How Does Fire Work at the Molecular Level?
Fire is described as a rapid burning of combustible material with the evolution of heat, and it is usually accompanied by flame. Also, it is one of the essential tools of the human race, control of which helped start it on the path towards civilization.
Source of Fire and Uses of Fire By Early Man
Undoubtedly, lightning was the original source of fire, and such type of fortuitously ignited blazes remained as the only source of fire for aeons. Peking man, about 500,000 BCE, was thought to be the undisputed earliest user of fire for a few years; however, evidence discovered in Kenya in 1981 and South Africa in 1988 indicates that the earliest powered fire use by hominids occurred about 1,420,000 years ago. Neolithic man did not learn reliable fire-making techniques until about 7000 BCE, in the form of saws, drills, and other friction-producing tools, or flint struck against pyrites. Then, even it was more convenient to put a fire alive permanently instead of reigniting it.
Uses of Fire
Let us look at the uses of fire below.
Gradually, the first human beings to control the fire learned its several uses. They had learned to use fire not only to cook their food and stay warm, but also to use it in fire drives in warfare or hunting, to gather berries, to kill insects, and to clear forests of underbrush so that game could be hunted more effectively by sight. They discovered that burning brush created better grasslands and, as a result, In Neolithic times, in the Middle East, about 7000 BCE, with the achievement of agriculture, there came a newer urgency to clear trees and brush.
The first agriculturists have made use of the fire in clearing the fields and producing ash to serve as fertilizer. This practice, which is known as slash-and-burn cultivation, persists in several tropical areas and a few temperate zones today.
Fire Manufacturing
The step ranging from the fire control to its manufacture is very great and needed hundreds of thousands of years. The variety and several inventions of fire of such manufacture are more difficult to imagine. There is no proof that human beings knew how to make fire until the Neolithic period. It is unknown whether a chance spark from striking flint against pyrites or a spark created by friction while drilling a hole in wood gave humans the idea to make fire; however, pyrites and flint, as well as fire drills, have been recovered from Neolithic sites in Europe.
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The friction method of producing fire was most common among prehistoric and primitive peoples. The simple fire drill, which is a pointed stick of hardwood twirled between the palms and then pressed into a hole on the stick's edge of softer wood, is mostly universal. The fire saw and the fire-plow are variations on the friction method very common in Australia, Indonesia, and Oceania.
Whereas the mechanical fire drills were developed by the ancient Egyptians, Eskimo, Asian peoples, including some American natives. A fire piston was a complex device that was invented and used in southeastern Indonesia, Asia, and the Philippines to create heat and fire by compressing air in a narrow bamboo tube. Around 1800, in Europe, a metal fire piston was independently invented. And, in 1827, the English chemist named John Walker invented the friction match that contains phosphorus sulfate, importantly similar to that which is in use now.
Types of Combustion
Complete Combustion
In the complete combustion type, the reactant burns in the oxygen and forms a limited number of products. When a hydrocarbon burns in the oxygen, primarily, the reaction will yield water and carbon dioxide. When these elements are burned, primarily, the products are the most common oxides.
Sulfur yields sulfur dioxide, carbon yields carbon dioxide, and iron yields iron (III) oxide. Nitrogen is not considered a combustible substance when oxygen is the oxidant. But still, fewer amounts of different nitrogen oxides (generally designated as NOx species) form when the air is oxidative.
Combustion is not certainly favourable to the maximum oxidation degree, and it may be temperature-dependent. For example, sulfur trioxide cannot quantitatively be produced by the combustion of sulfur.
Incomplete Combustion
If there isn't enough oxygen available for the fuel to fully react and emit both water and carbon dioxide, incomplete combustion occurs. Also, it happens when the combustion can be quenched by a heat sink, such as a flame trap or a solid surface. As this is the case with complete combustion, water can be produced by incomplete combustion; however, carbon monoxide, carbon, and hydroxide are produced instead of carbon dioxide (CO2).
For most of the fuels, such as coal, wood, diesel oil, pyrolysis takes place prior to combustion. In the incomplete combustion case, pyrolysis products remain unburnt and contaminate the smoke with noxious particulate gases and matter. Partially the oxidized compounds are also a major concern; partial oxidation of ethanol may produce harmful acetaldehyde, and carbon may produce toxic carbon monoxide.
