Class 8 Science Chapter 4 Combustion and Flame

Combustion and Flame

Combustion

Process of giving off heat by a substance after reaction with oxygen is called combustion. Sometimes, in the process of combustion, light is also given off along with heat. Combustion is a chemical process. For example; when petrol, LPG, kerosene, etc. reacts with oxygen, they give off heat.

Combustible Substance

Substances which go under combustion are known as combustible substances. Combustible substances are also known as fuel. For example; wood, charcoal, LPG, kerosene, petrol, diesel, etc.

Ignition Temperature:

Ignition temperature is the lowest temperature at which a substance catches fire. Ignition temperature is different for different substances. For example; LPG, petrol, natural gas, etc. catch fire at very low temperature and thus have low ignition temperature, while wood, coal, etc. have high ignition temperature.

Conditions for combustion

There are three conditions necessary for combustion: -
(a) Fuel – Fuel is the substance which undergoes combustion.
(b) Supply of air – Oxygen helps in combustion. Air contains about 29% of oxygen, thus supply of air makes the oxygen available which helps in combustion. Without oxygen, combustion will not take place.
(c) Ignition temperature – For catching fire, a combustible substance must reach its ignition temperature. If a combustible substance does not reach at or above its ignition temperature, it will not catch fire and combustion will not take place.
Thus, above three conditions are necessary for combustion to take place. If any one of the three will not be available, combustion will not take place.

Inflammable substance

Substances which catch fire at very low temperature and burn with flame are called inflammable substances. For example; LPG, Petrol, Spirit, etc.

Controlling the Fire – Fire extinguisher

Many a time; homes, forest, shops, etc. catch fire. In such cases, it becomes necessary to put off the fire otherwise it may cause huge monetary loss and loss of lives. Fire Brigade or Firemen are experts in controlling fire by using fire extinguisher.
To extinguish fire, at least any one out of three essential conditions for combustion must be removed. Supply of fuel, supply of air and ignition temperature; are the three essential conditions for combustion.
In case of fire in a building, the whole building becomes fuel. It is not possible to cut off the supply of fuel. Thus, firemen try to cut off air supply and or bring down the temperature of combustible material below their ignition temperature.

Fire extinguisher:

Water is one of the best, cheapest and oldest fire extinguishers. By pouring water over the combustible material, the temperature can be cooled down. Cooling down brings the combustible materials below their ignition temperature. In addition to this, water vapour surrounds the combustible material which stops the supply of air. Removal of these two conditions puts off the fire.
This is the cause that firemen generally pour water over the materials which have caught fire.
Controlling fire when electrical equipment is on fire: Water is not suitable in the case when electrical equipment or oil catches the fire. In the case of electrical equipment on fire, pouring water over them may prove disastrous because normal water is a good conductor of electricity. It may conduct electricity and can harm the persons who are trying to control the fire.
Controlling fire in the case of oil, petrol, etc.: Water is heavier than oil. So when water is poured over oil; oil comes on top and keeps on burning.
Carbon dioxide as fire extinguisher: Carbon dioxide does not support combustion and hence is considered as the best fire extinguisher.
Carbon dioxide is heavier than air and hence covers the material which is burning. By covering the material, supply of oxygen is stopped. This puts off the further combustion and fire is controlled.
Under high pressure, carbon dioxide liquefies and takes less space because of compression. Liquid carbon dioxide is stored in cylinders. A nozzle is attached with cylinder to release carbon dioxide. When nozzle is opened, carbon dioxide starts coming out from the cylinder because of high pressure. It expands and covers the combustible materials as blanket. This cuts off the supply of oxygen to the combustible materials. Because of expansion, the temperature of carbon dioxide decreases which decrease the temperature and brings down the combustible material below their ignition temperature. Thus, stoppage of supply of oxygen and bringing down the temperature below the ignition temperature of combustible materials put off the fire.
Sodium bicarbonate or potassium bicarbonate as fire extinguisher: Sodium bicarbonate (baking soda) or potassium bicarbonate releases carbon dioxide on heating. Thus, when powder of sodium bicarbonate or potassium bicarbonate is spread over or near the fire, they release carbon dioxide which covers the burning materials and cuts off the supply of oxygen to them. This puts off the fire.

