NCERT Textbook Questions
The conditions necessary for combustion to take place are:
- Presence of a combustible substance (fuel).
- Presence of oxygen (supporter of combustion).
- Attainment of ignition temperature (heating the substance to its lowest temperature at which it catches fire).
(a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel, used in homes is LPG (Liquefied Petroleum Gas).
(c) Fuel must be heated to its ignition temperature before it starts burning.
(d) Fire produced by oil cannot be controlled by water.
Combustion of fuels like petrol and diesel produces large amounts of unburnt carbon particles, carbon monoxide, carbon dioxide, sulphur dioxide and nitrogen oxides. These pollutants cause respiratory diseases and environmental damage (like acid rain). CNG (Compressed Natural Gas) burns completely and leaves behind no harmful residue. It produces very small amounts of pollutants. Therefore, it is a much cleaner fuel and its use has significantly reduced pollution in cities.
| Parameter | LPG | Wood |
|---|---|---|
| State | Gaseous (Liquefied) | Solid |
| Calorific Value | High (55000 kJ/kg) | Low (17000-22000 kJ/kg) |
| Pollution | Pollution free (Smokeless) | High pollution (Smoke) |
| Storage | Easy (Cylinders) | Requires more space |
(a) Water is not used to control fires involving electrical
equipment.
Ordinary water conducts electricity. If used on an electrical fire, water may conduct electric
current and cause shock or electrocution to the person fighting the fire.
(b) LPG is a better domestic fuel than wood.
LPG has a much higher calorific value than wood, meaning it produces more heat. It burns without
smoke and does not leave any ash, whereas wood produces a lot of smoke which causes respiratory
problems and leaves ash behind.
(c) Paper by itself catches fire easily whereas a piece of paper
wrapped around an aluminium pipe does not.
Paper has a low ignition temperature and catches fire easily. However, when wrapped around an
aluminium pipe, the heat supplied to the paper is transferred to the aluminium pipe by conduction.
As a result, the ignition temperature of the paper is not reached, and it does not burn.
A candle flame has three main zones:
- Outer Zone (Blue): Zone of complete combustion. It is the hottest part.
- Middle Zone (Yellow): Zone of partial combustion. It is moderately hot and luminous.
- Innermost Zone (Black): Zone of unburnt wax vapours. It is the least hot part.
The calorific value of a fuel is expressed in the unit kilojoule per kilogram (kJ/kg).
Carbon dioxide (CO2) is heavier than oxygen. It covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. Also, in most cases, it does not harm electrical equipment.
Green leaves contain a lot of moisture (water). This water absorbs heat and prevents the leaves from reaching their ignition temperature, making them difficult to burn. Dry leaves have no moisture, so their ignition temperature is reached quickly, and they catch fire easily.
A goldsmith uses the outermost zone (blue zone) of a flame for melting gold and silver. This is because it is the hottest part of the flame and undergoes complete combustion, providing the high temperature needed to melt these metals.
Extra Important Questions
Burning of coal and diesel releases sulphur dioxide gas. Petrol engines give off oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.
Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air is believed to cause global warming. This results in the rise in temperature of the atmosphere of the earth, leading to melting of polar glaciers and rise in sea levels.
Detailed Chapter Discussion: Combustion and Flame
1. What is Combustion?
We use different kinds of fuel for various purposes at home, in industry and for running automobiles. Examples include cow dung, wood, coal, charcoal, petrol, diesel, CNG etc. When a candle burns, it produces a flame, whereas coal burns without a flame. Let us study the chemical process of burning.
Definition: A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The substance that undergoes combustion is said to be combustible. It is also called a fuel. The fuel may be solid, liquid or gas. Sometimes, light is also given off during combustion, either as a flame or as a glow.
Combustible substances include wood, paper, kerosene oil, straw, charcoal, matchsticks, etc. Non-combustible substances include stone, glass, iron nails, etc. Oxygen (in air) is essential for combustion to take place. If air supply is cut off, combustion stops.
