Combustion and Flame

COMBUSTION AND FLAME

 

In villages, people generally use wood or cowdung cakes or crop residues to cook food. In towns, people use kerosene oil or coke to cook food. In cities, people use LPG gas. For running light vehicles, such as scooters and cars, we use petrol. For running heavy vehicles, we use diesel oil. In factories we use coal or fuel oil or natural gas.

The materials, such as wood, cowdung cakes, crop residues, kerosene oil, coke, LPG, petrol, diesel and natural gas have one common property, i.e., they produce heat on burning. It is this heat energy which is put to different uses.

The materials which produce heat energy on burning in air are called fuels. Furthermore, some materials on burning produce flame and some do not. For example, a candle or coal gas on burning produces a flame, but not the coal or charcoal.

Let us study the chemical process of burning and the types of flame produced during this process.

 

COMBUSTION

You have learnt that when magnesium is heated in air, it catches fire and burns with a dazzling white flame producing a large amount of heat energy. Similarly, when a strip of paper is brought near a candle flame, it burns to produce heat and light energy. The charcoal on heating burns to produce a large amount of heat and dull red light.

The materials which burn are commonly called combustible materials. The air or oxygen which helps in burning is called supporter of combustion, and the chemical reaction which takes place with the; release of heat and light energy is called combustion.

Definitions ;

(a)      Combustion : The chemical process in which a substance burns in air or oxygen, with the release of heat and light energy is called combustion.

(b)      Combustible material :A material which on heating in air or oxygen easily catches fire and releases heat and light energy is called combustible material.

Let perform the following activities to learn more about combustion.

Examples of combustible substances

Wood, paper, coal, charcoal, coke hydrogen, liquefied petroleum gas, natural gas, petrol, kerosene, diesel, furnace oil, alcohol, ether, benzene, etc., are examples of combustible substances.

It is not essentialthat light energy must be produced during combustion. For example, food that we consume is broken down into carbon dioxide and water by ite reaction with oxygen and produces heat energy. It is this heat energy which keeps our body warm. However, light energy is not produced during the combustion of food.

Supporter of Combustion :A substance (generally air or oxygen) which helps in the combustion of a combustible material is called supporter of combustion.

Combustion can take place in the absence of air. For example, sodium burns in the atmosphere of chlorine. Sodium and potassium burn in water. In fact, combustion is an oxidation process, in which chlorine, fluorine, bromine, sulphur, etc. could be oxidising the agents.

Similarly, combustion in case of fire crackers and all kinds of explosives take place in the absence of air. However, there is an internal supply oxygen due to the decomposition of oxidising agent, such as potassium chlorate, potassium nitrate, etc.

 

This show that air is essential for burning

Examples to prove that supporter of combustion is necessary for the combustion of a material.

1. When sand is poured over some burning material, the fire goes off. It is because the supply of air which supports the combustion is cut off.
2. If the clothes of a person catch fire, he is immediately wrapped in a thick blanket. The blanket cuts off the supply of air, i.e., the supporter of combustion, and hence, the fire is put off.

Let us perform another activity to learn more about combustion.

Ignition Temperature: From the above activity we can conclude that some minimum temperature is essential for a combustible substance, before it catches fire. This minimum temperature is calledignition temperature.

Definition :The minimum temperature to which a combustible substance must be heated, before it catches fire is called ignition temperature or kindling temperature.

Example: Ignition temperature of phosphorus is 35°C. So, unless the phosphorus is heated to 35°C, it will not catch fire. However, if temperature of air is 35°C or more, phosphorus will catch fire without heating.

Inflammable substances: The   substances   which  have  low  ignition temperature (generally less than 100°C) are called inflammable substances.

Examples : Alcohol, ether, carbon disulphide, “benzene, petrol, LPG, synthetic fibres, etc. are inflammable substances on account of their low ignition temperatures.

 

Conditions Necessary for Combustion

The following conditions necessary for producing and sustaining combustion.

