Fire is not a state of matter: rather, it is an exothermic chemical reaction accompanied by intense heat released during a rapid oxidation of combustible material. Fire may be visible as the brilliant glow and flames and may produce smoke.

Fires start when a flammable or combustible material with adequate supply of oxygen or other oxidizer is subjected to enough heat. The common fire-causing sources of heat include a spark, another fire (such as an explosion, a fire in the oven or fireplace, or a lit match, lighter or cigarette) and sources of intense thermal radiation (such as sunlight, a flue, an incandescent light bulb or a radiant heater). Mechanical and electrical machinery may cause fire when combustible materials used on or located near the equipment are exposed to intense heat from Joule heating, friction or exhaust gas. Fires can sustain themselves by the further release of heat energy in the process of combustion and may propagate, provided there is continuous supply of oxygen and fuel. Fires may become uncontrolled and cause great damage to and destruction of human life, animals, plants and property.

Fire is extinguished when any of the elements of so - called fire triangle-heat, oxygen or fuel - is removed. The unburnable solid remains of fire are called ash.

Exothermic Reaction

In chemistry, an exothermic reaction is one that releases heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:

Reactants = product + energy

When using a calorimeter, the change in heat of the calorimeter is equal to the opposite of the change in heat of the system. This means that when the solution in which the reaction is taking place gains heat, the reaction is exothermic.

In an exothermic reaction, the total energy absorbed in bond breaking is less than the total energy released in bond making.

The absolute amount of energy in a chemical system is extremely difficult to measure or calculate. The enthalpy change, ΔH, of a chemical reaction is much easier to measure and calculate. A bomb calorimeter is very suitable for measuring the energy change, ΔH, of a combustion reaction. Measured and calculated ΔH values are related to bond energies by:

ΔH=energy used in bond breaking reactions-energy released in bond making products

For an exothermic reaction, this gives a negative value for ΔH as a larger value is subtracted from a smaller value. For example, when hydrogen burns:

2H₂ + O₂ = 2H₂0
ΔH = -483,6kJ/mol of O₂

Examples of Exothermic Reactions

• Combustion
• Neutralization
• Adding water to concentrated acid
• Adding water to anhydrous copper(II) sulfate
• Thermite

Combustion

Combustion or burning is a complex sequence of chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames.

Since not every oxidation process results in the production of heat (for example, corrosion), the term combustion can only be applied to exothermic processes that occur at a rate fast enough to produce heat.

In a complete combustion reaction, a compound reacts with an oxidizing element, and the products are compounds of each element in the fuel with the oxidizing element. For example:

CH₄ + 2O₂ = CO₂ + 2H₂O + HEAT
CH₂S + 6F₂ = CF₄ + 2HF + SF₆ + HEAT

In most cases, combustion uses oxygen (O2) obtained from the ambient air, which can be taken as 21 mole percent oxygen and 79 mole percent nitrogen (N2). Thus, when methane (CH4) is combusted using air as the oxygen source, the first example equation above becomes:

CH₄ + 2O₂ + 7,52N₂ = CO₂ + 2H₂O + 7,52N₂ + HEAT

As can be seen, when air is the source of the oxygen, nitrogen is by far the largest part of the products of combustion.

The simple word equation for the combustion of a hydrocarbon in air is:

Typical Temperatures of Fires and Flames

• Oxyacetylene Flame (3,000 C or above)
• Oxyhydrogen Flame (2,000 C or above)
• Bunsen Burner Flame (Max. Setting) (1,300 - 1,600 C)
• Candle Flame (1,400 C)
• Blowtorch (1,300 C)
• Log fire (1,000 C ~)
• Heather Fire (500 - 1,000 C~)
• Paper — (approximately 235 C)

Fire Triangle

The fire triangle is a simple model, from the science of firefighting, for understanding the ingredients necessary for most fires. It has largely been replaced in the industry by the fire tetrahedron, which provides a more complete model, also described below.

The “triangle” illustrates the rule that in order to ignite and burn, a fire requires three elements — heat, fuel, and oxygen. The fire is prevented or extinguished by “removing” any one of them. A fire naturally occurs when the elements are combined in the right mixture (e.g., more heat needed for igniting some fuels, unless there is concentrated oxygen).

When a fire runs out of fuel it will stop. Fuel can be removed naturally, as where the fire has consumed all the burnable fuel, or manually, by mechanically or chemically removing the fuel from the fire. Fuel separation is an important factor in wild land fire suppression, and is the basis for most major tactics. Other fuels may also be chemically altered to prevent them from burning at ordinary temperatures, perhaps as part of a fire-prevention measure.

In short, it is possible to prevent fire by simply preventing one of those three elements to come together and form the triangle.

Fire Tetrahedron

The fire triangle is a useful teaching tool, but fails to identify the fourth essential element of fire: the sustaining chemical reaction. This has led to development of the fire tetrahedron: a triangular pyramid having four sides (including the bottom). In most fires, it does not matter which element gets removed; the fire fails to ignite, or it goes out. However, there are certain chemical fires where knowing only the “fire triangle” is not good enough.

Combustion is the chemical reaction that feeds a fire more heat and allows it to continue. With most types of fires, the old fire triangle model works well enough, but when the fire involves burning metals (known as a class-D fire in the American system of fire classifications, involving metals like lithium, magnesium, etc.), it becomes useful to consider the chemistry of combustion. Putting water on such a fire could result in the fire getting hotter (or even exploding) because such metals can react with water in an exothermic reaction to produce flammable hydrogen gas. Therefore, other specialized chemicals must typically be used to break the chain reaction of metallic combustion and stop the fire.