2. Ignition Temperature
Have you ever seen a matchstick burning by itself? No, we have to rub it on the side of the matchbox. Does a piece of paper burn when a burning matchstick is brought near it? Yes. Can you burn a piece of wood by bringing a lighted matchstick near it? No, we have to use paper or kerosene oil to start fire in wood. This shows that different substances catch fire at different temperatures.
Ignition Temperature: The lowest temperature at which a substance catches fire is called its ignition temperature. A combustible substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature.
Kerosene oil and wood do not catch fire on their own at room temperature. But if kerosene oil is heated a little, it catches fire. However, if wood is heated a little, it would still not catch fire. It means that ignition temperature of kerosene oil is lower than that of wood.
3. Inflammable Substances
The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Examples of inflammable substances are petrol, alcohol, Liquified Petroleum Gas (LPG), etc. These must be handled with great care.
4. How Do We Control Fire?
We often see fires in homes, shops or factories. Firemen extinguish fire by throwing water on it. Water cools the combustible material so that its temperature is brought below its ignition temperature. This prevents the fire from spreading. Water vapours also surround the combustible material, helping in cutting off the supply of air. So, the fire is extinguished.
Three Essential Requirements for Fire
- Fuel (combustible substance)
- Air (to supply oxygen)
- Heat (to raise the temperature of the fuel beyond the ignition temperature)
Fire can be controlled by removing one or more of these requirements. The job of a fire extinguisher is to cut off the supply of air, or to bring down the temperature of the fuel, or both.
5. Types of Fire Extinguishers
Water: It is the most common fire extinguisher. However, it works only when things like wood and paper are on fire. If electrical equipment is on fire, water may conduct electricity and harm those trying to douse the fire. Water is also not suitable for fires involving oil and petrol because water is heavier than oil, so it sinks below the oil, and oil keeps burning on top.
Carbon Dioxide (CO2): For fires involving electrical equipment and inflammable materials like petrol, CO2 is the best extinguisher. CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between fuel and oxygen is cut off, the fire is controlled. Also, it does not harm electrical equipment. It can be stored at high pressure as a liquid in cylinders.
6. Types of Combustion
- Rapid Combustion: When a substance burns rapidly and produces heat and light. Example: Gas burns rapidly when a burning matchstick is brought near a gas burner. Spontaneous combustion of phosphorus is another example.
- Spontaneous Combustion: The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause. Example: Coal dust in coal mines, Forest fires due to heat of the sun or lightning.
- Explosion: A sudden reaction that takes place with the evolution of heat, light and sound. A large amount of gas is liberated in the reaction. Example: Firecrackers.
7. Flame
Observe an LPG flame. Can you tell the colour of the flame? It is blue. What about a candle flame? It has different colours. Not all substances produce a flame during combustion. Only those substances which vaporise during burning give flames. For example, kerosene oil and molten wax rise through the wick and are vaporised during burning and form flames. Charcoal, on the other hand, does not vaporise and so does not produce a flame.
8. Structure of a Flame
A candle flame can be divided into three main zones:
| Zone | Colour | Combustion | Temperature |
|---|---|---|---|
| Outermost Zone | Blue | Complete combustion | Hottest part |
| Middle Zone | Yellow | Partial combustion | Moderately hot |
| Innermost Zone | Black | Unburnt wax vapours | Least hot |
Goldsmiths: They blow the outermost zone of a flame with a metallic blow-pipe for melting gold and silver, as it is the hottest part of the flame.
9. What is a Fuel?
The sources of heat energy for domestic and industrial purposes are mainly wood, charcoal, petrol, kerosene etc. These substances are called fuels. A good fuel is one which is readily available. It is cheap. It burns easily in air at a moderate rate. It produces a large amount of heat. It does not leave behind any undesirable substances.
There is probably no fuel that could be considered as an ideal fuel. We should look for a fuel which fulfills most of the requirements for a particular use.
