CBSE Class 8 Science Chapter 4 Combustion And Flame Detailed Notes Combustion
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 a combustible substance.
It is also called a fuel which can be solid, liquid, or gas. Sometimes, light is also given off during combustion, either as a flame or as a glow. For example, wood burns by producing a flame, but charcoal burns by creating light in the form of a glow.
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Fuel For Our Body
Food is a fuel for our bodies. During respiration, the digested food (glucose) is broken down by reaction with oxygen, producing carbon dioxide, water and heat. This heat energy is utilised by our body. Thus, respiration is a type of slow combustion of food.
Combustible and Non-Combustible Substances
Not all substances around us burn. Based on this, substances are divided into two types.
Combustible Substances The substances which burn in the presence of air or oxygen are called combustible substances. Example. petrol, LPG, kerosene, etc.
Non-combustible Substances The substances which do not burn in the presence of air or oxygen are called non-combustible substances. Example. Water, glass, sand, etc.
Necessary Conditions for Combustion
There are three necessary conditions for combustion. These are as follows
1. Presence of a Combustible Substance
The presence of a substance like petrol, LPG, wood, coal, etc. is necessary for the combustion process.
2. Presence of a Supporter of Combustion
A substance which helps in combustion is called a supporter of combustion, for Example. Oxygen (or air).. For example, when the clothes of a person catch fire, the person is covered with a blanket to extinguish the fire.
The blanket cuts off the supply of air, which is needed for the burning of clothes. This shows that air is necessary for combustion.
3. Ignition Temperature
The lowest temperature at which a substance catches fire and starts burning is called its ignition temperature.
Before a substance catches fire and starts burning, it must be heated to a certain minimum temperature by supplying heat. A combustible substance cannot catch fire if its temperature is lower than its ignition temperature.
For example, a matchstick does not catch fire on its own at room temperature. When matchstick rubbed on the rough side of a matchbox the heat produced increases its temperature to ignition temperature and its start burning.
History of Matchsticks
More than five thousand years ago, small pieces of pinewood dipped in sulphur were used as matches in ancient Egypt.
A mixture of antimony tri-sulphide, potassium chlorate and white phosphorus with some glue and starch was applied on the head of a match made of suitable wood.
These days, the head of the safety match contains only antimony trisulfide and potassium chlorate. When the match is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus.
This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulfide and start the combustion.
Inflammable Substances
The substances which have very low ignition temperatures and can catch fire easily with a flame are called flammable substances. Example. petrol, alcohol, Liquefied Petroleum Gas (LPG), etc.
CBSE Class 8 Science Chapter 4 How Can Fire Be Controlled?
We have learnt that there are three essential requirements for producing fire. These are fuel (combustible substance), a supporter of combustion (i.e., air or oxygen), and heat (to raise the temperature of the fuel beyond the ignition temperature).
Fire can be controlled in three ways given below
- By removing the fuel, but in most cases, fuel cannot be eliminated.
- By removing the heat to bring down the temperature.
- By cutting off the air supply to the burning substances.
Fire Extinguishers
Substances that are used to cut off the supply of oxygen to the fire are known as fire extinguishers, for example, water and carbon dioxide. etc
The most common fire extinguisher is water, but it works only when things like wood and paper are on fire. It extinguishes fire by cooling the burning substances. However, water cannot be used for extinguishing fire in electrical types of equipment.
Water conducts electricity, hence, it can cause an electrical shock if electrical equipment is on fire. Water is also not suitable for fires involving oil and petrol because it sinks below the oil, and oil keeps burning on the top.
For fires involving electrical equipment and flammable substances like petrol, carbon dioxide (CO2) is the best extinguisher.
CO2 is denser than air and thus forms a layer around the burning substances, i.e. it covers the fire like a blanket, due to which oxygen cannot reach the burning substances and the fire is controlled.
Carbon dioxide can be stored as a liquid at high pressure in cylinders (as shown in the figure below). It expands enormously in volume when released from the cylinder and cools down.
Another way to obtain CO2 is to release a large amount of powdered baking soda (sodium bicarbonate) or potassium bicarbonate over the fire. The heat of the fire releases CO2 gas from the baking soda, which cuts off the fire.
When an LPG cylinder catches fire, it is wrapped with a wet cloth. It lowers the temperature of the fire and cuts off its contact with oxygen.
Types of Combustion
There are various types of combustion. Three important types of combustion are as follows.
Rapid Combustion
The combustion in which a large amount of heat and light are produced in a short time, is called rapid combustion. An Example:
Immediate burning of cooking gas (LPG) in a gas stove when a burning matchstick lighter is brought near it.Burning of kerosene oil in a kerosene stove, etc.
Spontaneous Combustion
The combustion in which a material suddenly bursts into flames without any external heat is called spontaneous combustion.
The heat required to start the spontaneous combustion is produced internally by the chemical reaction of the substance with oxygen. Example. White phosphorus burns in the air at room temperature on its own.
Some spontaneous combustion can be very dangerous. For example,
The spontaneous combustion of coal dust has resulted in many disastrous fires in coal mines.
Sometimes, due to the heat of the Sun or due to the spark of lightning from the sky, spontaneous combustion of straw (dry grass) and forest wood takes place, which leads to forest fire.
