle flame luminous?

• The yellow (middle) zone is where incomplete combustion of wax vapour occurs due to limited oxygen. (Peela (madhya) zone mein wax vapour ka apoorn dahan hota hai kam oxygen ke karan).
• This incomplete combustion produces tiny unburnt carbon particles. (Is apoorn dahan se chhote bina jale carbon kan bante hain).
• These carbon particles get very hot and glow brightly, producing the yellow luminous flame. 🟡💡 (Yeh carbon kan bahut garam hokar chamakte hain, jisse peeli roshan jwala banti hai).

8. What is meant by the calorific value of a fuel? Why is hydrogen not commonly used as fuel despite having highest calorific value?

• Calorific Value: Heat energy produced by complete combustion of 1 kg of fuel (Unit: kJ/kg). Higher value means more efficient fuel.(1 kg indhan ke poorn dahan se mili heat energy. Zyada value matlab zyada efficient fuel).
• Hydrogen: Has highest calorific value (~1,50,000 kJ/kg).(Hydrogen ka value sabse zyada hai).
• But not common fuel because: It’s a gas, difficult & expensive to store/transport safely. It is highly inflammable and burns explosively.💥 (Lekin aam indhan nahi kyunki: Ise surakshit store/transport karna mushkil/mehenga hai. Bahut jvalansheel hai aur visphotak roop se jalti hai).

9. How does incomplete combustion lead to harm?

• Incomplete combustion happens when oxygen supply is insufficient. (Jab oxygen kam hoti hai tab apoorn dahan hota hai).
• It produces poisonous **Carbon Monoxide (CO)** gas instead of Carbon Dioxide (CO₂). (CO₂ ki jagah zehrili Carbon Monoxide (CO) gas banti hai).
• CO is dangerous because it reduces the oxygen-carrying capacity of our blood, which can be fatal ☠️ (e.g., sleeping in a closed room with burning coal). (CO khatarnaak hai kyunki yeh hamare khoon ki oxygen le jane ki kshamta kam kar deti hai, jo jaanleva ho sakti hai).
• It also releases unburnt carbon particles (soot) which cause air pollution and respiratory problems 😮‍💨. (Yeh bina jale carbon kan (kaalikh) bhi chhodta hai jo vayu pradushan aur saans ki samasyaein paida karte hain).

10. What is acid rain? How is it caused?

• Acid rain is rain water that has become acidic due to atmospheric pollution.🌧️💀 (Amliya varsha woh barish ka paani hai jo vayumandaliy pradushan ke karan acidic ho gaya hai).
• Burning fossil fuels (especially coal, diesel, petrol) releases oxides of **Sulphur (SO₂) and Nitrogen (NOₓ)** into the air.(Jeevashm indhan jalane se Sulphur aur Nitrogen ke oxide hawa mein jaate hain).
• These gases react with water vapour in the atmosphere to form **Sulphuric Acid (H₂SO₄) and Nitric Acid (HNO₃)**.(Yeh gasen hawa mein pani ki bhaap se react karke Sulphuric Acid aur Nitric Acid banati hain).
• These acids fall down with rain, making the rain acidic.(Yeh acid barish ke saath neeche girte hain, jisse barish amliya ho jaati hai).
• It is harmful to crops, buildings (like Taj Mahal), soil, and aquatic life.(Yeh faslon, imaraton, mitti aur jaliya jeevan ke liye haanikarak hai).

1. Explain the conditions under which combustion takes place.

Three conditions are essential for combustion (the ‘Fire Triangle’):

1. Presence of a Combustible Substance (Fuel): A material that can burn (like wood, paper, LPG) must be present. Non-combustible substances cannot burn. (Jalne wala padarth (Indhan) maujood hona chahiye).
2. Presence of a Supporter of Combustion (Air/Oxygen): Oxygen is necessary for most combustion reactions. Usually obtained from air (~21% oxygen). Cutting off the air supply stops the fire. 🌬️ (Jalne mein madad karne wala (Hawa/Oxygen) maujood hona chahiye. Hawa ki supply rokne se aag bujh jaati hai).
3. Attainment of Ignition Temperature (Heat): Every combustible substance has a minimum temperature at which it catches fire (its ignition temperature). Heat is required to raise the substance’s temperature to this point. 🔥🌡️⬆️ (Jvalan Taap tak pahunchna (Garmi). Har jalne wali cheez ka ek minimum temperature hota hai jis par woh aag pakadti hai. Uss temperature tak pahunchne ke liye garmi chahiye).

