Combustion And Flame

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.

Petrol has a lower ignition temperature. Kerosene molecules are larger than petrol molecules and hence don’t mix easily with oxygen present in air. Thus, it has higher ignition temperature than petrol.

There are mainly five types of fire extinguisher – Water, Foam, Dry Powder, CO2 and Wet Chemical.Water is one of the best, cheapest and most common fire extinguishers.

1) Water fire extinguisher

2) Foam fire extinguisher

3) Dry Powder fire extinguisher

4) CO₂ fire extinguisher and,

5) Wet Chemical fire extinguisher.Water is one of the best, cheapest and most common fire extinguishers.

Out of all these, the Water fire extinguisher is one of the best, cheapest and most common fire extinguishers.

Carbon dioxide does not support combustion and hence is considered as the best fire extinguisher for fires involving electrical equipment and inflammable materials like petrol.

When white phosphorus is left out open in the room temperature for sometime, it burns all by itself.

It is measured in units of energy per unit of the mass of the substance, such as: kJ/kg, J/Kg.

Coal has the lowest calorific value. Coal contains moisture. When coal burns the moisture in coal evaporates taking away some heat of combustion which is not available for our use.

CNG and methane have the highest calorific value. Both of them have calorific value of 50000 KJ/Kg.

Calorific Value is the term which is used to express the efficiency of a fuel.

Molten wax(Solid), Kerosene oil(Liquid) and LPG(gas) are the substances which burn by producing flames.

Charcoal does not vaporize and so does not produce a flame.

Charcoal does not vaporize and so does not produce a flame.

A flame consist of three zones. These are Innermost zone, middle zone, outer zone. The three zones of a flame have different colors and different temperature.

The outermost zone has the highest temperature in the flame. It is the hottest part of the flame. It is quite thin as compared to middle zone.

(a) innermost zone- The Innermost zone of a flame is dark or black

(b) middle zone- The middle zone of a flame is yellow

(c) outer zone- The outer zone of a flame is blue. It is a non-luminous zone

Incomplete combustion of fuels produces a very poisonous gas called carbon monoxide.

Incomplete combustion of fuels produces a very poisonous gas called carbon monoxide.

The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces the harmful products in very small amounts. CNG is a cleaner fuel.

LPG and petrol catch fire at very low temperature and thus have low ignition temperature, while wood and coal have high ignition temperature.

(a) ignition temperature

(b) air

(c) water

(d) kerosene

(e) calorific value

(f) flames

(g) pollution

A fuel is a very good source of heat. The heat energy produced by burning a fuel can be used directly to cook food, for running motor vehicles and factory machines, can be converted into electrical energy at thermal power stations. Wood and Petrol are examples of two common fuels.

Characteristics of ideal fuel-

1) It has a high calorific value

2) It burns easily in air at a moderate rate

3) It has proper ignition temperature

4) It does not produce any harmful gases or leaves any residue after burning.

The amount of heat produced by the complete burning or combustion of 1Kilogram of a fuel is called its calorific value. The calorific value of a fuel is expressed in the unit of Kilojoules per kilogram.

The calorific value of LPG is 55000KJ/Kg. It means when 1Kg of LPG is burned completely, then 55000 KJ of heat energy is produced.

A matchstick can light a tiny splinter of wood but not a big log of wood. A splinter of wood has a low ignition temperature. A burning matchstick can produce sufficient heat to reach the ignition temperature of the splinter of wood therefore a matchstick can light a splinter of wood directly.

The ignition temperature of wood or coal is very high, as it requires too much of time to get heated before burning. We use paper or kerosene to start fire because they have low ignition temperature which helps to catch fire immediately and helps the wood or the coal to reach its required ignition temperature.

Rapid combustion is the combustion in which a substance burns rapidly with the help of an external source and produces heat within a very short time. Eg:- Burning of LPG.

Spontaneous combustion is the combustion in which a substance burns spontaneously and produces heat and light without the help of external source of heat. Eg:- phosphorus burns spontaneously at room temperature.

Explosive combustion is the combustion in which a substance burns suddenly and produces heat, light and sound with the help of external source of heat or pressure. Eg:- explosion of crackers on applying heat or pressure.

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.

Combustion is a chemical process in which a substance reacts with oxygen and gives out energy during the process in the form of either heat or light or both. Rusting of iron is an exothermic process as heat is released during rusting. Hence, it is a kind of slow combustion.

As water is heavier than petrol therefore slips down permitting the petrol to rise to the surface and continue to burn. Besides, the existing temperature is so high that the water poured on the fire evaporates even before it can extinguisher the fire.

Water is conductive in nature and hence the electricity in the equipment could reach the extinguisher (the person dousing the fire using water) and can electrocute him/her.

To put an electrical fire out, we need to unplug or de-energise the equipment on fire. That will eliminate the electricity that is causing the fire. If one cannot unplug what is burning there is a special type of fire extinguisher.

Carbon dioxide is the best fire extinguisher to put out fire caused by inflammable materials like oil and petrol and electrical equipments. Carbon dioxide is heavier than air and it covers the fire and cuts off the supply of oxygen and puts out the fire. Carbon dioxide is stored at high pressure as liquid in cylinders.

