Combustion and Flame

(1) Combustion is a chemical process in which a substance reacts with oxygen to gives off heat and sometimes gives off light either in the form of flame or glow.

Eg:-

(i) Magnesium burns to form magnesium oxide and produces heat and light.

(ii) Coal also burns in air and produces heat and light.

Types of combustion

(i) Rapid combustion is a chemical process in which substance burns rapidly and produces heat and light.

Eg:- Gas burns rapidly in gas stove.

(ii) Spontaneous combustion is a combustion in which a material suddenly burns without heating from outside.  The heat required for the process is produced by slow oxidation of material. It happens at the room temperature. Spontaneous combustion takes place only in those materials which have very low ignition temperature.

Eg:-

(a) Phosphorus burns in air at room temperature.

(b) Coal dust has resulted in many disastrous fires in coal mines because of spontaneous combustion. Coal ignites spontaneously when exposed to oxygen and heats on its own if there is no ventilation for cooling.

(iii) Explosive combustion takes place when high temperature or high pressure is applied on the substance and heat, light, gas and sound produced by a rapid and violent oxidation reaction.

An explosive combustion is called explosion when it is caused by acceleration of chemical reaction.

Eg:-

(a) Crackers

(b) Bomb

(2) The substance that undergoes combustion is called combustible. It is also called fuel.

Eg:- Coal, charcoal, paper, wood etc.

Combustible and non – combustible substances are tabulated below:-

Material Combustible
Wood Yes – produces heat and flame of light
Paper Yes – produces heat and flame of light
Iron nails No – Not produces flame of light. Melts if continue heating for long time.
Kerosene oil Yes- produces heat and flame of light
Stone piece No – Not produces flame of light.
Straw Yes – produces heat and flame of light
Charcoal Yes- produces heat and flame of light
Matchsticks Yes- produces heat and flame of light
Glass No – Not produces flame of light. Melts if continue heating for long time.
Petrol Yes- produces heat and flame of light
Cloth Yes- produces heat and flame of light

Experiment to investigate conditions under which combustions happens.

Fix a lighted candle on the table or wooden plank. Now, put two blocks on the table. Put glass chimney on the candle. Now, observe flame. It will light well.

schoolhelpbygunjan.wordpress.com | Combustion and flame | diagram | Showing oxygen is essential for burning | NCERT | Class 8

Now, remove wooden planks and keep chimney on the lighted candle. Flame of candle flickers and produces smoke. It happens because air can not enter from below.

schoolhelpbygunjan.wordpress.com | Combustion and flame | Diagram | Showing oxygen is essential for burning | NCERT | Class 8

Now, put a glass plate over the chimney. Flame goes off because non availability of air.

schoolhelpbygunjan.wordpress.com | Combustion and flame | Diagram | Showing oxygen is essential for burning | NCERT | Class 8

It is clear from the experiment that air is very necessary for the combustion.

(3) The lowest temperature at which a substance catches fire is called its ignition temperature.

(i) If we place a piece of burning wood on an iron plate or tawa and cover that piece with a glass jar, the piece stops burning after some time. It happens because of lack of air in the jar.

(ii) A piece of paper takes time to burn when a burning matchstick is brought near it. It happens because paper takes time to reach its ignition temperature. Paper will never start burning if its temperature is lower than its ignition temperature.

(iii) We can not burn a piece of wood with a matchstick because wood will take time to reach its ignition temperature. A piece of wood will never start burning if its temperature is lower than its ignition temperature. Therefore, we will have to use paper or kerosene to burn a piece of wood.

Experiment to show that reaching up to ignition temperature is necessary for a substance to burn.

Take two paper cups. Fill 50 ml water in one cup and heat both cups separately. The cup without water burn easily because it reaches to its ignition temperature. But, cup with water does not start burning. Because, heat transferred to water by conduction and ignition temperature of paper does not reach in the presence of water. If we continue heating water can boil in the paper cup.

schoolhelpbygunjan.wordpress.com | Combustion and flame | Diagram | Heating water in a paper cup | NCERT | Class 8

(4) The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.

Eg:- Petrol, Liquified Petroleum Gas (LPG) etc.

