Combustion and Flame - Class 8 Notes & Olympiad Questions

Combustion

a) Combustion is a chemical process in which a substance reacts with oxygen in the air, producing heat, light, and sometimes other products. It's the process of burning, and it's essential for various things we use every day, like cooking, heating our homes, and running vehicles.
b) To understand combustion better, think of a simple example like lighting a matchstick. When you strike a matchstick against the rough surface of the matchbox, the heat generated from the friction causes the chemicals in the matchstick to react with oxygen in the air. This reaction produces heat and light, and you see a flame.
c) The key components needed for combustion are called the "fire triangle," which includes fuel (the substance that burns), oxygen (usually from the air), and heat (to raise the temperature of the fuel to its ignition point). If any of these components is missing, combustion cannot occur.

d) Combustion can be rapid, like when you burn paper or wood, or it can be slow, as in the process of rusting. Explosions are also a type of combustion that happens suddenly and rapidly.
e) It's essential to understand that not all substances burn with a visible flame. For example, when you burn charcoal or certain metals, you might not see a flame, but the process of combustion is still happening.
f) Combustion is a powerful and useful process, but it also has its challenges. Burning fuels can release harmful gases and pollutants into the air, contributing to issues like air pollution and climate change. That's why it's crucial to use fuels responsibly and find cleaner and more sustainable alternatives for our energy needs.

Combustible and Non-combustible Substances

Combustible and non-combustible substances are two categories of materials based on their ability to burn or support combustion.

Combustible Substances

Combustible substances are those materials that can burn or undergo combustion when exposed to heat or an ignition source in the presence of oxygen (air). During combustion, these substances react with oxygen to produce heat, light, and often various byproducts.
Examples of combustible substances include wood, paper, petrol, diesel, natural gas, kerosene, coal, and various organic materials.

The key characteristics of combustible substances are:

a) They have an ignition temperature, which is the minimum temperature required to start the combustion process.
b) Combustible substances release energy in the form of heat and light during the combustion process.
c) They can be used as fuels to provide energy for various purposes, such as cooking, heating, and powering vehicles.

Non-combustible Substances

Non-combustible substances, on the other hand, are materials that do not burn or support combustion, even when exposed to high temperatures or an ignition source in the presence of oxygen. These substances do not undergo any chemical reaction with oxygen that leads to burning. Instead, they remain stable and unchanged under the influence of heat.
Examples of non-combustible substances include metals like iron, copper, aluminium, glass, stone, sand, and many inorganic materials.

The key characteristics of non-combustible substances are:

a) They have very high ignition temperatures, making them resistant to catching fire.
b) Non-combustible materials are commonly used for constructing buildings and structures to ensure fire safety.
c) These substances are not used as fuels since they do not release energy through combustion.

It's essential to understand the distinction between combustible and non-combustible substances, especially in the context of fire safety and choosing appropriate materials for specific applications. Combustible materials should be handled with care and used responsibly to prevent accidental fires, while non-combustible materials provide a safer option for construction and other applications where fire resistance is a priority.

Conditions Necessary for Combustion

The conditions necessary for combustion, as you mentioned, are:

1. Presence of Combustible Substance

a) Combustion requires the presence of a material or substance that is capable of burning.
b) These substances are also known as fuels and can be in the form of solids, liquids, or gases.

2. Supporter of Combustion (Oxygen)

a) Oxygen gas, present in the air, acts as the supporter of combustion.
b) When a combustible substance reacts with oxygen, it undergoes a chemical process called oxidation or combustion, releasing energy in the form of heat and light.

3. Ignition Temperature

a) The ignition temperature is the minimum temperature at which a combustible substance starts burning or catches fire. If the temperature of the substance is below its ignition temperature, it will not burn, even in the presence of oxygen. However, once the substance reaches or exceeds its ignition temperature, it starts the combustion process and sustains the burning as long as the other conditions are met.
b) For example, Gasoline has a relatively low ignition temperature, which is why it can catch fire easily when exposed to a spark or high temperature.
Wood requires a higher temperature to ignite compared to gasoline, but it can still catch fire when exposed to enough heat.

These three conditions must be met simultaneously for combustion to occur. If any of these conditions are not present or are inadequate, combustion will not take place. Understanding these conditions is crucial for fire safety and also helps in controlling and preventing fires in various situations.

Inflammable Substance

a) Inflammable substances are those that have a low ignition temperature and can catch fire easily and burn with a flame. They are highly reactive with oxygen in the air, and when exposed to a source of heat or flame, they ignite rapidly. Due to their low ignition temperature, they can start burning with little external energy input.
b) Inflammable substances are widely used as fuels due to their ability to burn easily and produce a significant amount of heat energy. However, their high reactivity also makes them potentially dangerous, and proper safety measures should be taken when handling and storing them to prevent accidents and fires.

Common examples of inflammable substances include:

i) Petrol: Used as a fuel in automobiles and machinery.
ii) Liquified Petroleum Gas (LPG): Used as a household and industrial fuel.
iii) Kerosene: Used as a fuel for lamps, stoves, and heaters.
iv) Alcohol: Some types of alcohol, like ethanol, are highly inflammable.
v) Diesel: Used as fuel for diesel engines and vehicles.

