Cmbustion and Flame Notes

Combustion and Flame Notes

Combustionis the process of burning a substance in the presence of oxygen, resulting in the release of energy in the form of heat (and sometimes light). Flames are visible, gaseous parts of the combustion reaction. The color of the flame depends on the material burning and the temperature of the combustion. Examples of Combustion includes lighting a candle, burning wood in a fireplace and using petrol in car engines. Not all substances undergo combustion. Substances that easily catch fire are known as combustible substances, such as paper, coal, and wood. Substances that do not catch fire readily are non-combustible substances, such as rocks, iron, sand, water, and glass.

Conditions required for Combustion

For combustion to take place, three things are required, they are fuel, oxygen and ignition heat. The combination of fuel, oxygen, and is known as fire triangle. Combustion will not occur if one of these is absent. Fuel is substance provides energy for burning . Oxygen- found in the air and is necessary for combustion. Heat is the minimum temperature that can ignite fire required to ignite the fuel.

Ignition Temperature

It is the lowest temperature at which a combustible substance catches fire when heated in air. Iignition temperature vary for each substance.

Types of Combustion

Combustion can occur in different forms based on how fast and under what conditions a substance burns. 5 types of combustion are:

1. Rapid Combustion requires an external source of flame and the substances burns quickly to produce heat and light. Example: Burning of LPG, paper, or candle wax.

2. Spontaneous Combustion occurs when the temperature of a substance rises automatically to its ignition point. Substances under the influence of spontaneous combustion catches fire on its own without any external flame or spark. Example: Phosphorus, coal heaps, oily cotton rags,.

3. Explosion is seen in explosives like firecrackers, bombs, and crackers. Explosion leads to a sudden and rapid combustion that releases heat, light, and sound with the production of largevolumes of gas. Explosion demands very little time and energy release is instantaneous.

4. Complete Combustion This process occurs when there is a sufficient or excess supply of oxygen, allowing the fuel to be fully oxidized. In the complete cobustion, the fuel reacts with oxygen, producing carbon dioxide and water as the primary products, and releasing a significant amount of heat and light.

5. Incomplete Combustion is when there is limited oxygen available. Incomplete combustion is very harmful as it produces a lot of carbon monoxide or soot along with heat. Example: Burning of coal or wood in a closed room.

Flame and its structure

A flame is the visible part of the fire. Understanding the flame's structure helps in recognizing the process of combustion in different settings. Different substances produce flames with different colours. For example, wood and paper produce yellow flames, while some gases like methane produce a blue flame. The structure of a flame consists of three main zones:

A. Innermost Zone (Dark Zone): This is the part of the flame closest to the wick or fuel source. It is dark and contains unburnt wax or fuel particles. It is the least hot part of the flame.

B. Middle Zone (Luminous Zone): The middle part of the flame is yellow in color due to incomplete combustion. This zone is moderately hot and emits light.

C. Outermost Zone (Non-luminous Zone):

The outer zone is the hottest and burns with complete combustion, resulting in a blue flame.

It is the hottest part of the flame and is usually non-luminous (does not glow brightly).

Fuels and Their Efficiency

A fuel is a substance that produces heat and light when burnt. Fuels can be classified into three categories:

Solid Fuels: Wood, coal, charcoal
Liquid Fuels: Petrol, diesel, kerosene
Gaseous Fuels: LPG (liquefied petroleum gas), natural gas, biogas

Fuel Efficiency: The efficiency of a fuel is determined by its calorific value, which is the amount of heat energy produced when 1 kilogram of fuel is completely burnt. It is expressed in kilojoules per kilogram (kJ/kg).

Calorific Values of Different Fuels:

Fuel	Calorific Value (kJ/kg)
Cow dung cake	6000-8000
Wood	17000-22000
Coal	25000-33000
Petrol	45000
Kerosene	45000
Diesel	45000
Methane	50000
CNG	50000
LPG	55000
Biogas	35000-40000
Hydrogen	150000

Some fuels, like LPG and natural gas, are considered more efficient because they have higher calorific values, meaning they produce more energy with less fuel.