10. Fuel Efficiency
Suppose you were asked to boil a given quantity of water using cow dung, coal and LPG as fuel. Which fuel would you prefer? You would choose LPG because it produces more heat.
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 Some Fuels
- Cow dung cake: 6000-8000 kJ/kg
- Wood: 17000-22000 kJ/kg
- Coal: 25000-33000 kJ/kg
- Petrol: 45000 kJ/kg
- LPG: 55000 kJ/kg
- Hydrogen: 150000 kJ/kg
11. Harmful Effects of Burning Fuels
The increasing fuel consumption has harmful effects on the environment:
- Carbon Fuels: Wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.
- Incomplete Combustion: Incomplete combustion of these fuels 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 room.
- Global Warming: Combustion of most fuels releases carbon dioxide in the environment. Deforestation (cutting of trees) leads to less consumption of CO2 by plants, further increasing its level in the atmosphere. High concentration of CO2 contributes to global warming.
- Acid Rain: Burning of coal and diesel releases sulphur dioxide gas. It is extremely suffocating and corrosive. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.
Solution: 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.
50 Key Facts: Combustion and Flame
Combustion is a chemical process where a substance reacts with oxygen to give off heat and light. The substance undergoing combustion is called a fuel or combustible substance.
Substances that burn in air to produce heat and light are called combustible. Examples include wood, paper, coal, petrol, diesel, and kerosene.
Substances that do not burn in air are called non-combustible substances. Examples include stone, glass, iron nails, water, and sand.
Oxygen is essential for combustion. Without a continuous supply of oxygen (usually from air), a fire cannot sustain itself and will eventually go out.
The lowest temperature at which a substance catches fire is called its ignition temperature. A substance will not burn unless its temperature is raised to or above this point.
Substances with very low ignition temperatures that can catch fire easily with a spark or flame are called inflammable substances. Examples are LPG, petrol, and alcohol.
Three things are needed for a fire: fuel, oxygen (air), and heat (to reach ignition temperature). Removing any one of these can extinguish the fire.
Rapid combustion occurs when a substance burns quickly, producing significant heat and light. Burning of domestic gas (LPG) in a kitchen stove is an example.
Spontaneous combustion is when a substance bursts into flames without any apparent external cause (like a spark). Phosphorus burns spontaneously at room temperature.
An explosion is a combustion reaction that happens suddenly with the evolution of heat, light, and sound, and a large amount of gas. Firecrackers are a common example.
Only substances that vaporize during burning produce a flame. Wax and kerosene vaporize and form flames, while charcoal does not vaporize and only glows.
A candle flame has three zones: the innermost zone (dark, no combustion), the middle zone (yellow, partial combustion), and the outer zone (blue, complete combustion).
The outermost zone (blue zone) is the hottest part of the candle flame because complete combustion occurs there due to ample oxygen supply.
The innermost zone (black zone) near the wick is the least hot part. It consists of unburnt wax vapours because oxygen cannot reach this area effectively.
The middle zone is yellow and responsible for most of the light emitted by a candle. Carbon particles glow here due to partial combustion.
The amount of heat energy produced on complete combustion of 1 kilogram of a fuel is called its calorific value. It represents the efficiency of the fuel.
Currently, the standard unit for expressing calorific value is kilojoule per kilogram (kJ/kg). Higher calorific value means more energy per unit mass.
Burning fossil fuels releases Carbon Dioxide (CO2). High levels of CO2 in the atmosphere trap heat, leading to an increase in Earth's temperature, known as global warming.
Oxides of sulphur and nitrogen produced by burning coal and diesel dissolve in rainwater to form acids. This acid rain damages crops, soil, and historical monuments.
Incomplete combustion of fuels produces Carbon Monoxide (CO), a poisonous gas. It reduces the oxygen-carrying capacity of blood and can be fatal if inhaled in closed spaces.
