COMBUSTION AND FLAME
NOTES
Combustion
- A chemical process in which a substance reacts with oxygen to give off heat and light is called combustion.
- The burning of wood is an example of combustion.
Combustible and Non-Combustible Substances
- Substances which easily catch fire are combustible substances, such as paper, coal, wood.
- Substances which do not catch fire readily are non-combustible substances, such as sand, water, glass.
Types of Combustion
Let us now study the types of such reactions.
- Complete combustion: When the reaction takes place in the presence of abundant Oxygen, the substances combine with Oxygen to their maximum extent. Such reactions have heat and light as a visible by-product.
- Incomplete combustion: These are defined as the reactions that occur in the absence of sufficient oxygen because of which substances are unable to burn completely. Such reactions leave Soot in the container due to this process along with the formation of Carbon monoxide which is an air pollutant
- .Why is combustion so important?
Combustion reactions are a very important class of chemical reactions. Such responses are vital to our daily lives. The combustion reaction happens as the fuel and oxygen react, creating fire or heat and light. Combustion happens as the gasoline, most commonly a fossil fuel, reacts with the oxygen in the air to create heat. The heat produced by the combustion of fossil fuels is used in the operation of machinery such as boilers, furnaces, ovens and engines.
History of Wood and Candle Flame
Fuel
- Any substance, which upon combustion produces a usable amount of energy is known as fuel. For example, fossil fuels, biogas, nuclear energy, etc.
- Fuels can be solid, liquid or gas depending on their state.
- On the basis of their occurrence, it can be either natural or artificial.
Ignition Temp
The lowest temperature at which a combustible substance catches fire when heated in air is called its ignition temperature.
Inflammable Substances
The substances, which have very low ignition temperature and can easily catch fire with a flame are known as inflammable substances. Examples: diesel, LPG, acetone.
Fire
- Fire is the result of a chemical combustion reaction between oxygen and some sort of fuel.
- How long a fire lasts depends on how much fuel and oxygen are available.
Candle Flame
Fire Triangle
For the generation of fire, we need three things to be present simultaneously:-
- Some sort of fuel or combustible material.
- A heat source to raise the temperature of the fuel to its ignition temperature.
- Enough oxygen to sustain combustion. So, if we remove any one of these resources, the fire can be controlled.
Flame
- Flame is the visible and gaseous part of the fire.
- What we see as the flame is the light energy released due to the combustion of fuel.
Structure of Flame
- The outermost zone is the hottest among all zones and is blue in colour and this is due to complete combustion. It is the non-luminous part of the flame.
- The middle zone of the candle flame is moderately hot and is yellow in colour, and partial combustion of fuel takes place. It is the bright part of the flame.
- The innermost zone of the flame is the least hot and is black in colour. This is due to the presence of unburnt wax vapours.
Smoke
- Smoke is an example of solid (unburnt particles) dispersed in a gas (air).
- The black colour of smoke is due to the presence of unburnt carbon particles in the smoke.
Matchstick
Types of Combustion
- The type of combustion in which heat and light are released in a very short span of time is called rapid combustion. For e.g. combustion of L.P.G.
- The type of combustion in which substances catch fire on their own, without the application of heat is termed as spontaneous combustion. For e.g. forest fires.
To know more about Types of Combustion, visit here.
Working of a Matchstick
- The main component of the bulb of a matchstick is red phosphorus which turns into white phosphorus on heating.
- White phosphorus spontaneously ignites, thereby increasing the temperature of the wooden stem to the ignition point and the matchstick starts burning.
Calorific Value
Ideal Fuel
- The ideal fuel is cheap, easily available and readily combustible.
- It has high calorific value.
- It does not produce harmful gases or residues that pollute the environment.
Calorific Value and Efficiency of a Fuel
- 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).
- Efficiency is that proportion of energy released by a fuel combustion process which is converted into useful work.
- Calorific value is directly proportional to its efficiency. If the value is high, it’s efficiency will also be high. If the value is low, it’s efficiency would also be low.
Pollution
Harmful Products from Burning of Fuel
- The burning of fuels like wood, coal and petroleum products releases unburnt carbon particles in the air which causes respiratory problems.
- Incomplete combustion of fuels produces a very poisonous gas called carbon monoxide.
- The burning of fuels releases carbon dioxide in air in the environment which causes global warming.
Unburnt Carbon Particles
- Carbon fuels like wood, coal, candle, petroleum release unburnt carbon particles.
- These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.
CO Emission
- Carbon monoxide is a poisonous gas, which is produced by incomplete combustion of fuels.
- It is dangerous to burn coal in a closed room as the carbon monoxide produced can kill people sleeping in that room.
Global Warming
- The rise in the average temperature of the earth’s atmosphere due to the release of carbon dioxide on combustion of fuels is termed as global warming.
- Melting of polar ice-caps or change in the rainfall pattern are the consequences of global warming.