Harmful Effects of Burning Fuels

While burning fuels is essential for our day-to-day activities, it can also have harmful effects on the environment and health. Some of the key negative impacts are:

Air Pollution: Burning of fuels releases harmful gases like carbon dioxide (CO2), sulfur dioxide (SO2), and nitrogen oxides (NO?) into the atmosphere, leading to air pollution.

Global Warming: Excessive burning of fossil fuels increases the amount of CO2 in the atmosphere, contributing to the greenhouse effect and global warming.

Acid Rain: Pollutants like sulfur dioxide and nitrogen oxides react with water vapor in the atmosphere to form acids, which fall back to the earth as acid rain. This can damage crops, buildings, and aquatic life.

Respiratory Problems: Inhalation of polluted air can cause respiratory issues like asthma, bronchitis, and other lung diseases.

Fire Extinguishers

Fire extinguishers are devices used to put out small fires by cutting off the fire's supply of oxygen or by lowering its temperature.

Common types of fire extinguishers include:

Water extinguishers: Used for fires involving paper, wood, or cloth.

CO2 extinguishers: Effective for electrical fires or fires involving flammable liquids.

Foam extinguishers: Used for fires involving flammable liquids like petrol.

ensures complete combustion, making it ideal for melting gold and silver.

Question Answers

Question 1. List conditions under which combustion can take place.
Answer: The conditions for combustion are:

The presence of a combustible substance.
The presence of air with a sufficient amount of oxygen.
The temperature of combustible substances should be more than its ignition temperature.

Fill in the blanks.

(a) Burning of wood and coal causes __________of air.

(b) A liquid fuel, used in homes is__________ .

(d) The fire produced by oil cannot be controlled by___________ .

Answers:

(a) Burning of wood and coal causes pollution of air.

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

(d) The fire produced by oil cannot be controlled by water.

Question 3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer: The use of CNG in automobiles has reduced pollution in our cities because CNG burns with a blue smokeless flame and does not leave any ash after burning.

Question 4. Compare LPG and wood as fuels.
Answer:

LPG	Wood
1. It has a more calorific value.	1. It has less calorific value.
2. It is smoke-free fuel.	2. It gives out a lot of smoke which is quite dangerous.
3. It is easy to transport.	3. It is difficult to transport wood.
4. It is easily stored in cylinders.	4. It is difficult to store as it needs space to store.
5. It leaves no residue.	5. Ash is produced.

Question 5. Give reasons:
(a) Water is not used to control the fire involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminum pipe does not.

Answer:
(a) Water is not used to control the fire involving electrical equipment because water conducts electricity and thus, there is the possibility of the electric current electrocuting the person trying to extinguish the fire with water.

(b) LPG is a better fuel than wood because it is smokeless, does not leave any residue, has a high calorific value, and low ignition temperature.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not because the heat supplied to the paper is transferred to the aluminium pipe so the aluminium lowers the temperature of the paper. In the presence of aluminium, the ignition temperature of paper is not reached. Hence, it does not burn.

Question 6. Make a labeled diagram of a candle flame.

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

Answer: Kilojoules/kilogram (KJ/Kg).

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

Answer:CO2 forms a large around the burning substance and cuts off the supply of air (oxygen) to it and hence extinguishes fire.

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

Answer: The ignition temperature of dry leaves is much less than that of green leaves (due to the presence of water). Hence, 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:A goldsmith uses the outermost zone of a flame for melting gold, and silver because it the hottest zone of the flame.

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

Answer:The heat produced by burning 4.5kg of a fuel = 1,80,000 KJ.The heat produced by burning 1 kg of a fuel = 1,80,0004.5 = 40,000 KJ/kg. Therefore, caloric value of the fuel is 40,000 KJ/kg.

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

Answer: The process of rusting can be called combustion because it produces heat. This combustion takes place at a slow rate.

Question 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: The water of Rameshs beaker will get heated in a shorter time because the outermost part of the flame is the.