FAQs on Fire: Chemistry, Combustion & Applications
1. What is fire from a scientific perspective?
From a scientific perspective, fire is a chemical process known as combustion. It is a rapid, self-sustaining oxidation reaction that involves a fuel and an oxidant (usually oxygen from the air), which results in the evolution of heat and light. The visible part of the fire, the glowing hot gases, is called a flame.
2. What three conditions are essential for combustion to start and be sustained?
Three essential conditions, often referred to as the fire triangle, are required for combustion to occur:
- Fuel: A combustible substance that can burn, such as wood, paper, or petrol.
- Heat: An ignition source to raise the fuel to its ignition temperature.
- Oxygen: An oxidant, typically from the air, to support the chemical reaction.
Removing any one of these three elements will extinguish the fire.
3. What is the difference between complete and incomplete combustion?
The primary difference lies in the amount of oxygen available during the reaction.
- Complete Combustion: Occurs when there is a plentiful supply of oxygen. The fuel burns efficiently, producing primarily carbon dioxide (CO₂) and water (H₂O), and releasing maximum energy. It is often characterised by a blue flame.
- Incomplete Combustion: Occurs when the oxygen supply is limited. It produces harmful substances like carbon monoxide (CO), soot (carbon), and water. It is less efficient and often characterised by a yellow or orange, sooty flame.
4. What are the main types of combustion explained in chemistry?
Combustion can be classified into several types based on its speed and nature:
- Rapid Combustion: This is the most common type, where a substance burns quickly to produce significant heat and light, such as the burning of an LPG gas stove.
- Spontaneous Combustion: This type of combustion occurs without any external heat source. The substance ignites on its own when it reaches its ignition temperature due to internal heat generation, like phosphorus burning at room temperature.
- Explosion: An explosion is a very fast combustion process that takes place in a confined space, producing a large amount of heat, light, and sound, along with a rapid expansion of gases. The bursting of a firecracker is a prime example.
5. What is a flame, and why does it have different coloured zones?
A flame is the visible, gaseous part of a fire. The different colours in a flame are due to variations in temperature and the type of combustion occurring in different zones.
- Innermost Zone (Dark Zone): This is the coolest part around the wick, consisting of unburnt fuel vapours. No combustion occurs here.
- Middle Zone (Luminous Zone): This yellow-coloured zone experiences incomplete combustion of fuel particles, which glow and produce most of the light.
- Outermost Zone (Non-luminous Zone): This is the hottest part of the flame, where complete combustion occurs due to a good supply of oxygen. It is typically faint blue in colour.
6. How does the principle of the fire triangle help in extinguishing a fire?
Extinguishing a fire involves removing at least one of the three elements of the fire triangle (fuel, heat, or oxygen). For example:
- Removing Heat: Water is commonly used to cool the combustible material below its ignition temperature, thus breaking the triangle.
- Cutting off Oxygen Supply: A fire extinguisher releases carbon dioxide, or a fire blanket is used to cover the fire, displacing the oxygen and suffocating the flame.
- Removing Fuel: While difficult in most situations, this can involve shutting off a gas supply or creating a firebreak in a forest fire by clearing away trees and vegetation.
7. What makes a substance 'combustible' or 'non-combustible'?
A substance's ability to burn determines if it is combustible or non-combustible.
- Combustible substances are those that can catch fire and burn in the presence of air (oxygen). Examples include wood, paper, cloth, kerosene, and petrol. They have a relatively low ignition temperature.
- Non-combustible substances are those that do not burn on being exposed to flame. Examples include stone, glass, cement, and water. These materials have very high ignition temperatures or chemical structures that do not react with oxygen in this way.
8. Why does burning wood leave ash, but burning LPG does not?
This difference is due to the chemical composition of the fuels. Wood is a complex organic material containing carbon, hydrogen, and oxygen, along with non-combustible minerals. When wood burns, the carbon compounds turn into gases like CO₂, but the minerals do not burn and are left behind as ash. In contrast, LPG (Liquefied Petroleum Gas) is a mixture of simple hydrocarbons (like propane and butane) that completely vaporise and undergo complete combustion under normal conditions, producing only carbon dioxide and water vapour, leaving no solid residue.