(i)       There must be a combustible substance,

(ii)      There must be a continuous supply of oxygen or air.

(iii)     The temperature of combustible material should be above its ignition temperature.

Source of solar energy

Have you ever imagined what kind of combustible material is used by the sun which producesenormous amount of energy, Well our sun does not use conventional combustible materials nor does it use oxygen or air as supporter of combustion.  The 98% mass of sun is made of hydrogen which is at very high temperature. At such a temperature the hydrogen atoms fused in one another to form helium atoms and in the process release an enormous amount of heat and light energy. This energy is millions of time more then the energy produced by the combustion of coal. This process of production of energy in sun is callednuclear fusion.

TYPES OF COMBUSTION

(a)      Slow Combustion

When a combustible material burns at a slow or moderate rate, it is calledslow combustion.

During slow combustion there is not sufficient supply of air. Thus, combustion is never complete and as a result smoke or harmful gases are formed.

For example, cowdung cakes, wood, kerosene oil lamp, etc. exhibit slow combustion.

(b)      Rapid Combustion

When a combustible material (generally in the form of a gas) burns rapidly to produce heat and light, it is calledrapid combustion.

For example, LPG and CNG gas, when burnt in special stoves, exhibits rapid combustion. Similarly, kerosene oil pressure stoves and kerosene oil wick stoves exhibit rapid combustion.

During rapid combustion, there is sufficient supply of air and hence the combustion is complete.

(c)      Spontaneous Combustion

When a combustible material suddenly bursts into flames, without the application of apparent cause, spontaneous combustion takes place.

For example, when a small piece of phosphorus or sodium is kept in air for some time, it suddenly bursts into flames. It is because these materials initially form their oxides and produce heat energy. When their temperature rises above the ignition temperature, they suddenly burst into flame.

Underground coal mines always have very fine suspended particles in the air. When the amount of these suspended particles reaches at a certain threshold limit, the mixture of air and coal particles suddenly bursts due to spontaneous combustion. Such explosions not only cause extensive damage in the coal mines, but also kill a large number of miners. It is for the same reason that a good ventilation is provided in the coal mines to avoid such an explosion.

Similarly, explosions also occur in flour mills where the fine particles of flour are suspended in the air.

Forest fires are also the result of spontaneous combustion because there is a combustible material, a supporter of combustion and the right ignition temperature.

(d)      Explosive Combustion

When a mixture of combustible material and air (oxygen) completely burns in a very short span of time, in some closed space, explosive combustion takes place.

During explosive combustion, generally there is internal supply of oxygen or air.

For example, when the fuse of fire cracker is ignited, it decomposes potassium chlorate to form oxygen and raises the sulphur to its ignition temperature. Thus, the mixture of sulphur and carbon (in gunpowder) suddenly burns to produce a large amount of gases and produces sound, light and heat energy.

FIRE-FIGHTING

We have already learnt that combustion (fire) takes place only, if (i) there is a combustible material, (ii) continuous supply of air, and (iii) temperature of combustible material is higher than its ignition temperature.

If any of the above condition is not satisfied, the combustion will not take place, i.e., even initially combustion is taking place, it will die out. It is this principle which is used in firefighting. Let us discuss each of the above conditions in detail.

Fire Extinguisher

Fire extinguisher used by man

One of the most common fire extinguishers is discussed below :

Soda Acid Fire Extinguisher: It consists of a metallic cylinder from the bottom of which rises a plunger, with a sharp end. On the sharp end of the plunger is placed a sealed thin glass tube containing concentrated sulphuric acid. The test tube is surrounded by a fixed wire gauze cage. The cylinder is filled with solution of sodium bicarbonate. On the top of the cylinder is provided a nozzle which is sealed with wax.

In case of fire, the plunger is hit against the floor. The sharp tip of the plunger breaks the glass test tube. The acid in the test tube reacts with sodium bicarbonate to produce carbon dioxide gas.

