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Combustible and Non-combustible Substances

  • Last Updated : 05 Oct, 2021

We’ve seen coal burning in the air, releasing carbon dioxide and water into the atmosphere. When we ignite a candle in the air, CO2 is produced, along with heat and light. All of the examples are chemical reactions that result in the formation of new material as a result of the evolution of heat. Combustion is a chemical reaction in which a material combines with oxygen to produce heat. Combustible or fuel refers to a substance that may burn and can be solid, liquid, or gaseous. During combustion, light in the form of a glow or a flame can be produced.

Combustion is a chemical process that results in the production of heat and light. Fuel interacts with an oxidising agent to create energy, which is often in the form of heat and light. Fire is the most prevalent type of combustion. e.g. both a vehicle engine and a rocket engine use combustion to operate.

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Combustible and Non-combustible substances

Combustible Substances



A substance that burns in air and produces heat and light upon burning is called a Combustible substance. e.g. Diesel, petrol, kerosene. 

This is an important property especially when a substance is used for construction. It is also vital in processes that produce combustible substances as a by-product. Special precautions are usually required for such substances. The measures required are the installation of fire sprinklers from sources of ignition.

Non-Combustible Substances

Substances that don’t burn in the presence of air are called non-combustible substances. e.g. Glass, stones. 

Substances with low combustibility or which are non-combustible are selected for construction where there is a need to reduce fire risk like in apartment buildings. If there is the use of combustible substances, there is a greater chance of fire accidents and deaths. Fire-resistant substances are preferred for building materials and furnishings.

The extent of combustibility in air depends on the material’s volatility. This is in fact related to its composition-specific vapour pressure. This composition specific vapour pressure is temperature-dependent. This vapour quantity is increased by an increase in the surface area of the material that forms a mist. For example, Wood was finely divided wood dust may undergo an explosion and produce a blast wave. A piece of paper catches fire very easily while a heavy oak desk which is also made from wood like paper is harder to ignite.

Types of Combustion

  1. Rapid Combustion: Rapid combustion is a type of combustion in which a gas burns quickly and creates heat and light. Phosphorus, for example, burns in air at ambient temperature.
  2. Spontaneous Combustion: The form of combustion in which a substance spontaneously bursts into flames without any apparent cause is known as spontaneous combustion.
  3. Explosion: An explosion is a quick reaction that occurs as a result of the evolution of heat, light, and sound. The reaction produces a considerable volume of gas, which is then released. For instance, the use of firecrackers.

Conditions necessary for Combustion

For combustion to occur, the following criteria must be met:

  1. Presence of a combustible substance: Combustion is feasible only if the substance is combustible, that is, if it can burn. Wood, paper, kerosene oil, straw, matchsticks, and other materials are examples. Combustion cannot occur if there is no flammable substance present.
  2. Presence of a supporter of combustion: Combustion requires an adequate supply of a combustion supporter (e.g., oxygen).
  3. Attainment of ignition temperature: Only after a substance has reached a certain minimum temperature does it begin to burn. The ignition temperature of a substance is the temperature at which it burns in the presence of air. If the temperature of a substance is lower than the ignition temperature, it will not catch fire.

Fire triangles refer to the three circumstances listed above. Alcohol, gasoline, LPG, and nylon fibres are all extremely flammable. This is due to the low ignition temperatures of these compounds. Inflammable substances are those that are flammable. When storing or transporting combustible materials, more caution is required.



Important Characteristics of Combustible Substances

  1. Flashpoint: It is a measure of how easy it is to ignite the vapour of the material while it evaporates into the atmosphere. If it is low, it means that it has higher flammability.
  2. Vapour pressure:  It varies with its temperature. It is a measure of how much the vapour of the liquid is concentrated in the atmosphere as the liquid evaporates. It is an important criterion of the flashpoint. Higher vapour pressures lead to lower flash points and higher flammability.

Combustible Substances and their reactions

When a material reacts quickly with oxygen, it is called a combustion reaction (O2). Burning is a frequent term for combustion, and the substance that burns is commonly referred to as fuel. Carbon dioxide (CO2) and water vapour are the end products of a complete combustion reaction (H2O). In most cases, the reaction produces heat and light. For a complete combustion reaction, the generic equation is:

Fuel + O2 → CO2 + H2O

Hydrocarbons, which are molecules that exclusively include carbon and hydrogen, are found in the fuel that burns in a combustion reaction. Methane and ethane, for example. Natural gas is a regularly utilised fuel in home furnaces and gas stoves. Methane is the primary component of natural gas (CH4). The equation for methane combustion is as follows:

CH4 + 2O2 → CO2 + 2H2O

Combustible Substances and its relation with Ideal Fuel

  1. The perfect fuel would be inexpensive, abundantly available, and flammable.
  2. It should have a lot of calories.
  3. It does not pollute the environment with toxic gases or residues.
  4. It should be dry and low in moisture because dry fuel has a higher calorific value.
  5. A good fuel should have a moderate combustion speed.
  6. A good fuel’s combustion should not be explosive.

Sample Questions

Question 1: What is combustion?

Answer: 

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 or fuel which may be solid, liquid or gas. Sometimes, light as glow or a flame is also given off during combustion.

Question 2: What are the types of Combustion?

Answer:

The types of combustion are:



  1. Rapid Combustion: Rapid combustion is a type of combustion in which a gas burns quickly and creates heat and light. Phosphorus, for example, burns in air at ambient temperature.
  2. Spontaneous Combustion: The form of combustion in which a substance spontaneously bursts into flames without any apparent cause is known as spontaneous combustion.
  3. Explosion: An explosion is a quick reaction that occurs as a result of the evolution of heat, light, and sound. The reaction produces a considerable volume of gas, which is then released. For instance, the use of firecrackers.

Question 3: What are the conditions necessary for Combustion?

Answer:

For combustion to occur, the following criteria must be met:

  • Presence of a combustible substance: Combustion is feasible only if the substance is combustible, that is, if it can burn. Wood, paper, kerosene oil, straw, matchsticks, and other materials are examples. Combustion cannot occur if there is no flammable substance present.
  • Presence of a supporter of combustion: Combustion requires an adequate supply of a combustion supporter (e.g., oxygen).
  • Attainment of ignition temperature: Only after a substance has reached a certain minimum temperature does it begin to burn. The ignition temperature of a substance is the temperature at which it burns in the presence of air. If the temperature of a substance is lower than the ignition temperature, it will not catch fire.

Question 4: What are combustible and non-combustible substances?

Answer:

A substance that burns in air and produces heat and light upon burning is called Combustible substances. E.g. Diesel, petrol, kerosene. Substances that don’t burn in the presence of air are called non-combustible substances. E.g. Glass, stones. 

Question 5: What is the relation between combustibility and volatility of the material?

Answer:

The extent of combustibility in air depends on the material’s volatility. This is in fact related to its composition-specific vapor pressure. This composition specific vapor pressure is temperature dependent. This vapor quantity is increased by increase in the surface area of the material that forms a mist. For example: Wood where finely divided wood dust may undergo explosion and produce a blast wave. A piece of paper catches fire very easily while a heavy oak desk which is also made from wood like paper is harder to ignite.




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