If any one of these conditions is removed, combustion cannot occur or will stop.

2. Describe different ways to control fire.

Fire can be controlled by removing one or more essentials of the fire triangle (Fuel, Air, Heat):

• Removing Fuel: If possible, remove the combustible material. Example: Switching off gas supply for LPG fire.(Indhan hatana: Agar ho sake toh jalne wali cheez hata dein. Udaharan: LPG aag ke liye gas supply band karna).
• Cutting off Air Supply 🌬️❌: Use materials that prevent oxygen from reaching the fire.
  • Covering with a blanket (for small fires on person).(Kambal se dhakna).
  • Using sand 🏖️.
  • Using Fire Extinguishers, especially CO₂ type. CO₂ is heavy, forms a layer, cuts off oxygen.(Fire Extinguisher (khaaskar CO₂) ka istemal. CO₂ bhaari hai, parat banata hai, oxygen rokta hai).
• Bringing Down Temperature 🌡️⬇️: Cool the burning material below its ignition temperature.
  • Using Water 💧: Most common extinguisher for Class A fires (wood, paper). Water absorbs heat and vaporises, cooling the fuel. (*Not for electrical/oil fires*).(Paani ka istemal: Aam aag (lakdi, kagaz) ke liye. Paani garmi sokhta hai aur thanda karta hai. *Bijli/tel ki aag ke liye nahi*).
  • CO₂ from extinguishers also has a cooling effect.

3. Explain the different zones of a candle flame with a diagram (or detailed description).

(Diagram Description): A candle flame 🕯️ typically has three visible zones:

• Innermost Zone (Dark Zone):
  • Located around the wick. Contains unburnt wax vapours.(Batti ke aas paas. Bina jali mom ki bhaap hoti hai).
  • Least hot / Coolest part (because no combustion occurs here due to lack of oxygen). 🌡️⬇️(Sabse kam garam/thanda hissa (oxygen ki kami ke karan koi dahan nahi)).
  • Appears black/dark.
• Middle Zone (Luminous Zone):
  • Region where partial/incomplete combustion of wax vapours occurs due to limited oxygen. (Yahan seemit oxygen ke karan mom ki bhaap ka apoorn dahan hota hai).
  • Contains hot, glowing unburnt carbon particles, making this zone yellow and bright (luminous). 🟡💡 (Garam, chamakdar bina jale carbon kan hote hain, jisse yeh zone peela aur roshan hota hai).
  • Moderately hot.
• Outermost Zone (Non-Luminous Zone):
  • Region where complete combustion of wax vapours occurs due to plentiful supply of oxygen from surrounding air. ✅🌬️(Yahan aas paas ki hawa se bharpur oxygen milne ke karan mom ki bhaap ka poorn dahan hota hai).
  • Hottest part of the flame. 🌡️⬆️⬆️(Jwala ka sabse garam hissa).
  • Usually blue and faintly visible (non-luminous). Goldsmiths use this zone. 💍(Aam taur par neela aur halka dikhai deta hai (non-luminous). Sonar iska istemal karte hain).

4. What is a fuel? What are the characteristics of an ideal fuel?

Fuel: A substance that burns to produce heat energy.