2NaHCO3+  H2SO4Na2SO4   + 2H2O   +   2CO2

When the pressure of carbon dioxide exceeds a certain limit, the wax seal blows off. The solution within the cylinder along with carbon dioxide comes out with a great force through the nozzle. It is directed against the site of fire which in turn goes off.

Where to use soda acid fire extinguisher?

This extinguisher should be used against the fire caused by solid inflammable materials only. It is useless to extinguish fire caused by inflammable liquids such as petrol, because the solution coming out of cylinder being heavy, sinks in the oil and hence does not cut off air. It should not be used against electrical fires as the solution is a good conductor of electricity, and hence, user will get a severe electric shock.

 

Taking care of victims of fire

1. If the clothes of a victim catch fire, wrap him immediately in a thick blanket, so as to cut- off the supply of air. This will put off the fire.
2. Remove the victim to an open space so as to counter the effect of smoke and carbon monoxide.
3. Pour cold water over the victim’s body. This will remove the heat from his body and prevent the formation of boils.
4. Send the victim to a hospital for further medical treatment.

DISADVANTAGES OF INCOMPLETE COMBUSTION

Most of the common fuels contain a large percentage of carbon. During incomplete combustion, the following problems may arise.

(i)       During incomplete combustion, a part of the unburnt carbon passes into the atmosphere in the form of soot. This not only wastes the fuel, but also pollutes the atmosphere.

(ii)      During incomplete combustion, carbon monoxide is formed. This gas is highly poisonous in nature and can cause respiratory problems as well as death.

FLAMES

You must have seen, when you light a match stick, it burns with   a yellow flame. Similarly, the  candle burns with a yellow flame. The flame produced by the domestic gas (LPG) in the kitchen stove is blue. The question arises what is flame?

Flame: A region of burning gases is calledflame.

It is understandable that LPG is a gas. Therefore, the region in which it burns constitutes a flame. However, wax is a solid. How can a solid produce a flame?

When we light a candle with a matchstick, the heat of the burning matchstick not only melts wax, but also changes it into wax vapour. The wax vapour mixes with air and then catches fire to produce a flame.

Similarly, when the matchstick catches fire, the heat released due to the burning of chemicals on the match head, partly decomposes the wood to form wood gas. The wood gas then catches fire and produces a flame.

If you observe the flame of a lighted candle carefully, you will notice that it is not a simple flame.

At the base of the flame, quite close to the wick, the colour of the flame is blue. A region around the wick is dark grey in colour. Another region surrounding the dark grey zone is luminous and yellow in colour. There is another zone surrounding the luminous zone which is hardly visible. It is known as non-luminous zone. Let us study each of the zone in detail.

(i)       Blue zone :Near the base of flame, the fresh air rapidly mixes with wax vapour formed from the molten wax. A part of wax vapour almost completely burns and gives rise to blue flame, much the same way as the blue flame is seen on LPG gas burner.

(ii)      Bright and Luminous (Yellow) zone: It is the middle zone. Brightness of this zone is due to the glow of unburnt carbon particles.

(iii)     Dark inner zone : This zone consists of unburnt wax vapour given off by the molten wax. It is the coldest part of the flame. It is slightly dark because a part of wax vapour decomposes on heating to form carbon particles.

Different zones of candle flame

FUELS AND THEIR TYPES

Introduction

The prime necessities of a human being are food, clothing and shelter. None of these necessities can be available, unless we have heat energy to cook our food and other kinds of energy to run machines for manufacturing articles of clothing and shelter.

Fuel :Any substance which is easily available and bums in air at a moderate rate, producing a large amount of heat energy, without leaving behind any undesirable residue is called a fuel.

Note :Any substance which on burning in air produces heat energy is not a fad. For example, sulphur is easily available in nature and can burn in air to produce heat. However, it is not a fuel because on burning, it forms a poisonous gas, sulphur dioxide, which can cause serious respiratory problems and can be even fatal.

Calorific Value of a Fuel

The amount of heat energy produced on completely burning one kilogram of a fuel in pure oxygen is called the calorific value of a fuel.