Types of Combustion

1. Rapid Combustion

a) In this type of combustion, the substance burns quickly and produces a large amount of heat and light. The combustion is almost complete, meaning that the substance is entirely consumed by the reaction with oxygen.
b) An example of rapid combustion is the burning of LPG (Liquified Petroleum Gas) used in stoves and heaters. LPG burns rapidly and efficiently, providing a significant amount of heat for cooking and heating purposes.

2. Spontaneous Combustion

a) Spontaneous combustion refers to a type of combustion where a substance bursts into flames without the application of an external heat source. It occurs due to the substance's low ignition temperature, allowing it to catch fire on its own.
b) An example of spontaneous combustion is the burning of white phosphorus at room temperature. White phosphorus can ignite spontaneously when exposed to air due to its extreme reactivity with oxygen.

3. Explosion

a) In this type of combustion, the reaction occurs extremely fast and with a rapid release of heat, light, and the evolution of a large amount of gases and sound. Explosions are highly energetic reactions, usually involving a sudden increase in volume and pressure.
b) An example of an explosion is the ignition of fireworks or crackers, where the rapid combustion of certain chemical mixtures leads to the spectacular display of light, sound, and gas release.

Methods of Extinguishing a Fire

There are several methods of extinguishing a fire, and they typically involve one or more of the following principles:

Removing the Combustible Substance: One way to control fire is by removing the fuel source. If you take away the material that is burning, the fire will starve and eventually go out. For example, if a small fire is caused by burning papers, you can extinguish it by removing the papers from the fire's vicinity.

Lowering the Ignition Temperature: The ignition temperature is the minimum temperature at which a substance catches fire and starts burning. If you can lower the temperature of the fuel below its ignition temperature, it will not burn. Pouring water on a fire is an example of this method. The water absorbs heat from the burning material, cooling it down and preventing it from reigniting.

Cutting off the Supply of Air: Fire needs oxygen from the air to continue burning. By cutting off the supply of air, you can suffocate the fire and stop its spread. Fire blankets, sand, or fire extinguishers that release carbon dioxide (CO₂) are commonly used to cut off the oxygen supply and control the fire.

Using Fire Extinguishers: Fire extinguishers are specifically designed to extinguish different types of fires based on the fuel involved. Some common types of fire extinguishers include water-based extinguishers, foam extinguishers, dry powder extinguishers, and CO₂ extinguishers. Each type is suitable for specific fire scenarios and should be used according to the instructions provided.

Applying Chemicals: Some chemicals can chemically inhibit the combustion process, disrupting the chain reaction that sustains the fire. These chemicals can be sprayed on the burning material to control the fire.

It is essential to use the appropriate method of fire control based on the type and size of the fire and the available resources. Always prioritize safety and follow proper fire safety procedures to minimize risks and potential harm. In case of a large or uncontrollable fire, it is essential to call for professional firefighting help immediately.

Flame

a) Definition: A flame is a zone of burning gases that emits heat and light during the process of combustion. It occurs when a substance undergoes rapid oxidation in the presence of oxygen (usually from the air). The substances that vaporise and undergo combustion produce flames.
b) Example: When you light a candle, the wax in the candle melts and vaporises. The vaporised wax then reacts with oxygen in the air and produces a flame. In contrast, substances like coal burn without flames because they do not vaporise easily.
c) Structure of a Flame: A typical flame consists of three zones - the innermost zone, the middle zone, and the outermost zone. In the innermost zone, there is incomplete combustion, and unburnt carbon particles emit a dark glow. The middle zone is where complete combustion occurs, emitting a yellowish, luminous flame. The outermost zone is the hottest part of the flame, where the combustion is complete and emits a pale blue, non-luminous flame.
d) Importance of Flames: Flames are vital for many applications, including providing heat for cooking and heating, generating energy in power plants, and producing light in candles and lamps.
e) Colour of Flames: The colour of a flame can vary depending on the fuel being burned and the temperature of the flame. For example, a candle flame appears yellow due to the presence of unburnt carbon particles, while a gas flame (like in a Bunsen burner) appears blue due to the higher temperature and complete combustion.

Different Zones of a Candle Flame

The candle flame consists of three distinct zones, each with its own characteristics:

1. Outermost Zone

a) It is the hottest zone of the flame.
b) Complete combustion occurs in this zone, meaning that the fuel (wax vapours) fully reacts with oxygen from the air.
c) It is a non-luminous zone, which means it does not emit visible light.
d) The outermost zone appears blue in colour and is faintly visible. Its high temperature makes it less visible compared to the other zones.

2. Middle Zone

a) The middle zone is moderately hot.
b) Partial combustion occurs in this zone, resulting in the emission of visible light.
c) It is a luminous zone, meaning it emits light.
d) The middle zone appears yellow in colour due to the presence of glowing soot particles (unburnt carbon) from the partially combusted wax vapours.