Combustion and Flame

Types of Combustion: -

Combustion can be divided into three types – rapid combustion, spontaneous combustion and explosion.

Rapid Combustion:

Combustion in which a substance burns rapidly and produces heat and flame is known as rapid combustion, such as combustion of natural gas, LPG, petrol etc. This is usually attained by introducing external heat. Substances which undergo rapid combustion have lower ignition temperature.

Spontaneous Combustion:

When a substance suddenly starts burning into a flame; without the supply of any external cause (such as heating; the combustion is called spontaneous combustion. Substances; which have relatively low temperature of ignition generally burn by spontaneous combustion.

Examples:

• Phosphorous and sulphur start burning instantaneously; at room temperature.
• Haystacks, linseed oil, coal, pyrite, etc. sometimes start burning suddenly with flame because of increase in temperature.
• In coal mines, fire breaks out many a times because of combustion of coal dust.
• Fire often breaks out suddenly because of increase in temperature due to sun or friction.

Explosion: -

When combustion is accompanied by sudden production of heat, sound and large amount of gas, it is called explosion. Firecrackers and bombs are substances which show explosion.

Flame

All substances do not give flame while burning. Substances which vapourise while burning give flame and those which do not vapourise while burning do not give flame.
Fuels which burnt with flame are used to produce light.

Example –

Kerosene, wax, wood, etc. burn with flame as they vapourise during burning.
Coal and coke do not vaporize while burning and hence do not produce flame.

Structure of flame

Flame can be divided into three zones – innermost zone, middle zone and outer zone
Innermost zone: Zone near the wick of a candle is called the innermost zone of the flame. The innermost zone is the zone of unburnt wax and is generally of black colour. It is the least hot zone.
Middle zone: Zone in the middle of the flame is called the middle zone. Middle zone of flame is moderately hot and of yellowish colour. In this zone, fuel burns partially. Middle zone is the luminous zone, because partial combustion of carbon produces glow.
Middle zone produces black deposits of unburnt carbon particles.
Outer zone: Outer zone of flame is of bluish colour and the hottest part of the flame. In this zone, fuel is completely burnt. Because of complete combustion, it gives bluish colour.

Fuel

All combustible materials are not considered as fuel. Charcoal, wood, LPG, cowdung cake, natural gas, petrol, diesel, kerosene etc. are known as fuel.

Criteria for ideal fuel

• Easily and readily available
• Cheap
• Burns easily in air at moderate rate
• Produces large amount of heat
• Burns completely and does not leave any undesirable substance after burning.

Substances which meet above criteria are known as ideal fuel. But ideal fuel is not available in practical life.
Fuels which meet most of the criteria are considered as good fuel.

Fuel efficiency:

Efficiency of a fuel is measured by its calorific value.
Calorific value is the amount of heat produced by the complete burning of 1 kg of fuel. And hence calorific value of fuel is expressed in kilo joule per kg (k J/kg).
A fuel with higher calorific value is considered as an efficient fuel.

Burning of Fuel Produces Harmful Products

Since, no fuel is considered as an ideal fuel, thus they do not undergo complete combustion and produce unwanted substances. These substances have harmful effect on the humans and the environment. Some of the harmful effects of burning of fuel are as follows:
(a) Fuels; like, wood, petrol, diesel, coal, etc. release unburnt carbon particles. These unburnt carbon particles create pollution by mixing in air. They lead to diseases of respiratory system and many other related diseases.
(b) Incomplete combustion of carbon fuels produces carbon monoxide. Carbon monoxide is a poisonous gas which may kill a person.
(c) Most of the fuels release carbon dioxide gas. Excess carbon dioxide gas in the atmosphere leads to global warming.
(d) Burning of coal and diesel produce sulphur dioxide. Burning of petrol gives oxide of nitrogen. When these gases mix with rainwater, acids are formed. These acids fall along with rainwater and this is called acid rain. Acid rain is harmful for animals and plants. Acid rain is also harmful for buildings and monuments. The famous Taj Mahal has lost much of its sheen because of acid rain.
CNG is considered as a cleaner fuel. It releases harmful products in very small amount. Now-a-days CNG is being used to run vehicles in many towns, such as Delhi, Ahmadabad and Mumbai. This has helped in reducing the level of pollution.