Thus, a fuel which produces more heat energy per kilogram is said to have a higher calorific value. As matter of fact, more is the calorific value of a fuel more is the efficiency of the fuel.

The calorific value of the fuels is expressed kilojoules per kilogram (kj/kg). The calorific value of common solids, liquids and gaseous fuels is given below:

Calorific value of solid fuels

          Fuel                                                   Calorific value in kJ/kg

1 . Cow dung cake                              6000 – 8000

2. Wood                                             17000 – 22000
3. Coal                                              25000 – 33000

Calorific value of liquid fuels

          Fuel                                                   Calorific value in kJ/kg

1. Petrol                                           45000
2. Kerosene                                      45000
3. Diesel                                          45000

 

Calorific value of gaseous fuels

          Fuel                                                   Calorific value in kJ/kg

1 .  Methane                                       50000

2. Butane (LPG)                                50000
3. CNG                                             50000
4. Biogas                                          35000 -40000
5. Hydrogen         150000

Characteristics of a Good Fuel

(i)       It should be cheap and readily available,

(ii)      It should be easy to store,

(iii)     It should not produce any hazard during transportation.

(iv)     It should have a large calorific value,

(v)      It should produce a very small amount of residues, such as ash.

(vi)     It should not produce gases which pollute the air

(vii)    It should have low ignition temperature,

(viii)   It should burn at a slow rate and combustion should be controllable.

Classification of Fuels On the basis of physical state of fuel

Solid Fuels :The fuels which occur in a solid state at room temperature are called solid fuels.

Examples : Wood, agricultural residues, charcoal, coal, coke, paraffin wax.

Liquid Fuels :The fuels which occur in a liquid state at room temperature are called liquid fuels.

Examples : Liquefied hydrogen, petrol, kerosene oil, diesel oil, furnace oil, alcohol.

Gaseous Fuels :The fuels which occur in a gaseous state at room temperature are called gaseous fuels.

Examples : Water gas, producer gas, coal gas, compressed natural gas (CNG), gobar gas, hydrogen gas.

 

 

KEY POINTS

• Calorific Value : The amount of heat energy produced on complete combustion of 1 kilogram of fuel in pure oxygen.
• Combustible substance : A substance which on heating in air or oxygen catches fire easily with liberation of heat and light energy.
• Combustion : The chemical process in which substances burn in air or oxygen with the release of heat and light energy.
• Deforestation : Cutting down of excessive trees from the forest for wood or other commercial uses, thereby reducing the forest area is called deforestation.
• Explosion : When spontaneous combustion takes place in an enclosed space such that there is internal supply of air/oxygen with release of a large amount of sound, heat, light and mechanical energy it is called explosion.
• Flame : A region of burning gases is called a flame.
• Fuel: Any substance which is easily available and burns in air at a moderate rate, producing a large amount of heat energy, without leaving behind undesirable residue.
• Fuel efficiency : The heat produced by burning completely one kilogram of a fuel is called fuel efficiency.
• Global warming : The phenomenon due to which excessive heat is trapped in the atmosphere because of the excessive amount of carbon dioxide, such that it leads to melting of polar ice caps and changes in weather patterns is called global warming.
• Ideal fuel : A fuel which is cheap, easily available, easily stored, has highest calorific value and does not produce any harmful residues/gases on combustion is called ideal fuel.
• Ignition temperature : The minimum temperature to which a substance is heated before it catches fire.

 

WORKSHEET-1

1. What is combustion?
2. List conditions under which combustion can take place.
3. What is fuel? Give some examples of a fuel.
4. What are combustible substances? Give some examples.
5. What are non-combustible substances? Give some examples.

 

 

 

WORKSHEET-2

 

1. Explain various types of combustion?
2. Can the process of rusting be called combustion? Discuss.
3. Compare LPG and wood as fuels.
4. Explain how the uses of CNG in automobiles has reduced pollution in our cities.
5. Give reasons : –

(i)   Water is not used to control fires involving electrical equipment.