A small fire like a drum of kerosene on fire can be extinguished by throwing sand or soil over it. When sand is thrown over burning kerosene oil, the sand covers it like blanket. The sand cuts off the air supply to the burning kerosene oil due to which the fire gets extinguished.

Water extinguishes heat by cooling down the burning substance such as wood and paper. When water is thrown on these substances, it gets cooled below its ignition temperature and also stops burning. . The water vapour produced by the action of heat of fire on water surround the burning material and help in cutting off the supply of air.

When fire brigade arrives to the rescue, the fire man throws a strong stream of water on the building on fire, the burning material get cooled down to below their ignition temperature and fire is extinguished. The water vapour produced by the action of heat of fire on water surround the burning material and help in cutting off the supply of air. Fire-men extinguishes the fire by throwing water under pressure on the burning things.

Water extinguishes heat by cooling down the burning substance such as wood and paper. When water is thrown on these substances, it gets cooled below its ignition temperature and also stops burning. . The water vapour produced by the action of heat of fire on water surround the burning material and help in cutting off the supply of air.

Due to the burning of coal, the available oxygen gets depleted and it leads to incomplete burning of coal. Incomplete combustion of coal gives carbon monoxide gas. It is a very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that closed room

Flame is the zone of combustion of a combustible substance. Substances which vapourise during burning produce flames. Eg:- kerosene, wax etc.

Substances which vapourise during burning, produce flames, such as wax in the candle, Kerosene.

Substances which do not vapourise during burning, do not produce flames. Example- coal and charcoal.

Water extinguishes heat by cooling down the burning substance. When water is thrown on substances, it gets cooled below its ignition temperature and also stops burning. The water vapour produced by the action of heat of fire on water surround the burning material and help in cutting off the supply of air.

Carbon dioxide is the best fire extinguisher to put out fire caused by inflammable materials like oil and petrol and electrical equipments. Carbon dioxide is heavier than air and it covers the fire and cuts off the supply of oxygen and puts out the fire. Carbon dioxide is stored at high pressure as liquid in cylinders. When released from the cylinder, carbon dioxide expands enormously in volume and cools down. Carbon dioxide not only forms a blanket around the burning substance, it also cools down the burning substance.

If the clothes of a person catch fire, the person should be immediately covered with a blanket. When the burning clothes of a person are covered with a blanket, the supply of air to the burning clothes is cut off and hence the burning stops.

a) Solid Fuels: Wood, charcoal, coal, coke, cow-dung cakes.

b) Liquid Fuels: Kerosene, petrol, diesel, alcohol.

c) Gaseous fuels: Coal gas, natural gas

A candle flame has three main zones. They are:-

i) Outer zone ii) Middle zone iii) Inner zone

In the inner zone there are no burnt vapours of the fuel due to which the colour is black and is least hot(or coldest) part.

A goldsmith blows air into the kerosene lamp flame with a blow pipe so that complete combustion of fuel takes place and the temperature of the flame increases. He blow the outermost zone of a flame with a metallic blow pipe for melting gold and silver because this is the hottest part of the flame.

(a) In the middle zone partial combustion of the fuel takes place, so the colour of the flame is yellow and it is moderately hot part of the flame. It is the luminous part of the flame.

(b) In the outer zone complete combustion of the fuel takes place, so colour of the flame is blue and it is the hottest part of the flame. It is the non luminous part of the flame.

The use of petrol and diesel as fuels in automobiles is being replaced by CNG because

1) It produces very small amount of harmful gases.

2) It burns without smoke so it is a clean fuel.

Disadvantages of burning wood as fuel are as follow:

1) Burning of wood produces a lot of smoke which is very harmful for human being. It causes respiratory diseases such as bronchitis and asthma.

2) When trees are cut down to obtain wood for use as fuel, then all the useful substances which are obtained from trees are lost.

3) Deforestation is very harmful for environment.

LPG is a better domestic fuel than wood due to:

1) LPG has higher calorific value, so it produces much more heat on burning than an equal mass of wood.

2) It burns without leaving behind any smoke.

3) It burns completely without leaving behind any solid residue but wood leaves behind a lot of ash on burning.

When a burning candle is covered with an inverted gas jar, the oxygen supply gets cut-off. Oxygen is necessary for combustion to take place. It is a supporter of combustion. Hence the candle gets extinguished after sometime.

It is difficult to burn a heap of green leaves but dry leaves catch fire easily. The green leaves contain a lot of water. This water does not allow the green leaves to get heated to their ignition temperature and makes burning of green leaves difficult. Dry leaves do not contain water, they get heated to their ignition temperature easily and hence catches fire easily.

Those substances which can burn are called combustible substances. For Ex: Cloth, straw, cooking gas, kerosene oil, wood, coal, charcoal etc. A combustible substance is called as fuel.

Those substances which do not burn are called non-combustible substances. For Ex: Stone, cement, glass, bricks, soil, sand, water, iron nails, copper objects.