3. Innermost Zone

a) It is the coldest zone of the flame due to the lack of oxygen necessary for combustion.
b) The innermost zone is located close to the wick.
c) No combustion occurs in this zone as there is insufficient oxygen available for the burning of wax vapours.
d) It is a dark zone and does not emit visible light.

Fuel

a) Fuel is a substance that can undergo combustion and release energy in the form of heat and light when it burns. It is a source of usable energy that powers various applications in our daily lives, industries, and transportation.
b) The process of combustion involves the chemical reaction of the fuel with oxygen from the air, resulting in the production of heat, light, and other products. Different fuels have varying calorific values, which represent the amount of energy released per unit of the fuel.
c) Common examples of fuels include:

Types of Fuels

1. Solid Fuels

a) These are combustible substances that exist in a solid state at room temperature.
b) Examples include coal, wood, charcoal, and cow dung.
c) Solid fuels are often used in traditional cooking stoves, fireplaces, and some industrial applications.

2. Liquid Fuels

a) Liquid fuels are combustible substances that exist in a liquid state at room temperature.
b) Common examples are petrol (gasoline), diesel, kerosene, and various oils.
c) Liquid fuels are commonly used in transportation (vehicles, aeroplanes, ships) and for heating in oil-based furnaces.

3. Gaseous Fuels

a) Gaseous fuels are combustible substances that exist in a gaseous state at room temperature and pressure.
b) Examples include LPG (liquefied petroleum gas), natural gas (methane), biogas, and hydrogen.
c) Gaseous fuels are widely used in households for cooking and heating, as well as in industrial processes and power generation.

Calorific Value

a) The calorific value of a fuel is the amount of heat energy released when a specific quantity of the fuel undergoes complete combustion.
b) It is typically measured in kilojoules per kilogram (kJ/kg) or British thermal units per pound (BTU/lb).
c) Calorific value is an essential factor in determining the efficiency and energy content of a fuel.

Characteristics of a Good Fuel

1. High Calorific Value

a) Good fuel should have a high calorific value, meaning it should release a large amount of heat energy when burned completely.
b) Fuels with higher calorific values can produce more energy per unit mass, making them more efficient for various applications.

2. Economic

a) An ideal fuel should be economically viable, meaning it should be readily available and affordable.
b) Cost-effectiveness is crucial, especially when fuels are used on a large scale, such as in industries and for transportation.

3. Easy to Handle, Store, and Transport

a) Good fuels should be easy to handle, store, and transport without posing significant safety risks or logistical challenges.
b) Fuels that are easy to handle are more practical for widespread use.

4. High Ignition Temperature

a) Fuels should not ignite easily at room temperature or under normal conditions to prevent accidental fires.
b) They should require specific conditions, such as the presence of a spark or heat source, to initiate combustion.

5. Environmentally Friendly (Low Pollution)

a) An ideal fuel should burn cleanly, with minimal harmful emissions such as sulphur dioxide, nitrogen oxides, particulate matter, and greenhouse gases.
b) Low-pollution fuels help reduce the impact of combustion on air quality and climate change.

6. Low Residue

a) A good fuel should not leave behind a significant amount of residue, ash, or solid waste after combustion.
b) Low-residue fuels are more convenient to handle and result in less waste management.

Harmful Effects of Burning Fuels

1. Particulate Matter (Unburnt Carbon Particles)

a) The incomplete combustion of fuels can release particulate matter, which consists of tiny carbon particles that are not completely burned.
b) These particles can be inhaled deep into the lungs and cause respiratory problems, especially for people with asthma or other respiratory conditions.

2. Carbon Monoxide (CO)

a) Carbon monoxide is a colourless, odourless, and poisonous gas formed when fuels are not burned completely due to insufficient oxygen.
b) When inhaled, carbon monoxide binds to haemoglobin in the blood, reducing its ability to carry oxygen, leading to carbon monoxide poisoning.
c) High levels of carbon monoxide can be fatal and are a significant concern in enclosed spaces with inadequate ventilation, such as in poorly ventilated homes or garages with running vehicles.

3. Carbon Dioxide (CO₂)

a) Carbon dioxide is a greenhouse gas released during the complete combustion of most fuels.
b) The excessive accumulation of carbon dioxide in the atmosphere is a major driver of global warming and climate change.
c) The increased greenhouse effect leads to rising temperatures, changes in weather patterns, and various environmental impacts.

4. Oxides of Sulphur and Nitrogen

a) When certain fuels, like coal and diesel, are burned, they release oxides of sulfur (SO_x) and nitrogen (NO_x) into the atmosphere.
b) These gases can react with water vapour in the air to form sulfuric acid and nitric acid, respectively, resulting in acid rain.
c) Acid rain can have damaging effects on the environment, including harming aquatic life, forests, and buildings made of limestone or marble.
d) These harmful effects emphasize the importance of finding cleaner and more sustainable ways to meet our energy needs. The development and use of renewable energy sources and cleaner technologies can help mitigate the negative impacts of burning fuels on both human health and the environment.