(ii)  LPG is a better domestic fuel than wood.

(iii) Paper by itself catches fire easily whereas a piece of paper wrapped around

analuminium pipe does not.

 

 

WORKSHEET-3

 

1. What do you mean by ignition temperature?
2. What is calorific value of a fuel?
3. What are the characteristics of a good fuel?
4. What do you mean by global warming? Write its effects.
5. What do you mean by forest fire?

 

 

WORKSHEET-4

1. How do we control fire?
2. Define flame.
3. Describe the structure of a flame with a suitable diagram.
4. Which are the harmful products are formed due to burning of fuels?
5. What is the job of a fire extinguisher?

 

 

 

 

 

ASSIGNMENT-1

1. By giving two examples, explain what do you understand by the term combustible substance.
2. Definethe following terms:

(a)  combustion                                   (b)  ignition temperature

(c) inflammable substance,

3. State three conditions necessary for combustion.
4. What do you understand by the terms

(a) fuel,                                             (b)  calorific value?

5. State four characteristics of a good fuel,
6. Draw a diagram showing different zones of candle flame. Which zone contains:

(a) unburntvapour of wax                   (b)  very hot carbon particles?

7. How has the use of CNG in motor vehicles reduced pollution in the big cities? Explain,
8. Compare the LPG or CNG as a fuel with coal,
9. Why is water not used in controlling electrical fires?
10. Some paper is wound around

(i)   a wooden cylinder

(ii)  analuminium cylinder. Each of the cylinder is held over a bunsen flame for 10 seconds. What will you observe? Explain your answer,

11. Which zone of flame is used by goldsmiths to make gold ornaments and why?
12. Calorific value of cowdung cakes is 7000 J/kg. How much cowdung cakes are required to produce hea energy of 35000 J ?
13. 5 kg of a fuel produces 320,000 J of heat energy on complete combustion. Calculate the calorific value of fuel.
14. Is rusting of iron a combution process? Give reasons for your answer,
15. Two students A and B heated the same mass of water on a candle flame. The student A kept the beaker slightly above the flame of candle and student B kept the beaker close to wick of the candle. In which case, the water will get heated in shortest time and why?
16. Why is it easy to burn a heap of dry grass, compared to green grass?

 

ASSIGNMENT-2

Level – I

Straight Objective Type

17. During the combustion of a material, generally :
    (A)  heat energy is released                 (B) light energy is released
    (C) both (A) and (B)                            (D)     chemical energy is released

 

18. Which one of the following is an inflammable substance?
    (A) Wood                                            (B) Coke
    (C) Paper                                           (D) Kerosene oil

Multiple Correct Answer Type

19. Which is not considered as supporter of combustion?

(A)  O2                                                (B) H2

(C) Cl2                                                (d)  Br2

 

20. Which is/are the combustible substances?

(A)  wood                                             (B) LPG

(C) petrol                                           (D) kerosene

 

Assertion – Reason Type questions

21. Statement – 1: A good fuel should have high calorific value.

Statement – 2: The amount of energy consumed during reach is called calorific value.

 

22. Statement – 1: LPG is a liquid fluid.

Statement – 2: LPG contains butane.

Matrix Match Type

23. Match the statements in column A with those in column 8.

          Column A                                          Column B

(A) A kind of fossil fuel                       (p)  Cow dung

(B) The burning region of gases           (q)      Petrol

(C) A substance which burns in air on heating       (r)      Flame
(D) A gas produced during combustion,     (s)  Sulphur dioxide
which causes acid rain

Integer Answer Type

24. The percentage of noble gas by volume in air is?

 

25. Among given fuel. LPG, Petrol, Kerosene Oil & alcohol how many of them are liquid fuels?

 

Fill in the blanks Type

26. Forest fires are also an example of ……………… combustion.

 

27. During explosive combustion there is internal supply of ………. ignition.

Level – II

Straight Objective Type

28. A fire can be controlled by :
    (A) lowering the ignition temperature
    (B) cutting of the supply of supporter of combustion
    (C) any of the (A) and (B)
    (D) none of the above