An ideal fuel (though none exists perfectly) should have these characteristics:

• High Calorific Value: Produces large amount of heat per unit mass 🔥⬆️. (Prati ikai dravyaman par adhik matra mein garmi paida kare).
• Moderate Ignition Temperature: Neither too low (unsafe) nor too high (hard to light). (Uchit Jvalan Taap: Na bahut kam na bahut zyada).
• Moderate Rate of Combustion: Burns smoothly, not too fast (explosion) or too slow. (Theek Gati Se Jale: Na bahut tez na bahut dheere).
• Readily Available & Cheap: Easy to obtain and affordable 💰⬇️. (Aasani se uplabdh aur sasta).
• Easy to Handle & Transport: Safe and convenient to store/move 📦➡️. (Rakhne/le jane mein aasan aur surakshit).
• Clean Burning: Does not leave much ash or harmful residues ✨💨. (Zyada raakh ya haanikarak avashesh na chhode).
• Low Pollution: Produces minimal harmful gases 🌍💚. (Kam se kam haanikarak gasen paida kare).

5. Explain different types of combustion with examples.

Combustion types are based on speed and conditions:

• Rapid Combustion 🔥: Fast burning with heat and light. Needs external ignition source. Ex: Burning LPG, matchstick मैच . (Tezi se jalna garmi/prakash ke saath. Bahari ignition chahiye. Udaharan: LPG jalna).
• Spontaneous Combustion 💨➡️🔥: Substance bursts into flame suddenly *without* external heat source. Happens for substances with very low ignition temp. Ex: White Phosphorus in air, Coal dust fires. (Achaanak bina bahari garmi ke jal uthna. Kam jvalan taap wale padarth. Udaharan: Phosphorus, Koyla khan mein aag).
• Explosion 💥: Very fast combustion with sudden production of heat, light, sound 🔊, and large volume of gas. Often involves pressure buildup. Ex: Firecrackers 🎆. (Bahut tez dahan jisme achaanak garmi, prakash, awaz aur gas nikle. Aksar dabav badhne se. Udaharan: Patakhe).

6. What is meant by calorific value? Arrange wood, LPG, and hydrogen in increasing order of calorific value.

Calorific Value: It is the amount of heat energy produced when 1 kilogram (kg) of a fuel is completely burnt.

• It is measured in kilojoules per kilogram (kJ/kg). (Iski ikai kilojoule prati kg (kJ/kg) hai).
• A higher calorific value means the fuel produces more heat per kg, making it more efficient. 🔥⬆️ (Uchh calorific value ka matlab hai ki indhan prati kg adhik garmi paida karta hai, jo use adhik kushal banata hai).

Arrangement in increasing order of Calorific Value:

1. Wood (~17,000 – 22,000 kJ/kg)
2. LPG (~55,000 kJ/kg)
3. Hydrogen (~1,50,000 kJ/kg)

So, the order is: Wood < LPG < Hydrogen

7. Why is it advised never to sleep in a room with burning or smouldering coal fire in it?

It is extremely dangerous to sleep in a closed room with a burning or smouldering coal fire because:

• Coal is a carbon fuel. When it burns in insufficient supply of air (like in a closed room), it undergoes **incomplete combustion**. (Koyla ek carbon indhan hai. Jab yeh band kamre jaisi kam hawa wali jagah mein jalta hai, toh iska **apoorn dahan** hota hai).
• Incomplete combustion produces highly poisonous **Carbon Monoxide (CO)** gas instead of Carbon Dioxide (CO₂). (Apoorn dahan Carbon Dioxide (CO₂) ki jagah atyant zehrili **Carbon Monoxide (CO)** gas paida karta hai).
• Carbon Monoxide is odourless and colourless, so its presence is not easily detected. (Carbon Monoxide gandhheen aur rangheen hoti hai, isliye iski upasthiti ka aasani se pata nahi chalta).
• When inhaled, CO enters the bloodstream and binds strongly with haemoglobin (much stronger than oxygen). (Saans lene par, CO khoon mein pravesh karti hai aur hemoglobin ke saath mazbooti se jud jaati hai (oxygen se bhi zyada)).
• This drastically reduces the oxygen-carrying capacity of the blood, leading to oxygen starvation in body tissues and the brain. (Isse khoon ki oxygen le jane ki kshamta bahut kam ho jaati hai, jisse sharir ke uttakon aur dimaag mein oxygen ki kami ho jaati hai).
• This can cause dizziness, unconsciousness, and even death ☠️, especially while sleeping when the person might not realize the symptoms.