An ideal fuel should be cheap, readily available, burn easily at a moderate rate, have a high calorific value, and produce no harmful pollutants. (No fuel is perfectly ideal).
Fire extinguishers work by cutting off the air supply (smothering) or cooling the fuel below its ignition temperature, or both.
Water cools combustible materials below their ignition temperature. It is effective for wood and paper fires but dangerous for electrical and oil fires.
Carbon dioxide is heavier than oxygen and forms a blanket around the fire, cutting off oxygen supply. It is ideal for electrical and oil/petrol fires as it doesn't conduct electricity or damage equipment.
Modern safety match heads contain antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and red phosphorus. Friction converts red phosphorus to white phosphorus, igniting the head.
During extreme heat, dry grass in forests can catch fire spontaneously. This can spread rapidly to trees, causing uncontrollable forest fires that destroy large areas.
Respiration is a slow biological combustion process where food reacts with oxygen in the body to release energy. It differs from rapid burning as it happens at body temperature without flames.
Water can be boiled in a paper cup without burning the paper. The heat supplied is transferred to water by conduction, keeping the paper below its ignition temperature.
This older type of extinguisher contained sodium bicarbonate and sulphuric acid. When activated, they react to produce carbon dioxide gas which extinguishes the fire.
Wrapping a person whose clothes have caught fire in a thick woollen blanket cuts off the air supply, helping to extinguish the flames.
Hydrogen has the highest calorific value (150,000 kJ/kg) among common fuels. However, it is difficult to store and transport safely due to its explosive nature.
CNG (Compressed Natural Gas) is a cleaner fuel than petrol or diesel because it burns completely and produces very few pollutants, helping to reduce city pollution.
Using wood as fuel leads to deforestation. This has double negative effects: loss of trees (which absorb CO2) and release of CO2 and smoke when wood is burnt.
When fuel burns with insufficient oxygen (incomplete combustion), it produces soot (unburnt carbon particles) and carbon monoxide, instead of just carbon dioxide and water.
Goldsmiths use a metallic blowpipe to direct the outermost (non-luminous) zone of the flame onto gold/silver. This zone is the hottest and ensures quick melting.
Examples of solid fuels include wood, coal, charcoal, and cow dung cakes. They generally leave ash residue after burning.
Examples of liquid fuels include kerosene, petrol, and diesel. They are generally more volatile and easier to transport than solid fuels.
Examples of gaseous fuels include Natural Gas (CNG), Biogas, and LPG. They burn cleanly and can be transported via pipelines.
The first matches were made of pine wood dipped in sulphur in ancient Egypt. The modern safety match was developed much later to be safe for use.
White phosphorus is dangerous because it ignites spontaneously in air at around 35 degrees Celsius. It was used in early matches but discontinued due to toxicity.
Potassium chlorate is an oxidizer used in safety match heads. It provides oxygen to help burn the antimony trisulphide when ignited by the friction heat.
Combustion releases chemical potential energy stored in the bonds of fuel molecules as heat and light energy.
Soot consists of fine, unburnt carbon particles. It is black and powdery and is produced when carbon-based fuels don't burn completely.
Only gaseous fuels or solid/liquid fuels that vaporize form flames. Charcoal burns as a solid and glows but produces no flame.
In a candle or oil lamp, the liquid fuel is drawn up the wick by capillary action. The height of the wick determines the size of the flame.
Burning fuels releases microscopic particles into the air known as SPM. These can reduce visibility (smog) and cause lung damage.
Magnesium burns to form magnesium oxide and produces heat and light. This is a combustion reaction similar to burning charcoal.
In the sun, heat and light are produced by nuclear reactions, not by ordinary chemical combustion. It is a different process entirely.
The fire triangle is a simple model for understanding the ingredients necessary for most fires: Heat, Fuel, and Oxygen.
Pouring water on an oil fire is dangerous because water sinks, boils instantly, and can splash burning oil, spreading the fire. Use sand or a fire blanket instead.