Heat and light (flame or glow)
There are three types of combustions:
a) Rapid combustion
b) Spontaneous combustion
The lowest temperature at which a combustible substance catches fire without the need for an external flame is called its ignition temperature.
For combustion to occur the following conditions are necessary:
a) Presence of a combustible substance
b) Presence of oxygen
c) Enough heat to raise the temperature of the combustible substance to its ignition temperature
The outermost zone or non-luminous zone is the hottest part of a flame. Due to this reason, goldsmiths use this zone for melting gold and silver.
Carbon dioxide being heavier than oxygen forms a blanket around the burning substance, thereby cutting off the oxygen supply from the burning material and extinguishing the fire.
Unlike dry leaves, green leaves contain water which absorbs heat and makes it more difficult for the ignition temperature to be reached.
The water conducts heat away from the paper cup. This heat raises the temperature of the water to its boiling point and only after all the water has been converted into vapour can the ignition temperature of the paper cup be reached.
In rusting, though heat is produced there is no flame or glow. Hence it cannot be considered as combustion.
Substances that burn in air to produce heat and light are called combustible substances. For example - wood, coal, kerosene, petrol, cardboard, straw etc.
Substances that do not burn in air are called non-combustible substances. For example - stone, cement, glass, iron nails etc.
The water in Rahul’s beaker will boil first because he has kept the beaker in the outermost zone which is the hottest part of the candle flame.
For a substance to keep burning, a constant supply of air or oxygen is required. A blanket wrapped around a burning object cuts off the supply of air or oxygen, thereby extinguishing the fire.
Carbon dioxide is the best fire extinguisher to put out fires caused by flammable materials such as petrol. Being heavier than air, it covers the fire cutting off the supply of oxygen and putting out the flames.
Kerosene oil is a highly flammable substance with a low ignition temperature. Due to this it easily catches fire on being heated and poses a fire risk.
When water is poured over a burning material, it absorbs heat from the material. As a result, the temperature of the burning material falls below its ignition temperature and put outs the fire.
When a candle is covered with a glass tumbler, the carbon dioxide formed descends on the candle flame and extinguishes it.
In general, any fire extinguisher uses any or all of the following methods.
a) Removing the combustible substance
b) Cooling the burning substance to below its ignition temperature.
c) Cutting off the supply of air/oxygen.
There are three different types of combustion as explained below:
a) Rapid Combustion: In this type of combustion, the combustible substance burns rapidly and produces both heat and light. This is called rapid combustion. For example, LPG burning on a kitchen stove.
b) Spontaneous Combustion: Spontaneous combustion occurs due to internal exothermic reactions such as fermentation, which bring the fuel to its ignition point. For example, stacks of hay or compost can self-ignite through heat produced by bacterial fermentation.
c) Explosion: When combustion takes place with a sudden release of heat, light, sound and a large, rapidly expanding volume of gas, it is called explosion. Examples are crackers and bombs.
Spontaneous combustion occurs due to internal exothermic reactions such as fermentation, which bring the fuel to its ignition point. For example, stacks of hay or compost can self-ignite through heat produced by bacterial fermentation.
In an explosion, the introduction of heat or pressure causes a substance to burn suddenly and expands with the generation of a large amount of heat, light and sound. For example, the explosion of crackers on applying heat or pressure. Spontaneous combustion can sometimes lead to an explosion.
(a) Water is not used to control fires involving electrical equipment because water may conduct electricity and harm those trying to put out the fire.
(b) Paper wrapped around an aluminium pipe does not catch fire on heating because it is unable to attain its ignition temperature due to the transfer of heat to the aluminium pipe, which is a good conductor of heat.
A substance does not catch fire until its temperature is raised to its ignition temperature. A matchstick does not catch fire on its own because its ignition temperature is higher than room temperature.
(a) The middle zone is luminous due to partial combustion of wax and unburnt carbon vapour. These unburnt carbon particles impart a pale-yellow colour to the flame.
(b) The innermost zone is black in colour and is the coldest zone because it contains wax that has been vaporized but is unburnt. No combustion takes place here as no oxygen is available.
(c) The outermost zone is called the zone of complete combustion. Here, carbon and hydrogen are completely oxidized to carbon dioxide and water vapour.
Complete combustion of a combustible substance gives rise to a blue coloured flame, whereas partial combustion of a combustible substance gives rise to unburnt carbon particles which glow with heat and impart a yellow colour to the flame.
During extremely hot summers, the high temperature sometimes reaches the ignition temperature of dry grass in the forest, due to which the dry grass catches fire. The fire from burning grass spreads to bushes and trees and very soon the whole forest is on fire.
No, it does not burn. This is because wood has a high ignition temperature which is not reached by the small amount of heat in a matchstick flame.
