Chapter 6 – Combustion and Flame

You would have noticed that not all substances catch fire at the same temperature. Think of a matchstick, forest fires, or piece of paper. The lowest temperature at which a substance catches fire is called its ignition temperature.

Substances with very low ignition temperature can catch fire very easily and are called inflammable substances. Some examples of inflammable substances include petrol, LPG, diesel, etc.

We have all seen fire being put out by water in movies and TV shows. What exactly does water do to fire?

• In order to control a fire at home or any other place, water is used to douse it. By doing so, water is essentially cooling the combustible substance and reducing its ignition temperature to prevent it from spreading.
• The water vapour that forms when water is poured over a combustible substance cut-off the air supply and helps in extinguishing a fire.
• There are other ways to put out a fire, such as using carbon dioxide stored in pressurized cylinders(forms sodium and potassium bicarbonate over the fire), putting sand or soil(only for small fires), etc.
• Fire extinguishers are typically used to put out fires that arise from petrol and diesel( water is ineffective here as it is heavier than these substances and settles down while oil continues to burn on top) and those arising from electrical short circuits(as throwing water can result in giving a person shock as the water we get in our homes can conduct electricity).
• For a fire to arise, there are three main requirements– fuel, air, and heat. A fire can be brought under control by eliminating one or more of these factors.

You may have noticed how different substances burn in different ways. Based on this, there are two different types of combustion:

• Rapid combustion

Substances that burn rapidly to give heat and light are said to show rapid combustion.

• Spontaneous Combustion

Some substances catch fire suddenly, without any apparent cause, and are said to show spontaneous combustion. A great example of this type of combustion is substances like phosphorus that catch fire at room temperature.

Chemical Reaction and Combustion

Another example of combustion is the explosion of a cracker. When we ignite a cracker, a chemical reaction takes place inside it that leads to the combustion of substance inside and results in the liberation of a large amount of gas, along with heat and light. This type of reaction is called an explosion.

The flame is the visible, gaseous part of a fire. It is the light energy emitted during the process of fuel combustion. Substances that vaporize during burning give out flames—for example, kerosene and wax. Substances like coal that do not evaporate on burning do not give our flames while burning.

Structure of Flame

Did you know a candle flame has many zones? Observe a goldsmith. You will notice how he blows into the outermost zone of the flame. This is because the outermost zone of the flame is the hottest part of the flame. 

Different zones of a candle flame are:

• Outermost: Zone of Combustion: Hottest part of the flame
• Middle Zone: Zone of Partial Combustion: Moderately hot
• Innermost Zone: Zone of Unburnt Wax Vapours: Least hot

Substances that are used to generate energy for domestic and industrial purposes are called fuels. Commonly used fuels are petrol, diesel, kerosene, LPG, coal charcoal, etc.

• A good fuel is readily available, is cheap, burns quickly in the air at a moderate rate, produces large amounts of heat, and does not leave behind any residue.
• However, there is no ideal fuel that fulfils all the above requirements.

Another property of fuels that makes them more or less popular is their efficiency. 

• The amount of heat produced on the complete combustion of 1kg of fuel is called its calorific value. It is represented as kilo Joule per Kg or kJ/kg.
• Fuel efficiency is the proportion of energy produced on combustion of the fuel that is converted to useful work.
• For example, efficient fuels used in vehicles can release more energy even in lesser quantities and result in the vehicle going a longer distance than inefficient fuels that can take the vehicle only a short distance. 
• With an efficient fuel, we can use lesser amounts of it whereas, we use more quantity of inefficient fuel to go the same distance. Using efficient fuels, we are effectively burning a lesser amount of fuel and contributing lesser pollutants to the environment.
• For example, diesel as fuel is said to be more efficient than petrol.

We are all aware that the burning of fuels is harmful to us and our environment.

• Carbon-based fuels such as coal, petroleum, and wood give out unburnt carbon particles on the combustion that is the major pollutant in the Earth’s atmosphere today and leads to respiratory diseases like asthma.
• Incomplete combustion of fuels releases carbon monoxide into the air, which is a poisonous gas. Sleeping in a closed room with burning coal can lead to carbon monoxide poisoning, which is fatal.
• Combustion of most fuels releases carbon dioxide into the atmosphere, the major cause of global warming. 
• Burning of these fuels also releases sulphur dioxide, a corrosive and suffocating gas. When this sulphur dioxide combines with nitrogen oxides and rainwater, it results in acid rain formation.

1. What is the difference between combustion and flame?

The chemical process by which a substance reacts with oxygen in the air to give out heat and light is called combustion. A flame, however, is the visible gaseous portion of the fire.

2. What type of combustion is fire?

Fire is an example of rapid combustion as it is characterised by the release of large amounts of heat and light in the form of a flame.

3. What are the 3 types of combustion?

Based on the nature of the process, combustion is of three types:

• Rapid combustion
• Spontaneous combustion and
• Explosion

4. What is a flame in chemistry?

The flame is the visible gaseous portion of the fire. It is made up of three layers– with the outermost layer being the hottest and the innermost being least hot.

5. What are the 2 types of combustion?

Two types of combustion are rapid and spontaneous combustion. Rapid combustion is when a substance burns to release large amounts of heat and light. A substance that burns in the air without an apparent cause is said to show spontaneous combustion.

6. Does charcoal burn with a flame?

Charcoal does not burn with a flame because it does not evaporate on burning. It is notorious for leaving an unburnt residue that causes air pollution and respiratory diseases.

Curious to know more about combustion and flame and how they form an integral part of our daily lives? Check out more information on the MSVGo app or our website.