8. Compare LPG and Wood as fuels.

Comparison between LPG and Wood:

• State: LPG is a gas (stored as liquid under pressure). Wood is a solid.
• Calorific Value: LPG has a much higher calorific value (~55,000 kJ/kg) than wood (~17,000 kJ/kg). LPG produces more heat per kg. 🔥⬆️ (LPG ka ushmiy maan lakdi se bahut zyada hai).
• Pollution: LPG burns cleanly without smoke and leaves no residue. Wood produces a lot of smoke and leaves ash, causing air pollution. 💨 (LPG bina dhuein ke saaf jalti hai, koi avashesh nahi. Lakdi bahut dhuan aur raakh chhodti hai).
• Storage/Transport: LPG stored in cylinders, easy to transport/use. Wood is bulky, harder to store/transport. 📦 (LPG cylinder mein store hoti hai, le jana/istemal karna aasan. Lakdi bhaari hai).
• Ignition: LPG ignites easily. Wood takes time and effort to ignite.
• Environmental Impact: Cutting wood leads to deforestation 🌳❌. LPG is a fossil fuel but cleaner burning. (Lakdi kaatne se jungle khatam hote hain. LPG fossil fuel hai par saaf jalti hai).

Overall, LPG is a much more efficient and cleaner fuel than wood.

9. Goldsmiths use the outermost zone of the flame for melting gold and silver. Why?

Goldsmiths use the outermost zone because:

• This zone undergoes **complete combustion** due to sufficient oxygen supply from the air. ✅🌬️ (Is zone mein hawa se paryapt oxygen milne ke karan **poorn dahan** hota hai).
• Complete combustion results in the **highest temperature** in this zone compared to the middle or inner zones. 🌡️⬆️⬆️ (Poorn dahan ke karan is zone mein sabse **adhik taapmaan** hota hai).
• Gold and silver have high melting points. (Sone aur chandi ke galnaank uchh hote hain).
• The high temperature of the outermost zone is required to effectively melt these metals for shaping them into ornaments 💍. (In dhaatuon ko pighlaakar abhooshan banane ke liye sabse bahari zone ke uchh taapmaan ki avashyakta hoti hai).
• This zone is also typically non-luminous (blueish), allowing the goldsmith to see the metal clearly while working. (Yeh zone aam taur par non-luminous (neela) hota hai, jisse sonar kaam karte samay dhaatu ko saaf dekh sakta hai).

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

No, rusting cannot be called combustion, although both are oxidation processes (involve reaction with oxygen). Here’s why:

• Speed: Combustion is a **rapid** process that produces heat and light quickly 🔥. Rusting is a very **slow** process that happens over days, months, or years ⏳. (Gati: Dahan tez prakriya hai. Jang lagna bahut dheemi prakriya hai).
• Heat & Light Production: Combustion typically produces noticeable heat and often light. Rusting produces a very small amount of heat, which is usually not noticeable, and produces no light. (Garmi/Prakash Utpadan: Dahan mein garmi/prakash dikhta hai. Jang lagne mein bahut kam garmi nikalti hai, prakash nahi).
• Process: Combustion is a rapid oxidation giving off significant energy quickly. Rusting is a slow electrochemical oxidation involving iron, oxygen, AND water/moisture 💧. (Prakriya: Dahan tez oxidation hai. Jang lagna dheemi electrochemical oxidation hai jisme loha, oxygen, AUR paani/nami shamil hai).
• Flame: Combustion often involves a flame (if substance vaporises). Rusting never involves a flame.

Therefore, while rusting involves oxygen, it lacks the rapidity and significant energy release (as heat and light) characteristic of combustion.