Combustion and Flame

NCERT Solution

Question: 1. List conditions under which combustion can take place.

Answer: - Conditions under which combustion can take place are as follows:

(a) Presence of oxygen

(b) Presence of an inflammable substance

(c) Ignition temperature

Question: 2. Fill in the blanks:
(a) Burning of wood and coal causes …………….of air.

Answer: Pollution

(b) A liquid fuel, used in homes is …………….

Answer: - Kerosene

(c) Fuel must be heated to its ………… …………….before it starts burning.

Answer: Ignition temperature

(d) Fire produced by oil cannot be controlled by ……………...

Answer: Water

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

Answer: CNG is a cleaner fuel; compared to petrol and diesel. CNG produces lesser amount of harmful gases on burning. Due to this, CNG is now being used in automobiles in many cities; which has helped in reducing pollution in our cities.

Question: 4. Compare LPG and wood as fuels.

Answer: Wood has traditionally been used as kitchen fuel and is still predominantly being used in rural areas. Burning of wood creates many air pollutants which can result in respiratory problems. Moreover, incomplete oxidation during burning of wood creates carbon monoxide which is a poisonous gas. LPG is much better because it burns without giving smoke. It produces lesser amount of air pollutants. Complete oxidation during burning of LPG does not lead to carbon monoxide production.

Question: 5. Give reasons:
(a) Water is not used to control fires involving electrical equipment.

Answer: Pure water is a bad conductor of electricity but normal water contains many salts and hence is a good conductor of electricity. Trying to douse fires; involving electrical equipment; with water can result in electric shock. Due to this, water is not used to control fires involving electrical equipment.

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

Answer: LPG is a better domestic fuel than wood because of several reasons. Unlike wood, LPG burns without smoke. This makes the life of housewives more comfortable and they do not have to worry about blackening of pots and pans. Moreover, use of LPG as domestic fuel also rules out the chances of getting respiratory disorders which may happen when someone uses wood as kitchen fuel. Storage and transportation of LPG is easier compared to that of wood.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Answer: The ignition temperature of paper is lower compared to that of aluminium. When paper is wrapped around an aluminium pipe; the ignition temperature increases. That is why paper itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Question: 6. Make a labelled diagram of a candle flame.

Answer:

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

Answer: The calorific value of a fuel is expressed in kilo joule per kg (kJ/kg).

Question: 8. Explain how CO2 is able to control fires.

Answer: Carbon dioxide is heavier than oxygen. Due to this, carbon dioxide forms a blanket around the burning material and makes a barrier between the burning material and oxygen. As oxygen supply is cutoff, it helps in extinguishing the fire.

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

Answer: Green leaves contain moisture and as a result, the ignition temperature of green leaves is much higher than that of dry leaves. Due to this, it is difficult to burn a heap of green leaves but dry leaves catch fire easily.

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

Answer: The outermost zone of a flame is the hottest zone. Due to this, goldsmith uses the outermost zone of a flame for melting gold and silver.

Question: 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.

Answer: Total mass of fuel = 4.5 kg

Heat produced by burning the given mass of fuel = 180,000 kJ.

We know that calorific value of fuel

Hence, the calorific value of the given fuel = 40,000 kJ/kg

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

Answer: Although the process of rusting also involves oxidation but it cannot be termed as combustion. The reason for this is that combustion is defined as a process in which oxidation is accompanied by heat and heat is not produced during rusting.

13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in 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?
Answer: Since Abida has kept the beaker in the luminous zone of the flame, the beaker will take more time to get heated. On the other hand, Ramesh has kept the beaker in the non-luminous zone of the flame so his beaker will be heated in shorter time