In the exothermic chemical process of combustion, fire is the fast oxidation of a substance, generating heat, light, and other reaction products. The conversion of the weak double bond in molecular oxygen, O2, to the stronger bonds in the combustion products carbon dioxide and water releases energy (418 kJ per 32 g of O2), making fire hot. The bond energies of the fuel play only a tiny part in this. Flames are created at a certain moment in the combustion reaction termed the ignition point. The visible part of the fire is the flame. Carbon dioxide, water vapor, oxygen, and nitrogen are the primary components of flames. The gases may get ionized and generate plasma if heated sufficiently. The color of the flame and the strength of the fire will vary depending on the things alight and any contaminants outside.

Conflagration is the most frequent type of fire, and it has the ability to cause bodily harm via burning. Fire is a significant phenomenon that has an impact on ecological systems all around the world. Fire has a number of beneficial benefits, including promoting development and sustaining diverse ecological systems. Its negative consequences include danger to life and property, pollution of the atmosphere, and poisoning of the water supply. Heavy rains may exacerbate soil erosion by water if a fire destroys protecting plants. Furthermore, when vegetation is burned, the nitrogen it contains is released into the atmosphere, as opposed to components like potassium and phosphorus, which remain in the ash and are swiftly recycled back into the soil. The loss of nitrogen caused by a fire reduces soil fertility over time, but this fertility can be recovered as molecular nitrogen in the atmosphere is “fixed” and converted to ammonia by natural phenomena such as lightning and “nitrogen-fixing” leguminous plants such as clover, peas, and green beans.

Humans have utilized fire in ceremonies, agriculture for land clearing, cooking, creating heat and light, signaling, propulsion, smelting, forging, trash incineration, cremation, and as a weapon or means of destruction.

Fires start when a flammable or combustible material, in combination with a sufficient amount of an oxidizer such as oxygen gas or another oxygen-rich compound (though non-oxygen oxidizers exist), is exposed to a source of heat or ambient temperature above the flashpoint for the fuel/oxidizer mix and is able to sustain a rate of rapid oxidation that results in a chain reaction. The fire tetrahedron is a popular name for this shape. Without all of these components in situ and in the proper quantities, fire cannot exist. A flammable liquid, for example, will only begin to burn if the fuel and oxygen are in the proper quantities. Some fuel-oxygen mixtures may need the use of a catalyst, which is a material that is not burned in any chemical process during combustion but facilitates the burning of the reactants.

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