A chemical process in which a substance reacts with the oxygen to give heat and light is called combustion. The light which is given off during combustion can be in the form of flame or as a glow. The substance which undergoes combustion is said to be combustible. It is called fuel.

There are three conditions which are necessary for combustion to take place:

1) Presence of combustible substance.

2) Presence of supporter of combustion.

3) Heating the combustible substance to its ignition temperature.

In the middle zone, partial combustion of the fuel takes place, so the colour of the flame is yellow and it is moderately hot part of the flame. It is the luminous part of the flame.
It is given in the following picture:

Global warming is the rise in temperature of earth atmosphere caused by the excessive amount of carbon dioxide in the air. Due to rise in the temperature of atmosphere, the ice in polar regions will melt very fast, producing a lot of water. Burning of fuels releases carbon dioxide in air in the environment. Increased percentage of carbon dioxide in air is causing global warming. Carbon dioxide gas in the air trap sun’s heat rays by producing green house effect. Rise in water may cause a rise in the sea level leading to floods in coastal areas. The low-lying areas may be completely submerged under water leading to loss of life and property.

Burning of coal, petrol and diesel produces sulphur dioxide which goes into the air. It is an extremely suffocating and corrosive gas. It may damage our lungs. The burning of petrol and diesel in the engines of vehicles also releases nitrogen oxides into the air. These oxides produced by the burning of fuels dissolve in rain water and form acids. The rain water containing acid is called acid rain. It is harmful for crops, soil and damages buildings.

Spirit/petrol has the lowest ignition temperature and can catch fire easily.

Asbestos is a non combustible substance and cannot catch fire on its own.

White phosphorus undergoes spontaneous combustion. Hence it is not used in making matchsticks now days.

white phosphorus undergoes spontaneous combustion. It catches fire on its own at room temperature.

Carbon dioxide is lighter than air and forms a layer around the burning substance. It covers the fire like a blanket due to which fresh air cannot reach the burning substance.

fires in underground coalmines occur due to spontaneous combustion of coaldust.

Biogas has calorific value of 40000Kj/Kg.

Hydrogen gas has the highest calorific value.

Due to the burning of coal, the available oxygen gets depleted and it leads to incomplete burning of coal. Incomplete combustion of coal gives carbon monoxide gas. It is a very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that closed room

Coke does not vapourise during burning and hence do not produce flame.

CNG

Burning of fuels releases carbon dioxide in air in the environment. Increased percentage of carbon dioxide in air is causing global warming. Carbon dioxide gas in the air trap sun’s heat rays by producing green house effect.

Carbon monooide does not contribute to the formation of an acid rain.

The use of petrol and diesel as fuels in automobiles is being replaced by CNG because it produces very small amount of harmful gases.

production of heat and light from hydrogen in the sun is a nuclear reaction.

The green leaves contain a lot of water. This water does not allow the green leaves to get heated to their ignition temperature and makes burning of green leaves difficult.

If the clothes of a person working in the kitchen catch fire, the person is immediately covered with a blanket. When the burning clothes of a person are covered with a blanket, the supply of air to the burning clothes is cut off and hence the burning stops.

In the outer zone complete combustion of the fuel takes place and the colour of the flame is blue and is the hottest part of the flame.

It has three zones.

To obtain carbon dioxide for extinguishing a fire is to release a lot of dry powder of chemicals like sodium bicarbonate(baking soda) or potassium bicarbonate over the fire.

As it is a closed tank full of petrol, there will be no supply of air. Hence the petrol will not catch fire as air is necessary for the combustion of petrol.

A piece of paper wrapped around aluminium pipe does not catch fire easily. This is because aluminium, being a metal, is a good conductor of heat and hence heat is conducted away. Also, 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.

The yellow part of the flame is the middlemost part where Abida has kept the beaker. The fuel vapours burn partially in the middle zone because there is not enough air of burning in this zone. The partial burning of fuel in the middle zone produces carbon particles. It has moderate temperature. Hence, Abida’s beaker will take time to heat up. Ramesh’s beaker is kept in the outermost part of the flame. In this zone, complete combustion of the fuel takes place because there is plenty of air around it. The outermost zone has the highest temperature in the flame. It is the hottest part of the flame. Hence Ramesh’s beaker gets heated in a shorter time.

(a) Sodiumbicarbonate (sodiumhydrogencarbonate)

(b) The heat of fire decomposes sodium bicarbonate to produce carbondioxide gas. This carbon dioxide covers the fire like a blanket and cuts off supply of fresh air to the burning substance. Due to this the fire gets extinguished

(c) Potassium bicarbonate(or Potassium hydrogen carbonate)

(a) Burning of white phosphorus in air at room temperature is a spontaneous combustion. The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.

(b) Burning of LPG is combustion in which a substance burns rapidly and produces heat and light with the help of external heat.

(c) Ignition of a cracker is a combustion in which a substance burns suddenly and produces heat, light and sound with the help of heat or pressure.

(d) Burning of coal dust in a coalmine is a spontaneous combustion. The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.