Coal has a high ignition temperature which cannot be reached by the small amount of heat in a matchstick flame. It needs a steady supply of heat from burning a combustible substance like paper or kerosene for some time to raise the coal temperature to its ignition point.
Kerosene oil is a liquid that, upon being heated, vapourises and the burning of the vapours produce a flame; charcoal is a solid that does not vapourise on being heated and so does not produce a flame. It only glows during combustion.
A substance that is burnt to produce heat is called a fuel. Fuels can be in the form of solids, liquids or gases. We use fuels in our daily lives for various applications where heat is needed. Examples of fuels are:
Solid fuels: Wood, coal, charcoal
Liquid fuels: Petrol, diesel, kerosene
Gaseous fuels: Natural gas, biogas, coal gas
The calorific value of a fuel is expressed in kilo joule per kg (kJ/kg).
Sulphur dioxide and nitrogen dioxide are the gases that dissolve in rain water to form acid rain.
Global warming leads to melting of polar glaciers, rise in sea level that can cause coastal flooding and changes in weather patterns.
The amount of heat energy produced on complete combustion of 1 kg of a fuel in the presence of oxygen is called its calorific value. The calorific value of a fuel is expressed in kilojoule per kg (kJ/kg).
A calorific value of 17000 kJ/kg -22000kJ/kg means that when 1kg of wood is burnt completely in the presence of oxygen, it produces 17000-22000 kJ of heat energy.
The calorific value of a fuel is the amount of heat energy produced on complete combustion of 1 kg of a fuel. The calorific value of a fuel is expressed in kilo joule per kg.
Thus, the calorific value of the fuel
The burning of coal and diesel releases sulphur dioxide gas which is a suffocating and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen as well. When these oxides of nitrogen and sulphur dissolve in rain water, the rain water becomes acidic. Such rain is called acid rain. It is very harmful for crops, buildings, aquatic animals and even to human skin.
LPG is a better domestic fuel than wood due to the following reasons.
(a) LPG has a higher calorific value than wood. This means that LPG produces more heat on burning than an equal mass of wood.
(b) LPG burns without producing any smoke or leaving any solid residue. But when wood is burnt it produces a lot of smoke and leaves behind a solid residue called ash.
Petrol and diesel produce a lot of unburnt carbon particles and emit carbon dioxide and oxides of nitrogen and sulphur upon burning. These gases are poisonous and cause environmental hazards such as global warming and acid rain. In comparison, CNG is a clean fuel. It produces very small quantities of these harmful products. Since the burning of CNG does not cause air pollution, it is considered a better fuel than petrol and diesel.
Total mass of fuel burnt = 2.5 kg
Heat produced by burning the given mass of fuel = 120,000 kJ
We know that calorific value of a fuel
= Heat produced by burning the given mass of fuel/ Total mass of fuel burnt
= 120,000 kJ/2.5 kg
= 48,000 kJ/kg
Hence, the calorific value of the given fuel = 48,000 kJ/kg
The burning of fuel causes the following harmful effects on the environment:
(i) The burning of fuels like wood, coal and petroleum releases unburnt carbon particles that cause respiratory diseases like asthma.
(ii) Incomplete combustion of fuels releases carbon monoxide gas. This is a highly poisonous gas that can even be lethal to humans if inhaled in large quantities.
(iii) Burning of most fuels release carbon dioxide gas. Excess of carbon dioxide in the atmosphere causes trapping of heat, leading to a rise leading to a rise in the earth’s temperature. This is called global warming. It causes melting of polar ice, rise in sea level and consequent flooding of coastal areas.
(iv) The burning of coal and diesel releases sulphur dioxide gas. Moreover, petrol engines give off gaseous oxides of nitrogen as well. When these oxides of nitrogen and sulphur dissolve in rain water, the rain water becomes acidic. This is called acid rain. It is very harmful for crops, buildings, aquatic animals and also our skin.
Normally, carbon dioxide, which is a molecule with one atom of carbon bonded to two oxygen atoms, is formed from the combustion of coal. But with the limited oxygen in closed room, another oxide of carbon known as carbon monoxide begins to be formed; the carbon atoms in whose molecules are each bonded to a single oxygen atom. Carbon monoxide is a colourless, odourless, highly toxic gas which can cause tissue damage and even death if inhaled in large quantities. Because of the dangerous possibility, it is not advisable to burn coal in a closed room.
Petrol has a low ignition temperature and catches fire easily. Therefore, it is known as inflammable substance.
Petrol is not used as a domestic fuel because it has a very low ignition temperature. Due to this a can full of petrol can catch fire very easily on being lighted with a matchstick and can burn explosively, thereby increasing the chances of accidental fires in the household.
Among the three fuels, LPG is the most efficient fuel and wood is the least efficient one. LPG has the highest calorific value compared to the other fuels. The higher the calorific value the higher is the fuel’s efficiency. Therefore, LPG should be preferred to boil water faster.