Acid Rain
- Acid rains are caused by emissions of sulphur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acid.
- It has a very harmful effect on plants, land and aquatic animals and infrastructure.
Fire Control
Fire can be controlled by removing any or all of the factors of combustion, i.e. fuel, oxygen (air) and ignition temperature (by lowering the temperature).
Fire Extinguisher
- The fire extinguisher is a device used by the fire brigade to control fire.
- The role of the fire extinguishers is to cut off the supply of oxygen or bring down the temperature of the fuel or both.
- NCERT EXCERCISES
1. List conditions under which combustion can take place.
ANSWER-
The burning of a substance in the presence of oxygen is defined as combustion.
The conditions under which combustion can take place are
- The presence of air or oxygen.
- The presence of fuel plays a significant role.
- Ignition temperature is maintained (It is defined as the substance that catches fire at its lowest temperature.)
- 2. Fill in the blanks.
(a) Burning of wood and coal causes Pollution of air.
(b) A liquid fuel, used in homes is Kerosene.
(c) Fuel must be heated to its Ignition Temperature before it starts burning.
(d) The fire produced by oil cannot be controlled by Water
3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Soln:
CNG played an important role in reducing pollution among automobiles for the following reasons
- CNG is comparatively a cleaner fuel.
- The CNG can be an alternative for diesel, petrol and propane/LPG.
- It usually contains a few undesirable gases than the other fuels mentioned above.
- The combustion of fuels like petroleum causes many unburnt carbon particles along with carbon monoxide, which leads to respiratory diseases.
4. Compare LPG and wood as fuels
Soln:
Wood
- It is considered as a traditional fuel used for both domestic and industrial purposes.
- Wood produces a lot of smoke which pollutes the atmosphere and cause respiratory diseases.
- The usage of wood to a large extent causes deforestation.
- The calorific value of wood ranges between 17000 to 22000 kJ/kg
- However, wood may be used as a furnace, stove or fireplace indoors while it is used for a campfire, furnace outdoors.
LPG
- The usage LPG (Liquefied petroleum gas) has replaced wood.
- It doesn’t release smoke and other pollutants.
- It is a cleaner fuel.
- The fuel efficiency of LPG is more than that of wood.
- The calorific value of LPG is 55000 kJ/kg.
- Hence, LPG is the most preferred choice.
5. Give reasons.
(a) Water is not used to control fires 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 aluminium pipe does not.
Soln:
a)
- Water is a good conductor of electricity.
- If added to an electrical fire, the water would just spread the electricity further.
- The person dousing the fire might get an electric shock
b)
- LPG being a cleaner fuel than wood doesn’t release smoke and other pollutants.
- Wood, on the other hand, releases a lot of smoke and fumes polluting the atmosphere causing pollution and leading to respiratory diseases.
- Hence, LPG is a better domestic fuel than wood.
c)
- The paper by itself catches fire easily because of its low ignition temperature.
- The piece of paper wrapped around an aluminium pipe doesn’t catch fire because aluminium is a good conductor of electricity.
- While the paper wrapped around an aluminium pipe results in an increase in ignition temperature. So, there is a transfer of heat from paper to the aluminium pipe. Hence it doesn’t catch fire.
- 6. Make a labelled diagram of a candle flame.
7. Name the unit in which the calorific value of a fuel is expressed.
Soln:
Calorific value is defined as the energy contained in the fuel. It is expressed in the form kJ/kg
kJ=kilo joules and kg=kilogram
8. Explain how CO2 is able to control fires.
Soln:
CO2 is a non-combustible gas and extinguishes fire in two ways:
(i) It is heavier than oxygen and it covers the fire like a blanket and cuts off the contact between oxygen and fuel.
(ii) In cylinders, CO2 is kept in the liquid form. When released, it expands enormously. This brings down the temperature of the fuel, which helps in controlling the fire.
9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Soln:
A heap of green leaves contains a lot of moisture in it, hence its ignition temperature is high. Therefore, it does not catch fire easily.
But dry leaves have no moisture content in it, hence its ignition temperature is low. Therefore, it catches fire easily.
10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Soln:
The goldsmith mainly uses non-luminous flame which is termed to be the outermost part of the flame. This part of the flame is used because the outermost flame undergoes complete combustion and is considered as the hottest part of the flame.
11. In an experiment, 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.
12. Can the process of rusting be called combustion? Discuss.
Soln:
No, because rusting is an exothermic process as heat is liberated during rusting. On the other hand, combustion is a chemical process in which a substance reacts with oxygen to release energy in the form of heat or light.
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?
Soln:
The water placed in the outermost part of the flame will be heated in a short time since it is a non-luminous flame and is regarded as the hottest part of the flame. So Ramesh’s beaker will be heated first. However, Abida who placed the beaker in the luminous flame (yellow flame) is comparatively less hot.
No comments:
Post a Comment