(5) Fire men control fire by using water on the fire. In this way, they cut supply of air, they cools the combustible materials so that its temperature brought below its ignition temperature. This prevents the fire from spreading.

(6) Water is the most effective extinguisher but it does not work when electrical goods are on fire. Water may conduct electricity and harm fire brigade persons trying to extinguish fire.

Water also not work if fire is involving oil or petrol. Water is heavier than oil. Therefore, it sinks below the oil and oil keeps burning above.

CO2 gas is the best option for the fire by electrical equipment. Being of heavier than the oxygen, carbon –di – oxide covers the fire and cut the supply of oxygen which is very necessary for fire. Therefore, fire gets controlled. Co2 does not harm the electrical appliances. We can get Co2 immediately from the fire extinguisher cylinder in which gas is stored at high pressure in liquid form. It spreads and cools down immediately after it is released. Therefore, it is the best to use fire extinguisher because it cut off the oxygen and cools the temperature of fuel.

(7) A hot burning gaseous part of the substance is called flame.

Eg:- Wax, kerosene etc. provide flame when burns.

Materials forming flame on burning

SN. Material Forms flame Does not form flame
1. Candle Yes
2. Magnesium Yes
3. Camphor Yes
4. Kerosene Stove Yes
5. Charcoal Yes

Structure of the flame

The substances that vaporise during burning give flames.

Eg:- Kerosene oil rise through the wick and vaporised during burning. Therefore produces flame.

Charcoal does not vaporise during burning. Therefore, it does not produce flame.

Zones of the flames – There are three zones of the flames.

schoolhelpbygunjan.wordpress.com | Combustion and flame | Diagram | Different zones of candle flame | NCERT | Class 8

(i) The zone near the wick is called dark zone or inner most zone of the flame. The wax of the candle melts and rises up through wick (Capillary action). The wax vaporised during burning and forms flame. The dark zone is the least hot part of the flame.

(ii) The zone above the dark zone having yellowish orange colour is known as luminous zone. The fuel particles or wax particles (In our case) starts burning in this zone. But, fuel burns partially because of less oxygen available in this zone. Therefore, this zone has unburnt carbon particles of fuel or wax.

Experiment-

Hold a glass plate in the luminous zone for 10 seconds. A circular blackish ring is formed on the glass plate which shows the unburnt carbon particles present in the luminous zone of the flame.

schoolhelpbygunjan.wordpress.com | Combustion and flame | Diagram | Showing unburnt carbon particles in the luminous zone | NCERT | Class 8

(iii)The outer most zone which is black in colur is known as non – luminous zone. This is the hottest part of the flame. In this part, full combustion takes place because of plenty of oxygen.

Experiment

Hold a copper wire just in side the flame. The portion of copper wire which is just outside the flame gets red hot which shows that outer most flame has the highest temperature of the flame.

schoolhelpbygunjan.wordpress.com | Combustion and flame | Diagram | Showing non - luminous zone of flame is the hottest zone | NCERT | Class 8

(8) Goldsmith blows the outer most zone to melt gold and silver because combustion of unburnt fuel takes place (full combustion) and outermost part has highest temperature.

(9) Fuel is the source of heat energy for the domestic and industrial purposes.

Eg:- Wood, kerosene oil, coal etc.

(10) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.

The calorific value of a fuel is expressed in a unit called kilojoule per kg (kj/kg).

Calorific values of different fuels

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

(11) Burning of fuels leads harmful products

(i) Carbon fuels like wood, coal, petroleum releases unburnt carbon particles are responsible for respiratory diseases like asthma.

(ii) Incomplete combustion of these fuels produces carbon -mono- oxide gas which is very poisonous gas. Production of carbon mono oxide can kill a person sleeping in the room leaving coal on fire in a closed room.

(iii) Burning of fuels is a cause of global warming.  The most of fuels produce carbon – dioxide after burning which increases concentration of the co2 in the environment. It results in global warming.

(iv) Burning of coal and diesel releases Sulphur dioxide gas. It is an extremely suffocating and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rains are called acid rains. It is very harmful for crops, buildings and soil.

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