 

29. Which one is an example of spontaneous combustion?
    (A) Phosphorus catches fire in air (B)  Charcoal burns with a dull glow
    (C)  LPG gas burns with a blue flame   (D) Candle burns with a yellow flame

Multiple Correct Answer Type

30. Which is/are correct?

(A)  2NaHCO3 + H2SO4 —Na2SO4 + 2H2O + 2 CO2

(B) 2Na + Cl2 —NaCl

(C) Cl2 + H2 — 2HCl

(D) none of these.

 

31. Which is/are the characteristics of a good fuel?

(A)  It should be easy to store.

(B) It should have low ignition temper.

(C) It should be cheap.

(D) It should have high calorific value.

Assertion – Reason Type questions

32. Statement – 1: There are three types of fuels based on physical state.

Statement – 2: Solid fuel, liquid fuel & gaseous fuels one types of fuels based on physical state.

 

33. Statement – 1: Methane has calorific value 50,000 Kj/Kg.

Statement – 2: Hydrogen has 3 times more calorific value than CH4.

Matrix Match Type

34. Match the statements in column A with those in column 8.

          Column – I(Fuel)                                 Column – II( Number of Carbon Atoms)

(A)  Methane                                       (p)  2

(B)  Butane (LPG)                                (q)  4

(C)  CNG                                             (r)  1

(D) Biogas                                          (s)  5

Integer Answer Type

35. The ratio of calorific value of H2to that of CH4is?
36. Water gas contain how many moles of CO?

Fill in the blanks Type

37. A good fuel should have low …………. temperature.

 

38. It is not essential that light is produced during ………

 

Level – III

Straight Objective Type

39. The hottest part of a candle flame is :
    (A) blue zone (B) invisible zone
    (C) visible zone                                   (D) dark inner zone

 

40. Incomplete combustion of carbon produces

(A)  CO                                               (B)  CO2

(C)  C2O3                                             (D) none.

Multiple Correct Answer Type

41. Which are is non-poisonous?

(A)  CO                                               (B) CO2

(C) NO2                                              (D) SO2.

 

 

42. Which are not produced when CO combine with O2?

(A)  CO2                                              (B) NO2

(C) SO2                                              (D) SO3.

 

Assertion – Reason Type questions

43. Statement – 1: Biogas is a gaseous fuel.

Statement – 2: It contain CH4

 

44. Statement – 1: Coal gas on burning produces a flume.

Statement – 2: Coal / charcoal do not produces it.

 

Matrix Match Type

45. Match the statements in column A with those in column 8.

          Column – I(Physical state of fues)      Column – II (Examples)

(A)  Solid fuels                                    (p)  wood

(B)  Liquid fuels                                  (q)  Petrol

(C)  Gaseous fuels                               (r)  Coal gas

(D) Bio gas                                         (s)  High calorifre value

Integer Answer Type

46. Benzene contains how many number of c-atoms.

 

 

47. LPG contains butane which contain how many number of c-atoms?

Fill in the blanks Type

48. Food that we consume is broken down into ……… the

 

49. Oxygen is the supporter of …………

 

ANSWER KEY

Level – I

17. (C) 18. (D)
18. (C, D) 20. (A, B, C, D)
19. (C) 22. (A)
20. (A -q); (B -r); (C – p), (D – s) 24. 1
21. 4 26. Spontaneous
22. O2

Level – II

28. (C) 29. (C)
29. (A, B, C,) 31. (A, B, C, D)
30. (A) 33. (A)
31. (A -r); (B -q); (C – s); (D – p, r) 35. 3
32. 1 37. ignition
33. combustion

Level – III

39. (C)
40. (A)
41. (B, C, D) 42. (B, C, D)
42. (A) 44. (A)
43. (A – p); (B – q); (C – r); (D – s) 46. 6
44. 4 48. CO2
45. combustion.