3. Explosions
A sudden combustion in which a large amount ofheat, light, and sound are produced is called an explosion. A large amount of gases formed is released quickly in an explosion, for Example.
When a cracker is ignited, an explosion takes place with the evolution ofheat, light, and sound. An explosion can also take place if pressure is applied to the cracker.
CBSE Class 8 Science Chapter 4 Flame
A flame is a region where combustion of a gaseous substance or vapour takes place.
The substances which vaporise during burning give flames, an Example. Kerosene oil and molten wax rise through the wick and are vaporised during burning and form flames.
While charcoal does not vaporise and so does not produce a flame, it just glows.
Given below is the list of some materials that either form or do not form a flame on burning.
Types Of Flame
Based on the availability of oxygen, a flame can be of the following types
- When the supply of oxygen is not sufficient, the fuels burn incompletely, producing mainly a yellow flame. For example, the colour of a candle flame is yellow.
- When the supply of oxygen is sufficient, the fuels burn completely, producing mainly a blue flame. For example, in LPG stoves, the LPG burns with a blue flame.
Structure of a Flame
A flame consists of three zones.
Innermost Zone: It is a dark or black coloured zone, which consists of hot, unburnt vapours of combustible material. It is the coolest part of the flame.
Middle Zone: It is the bright and luminous zone that produces a moderate temperature. Here, partial combustion of the fuel takes place.
Outermost Zone: It is a non-luminous zone. Here, the fuel’s complete combustion occurs because there is plenty of air around it. It has the highest temperature in the flame, i.e. is the hottest part of the flame.
This is the reason why goldsmiths blow the outermost zone of a flame with a metallic blow pipe for melting gold and silver to form the desired shapes for making jewellery.
CBSE Class 8 Science Chapter 4 Fuel
A substance which produces heat on burning is called a fuel. For example. Wood, charcoal, petrol, kerosene, LPG, natural gas, etc. These are the sources of heat energy. These fuels can be in solid form. Liquid or gaseous state.
A good fuel has the following characteristics
- Readily available
- Cheap
- Burns easily in air at a moderate rate.
- Produces a large amount of heat.
- Does not leave behind any undesirable substances.
- Low ignition temperature
Ideal Fuel
An ideal fuel is readily available, cheap and has a high calorific value and produces less amount of I pollution. There is probably no fuel that could be considered an ideal fuel.
CBSE Class 8 Science Chapter 4 Fuel Efficiency
Different fuels produce different amounts of heat on burning. The amount ofheat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value ofa fuel is expressed in a unit called kilojoules per kg (kJ or kg).
Calorific values of some fuels are given below
Burning of Fuels Leads to Harmful Products
The increasing fuel consumption has harmful effects on the environment because burning fuels produces harmful products that pollute the air around us.
These are as follows.
- Hunting of carbon fuels like wool, coni nod petroleum rclc.tse unburnt carbon particles in the air, which are not a dangerous pollutant, are causing respiratory diseases such as asthma.
- Also, the use of wood as a fuel leads to deforestation, which is quite harmful to living beings.
- Incomplete combustion of carbon fuels gives carbon monoxide gas, which is a very poisonous gas. Excessive inhaling of this gas can kill a person.
It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill a person sleeping in that room. So, we should never sleep in a room with a burning or smouldering coal fire in it.
- Burning of most fuels releases carbon dioxide into the air. Increased concentration of carbon dioxide in the air is believed to be the cause of the rise in temperature of the atmosphere of Earth.
- This is called global warming. Due to this, the ice in polar regions will melt very fast, producing a lot of water. It may cause a rise in the sea level, leading to floods in coastal areas.
- Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas. It may damage our lungs. Moreover, petrol and diesel engines give off gaseous oxides of nitrogen.
These oxides dissolve in rainwater and form acids. Such rain is called acid rain. It is very harmful for crops, buildings, and soil.
Compressed Natural Gas (CNG)
The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas) because it produces harmful products in very small amounts. CNG is a cleaner fuel.
Acid rain. When oxides of sulphur and nitrogen react with water molecules in the atmosphere, they form an acid. This acid mixes with rainwater and causes acid rain.
Calorific value is the amount of heat energy produced on the complete combustion of 1kg of fuel.
Combustion is a process in which a substance reacts with oxygen and gives off heat or light.
Deforestation, the Cutting of trees, leads to deforestation, which is quite harmful to the environment.
Explosion: When a cracker is ignited, a sudden reaction takes place. A large amount of gases is evolved in the process along with heat, light and sound. Such a reaction is called an explosion.
A flame is a region where combustion of gaseous substances or vapour takes place.
Fire extinguishers. These are used to block the contact of fuel with the promoter of combustion, such as oxygen.
Fuel is a combustible substance that catches fire easily and produces heat energy.
Fuel efficiency is the amount of heat that a fuel produces for a fixed amount of mass.
Global warming It is the process of rise in the average temperature of the Earth’s atmosphere due to the release of carbon dioxide on combustion of fuels.
Ideal fuel: An ideal fuel is cheap, readily available, readily combustible, and easy to transport.
Ignition temperature is the minimum temperature that must be reached before a substance starts burning.
Inflammable substances, these substances have a lower ignition temperature and burn easily with slight heating.