Triangle of Fire

Fire is the combination of the three elements which consists of Oxygen, Fuel and Heat.  If some elements of fuel, oxygen and heat are available in any condition they can not produce Fire, Fire can be produced in only that condition when these three elements will be combined in a specific conditions.

Chemistry

Fires start when a flammable and/or a combustible material, in combination with a sufficient quantity of an oxidizer such as oxygen gas or another oxygen-rich compound (though non-oxygen oxidizers exist that can replace oxygen), is exposed to a source of heat or ambient temperature above the flash point for the fuel/oxidizer mix, and is able to sustain a rate of rapid oxidation that produces a chain reaction. This is commonly called the fire tetrahedron. Fire cannot exist without all of these elements in place and in the right proportions. For example, a flammable liquid will start burning only if the fuel and oxygen are in the right proportions. Some fuel-oxygen mixes may require a catalyst, a substance that is not directly involved in any chemical reaction during combustion, but which enables the reactants to combust more readily.

Once ignited, a chain reaction must take place whereby fires can sustain their own heat by the further release of heat energy in the process of combustion and may propagate, provided there is a continuous supply of an oxidizer and fuel.

If the oxidizer is oxygen from the surrounding air, the presence of a force of gravity, or of some similar force caused by acceleration, is necessary to produce convection, which removes combustion products and brings a supply of oxygen to the fire. Without gravity, a fire rapidly surrounds itself with its own combustion products and non-oxidizing gases from the air, which exclude oxygen and extinguish it. Because of this, the risk of fire in a spacecraft is small when it is coasting in inertial flight. Of course, this does not apply if oxygen is supplied to the fire by some process other than thermal convection.

Fire can be extinguished by removing any one of the elements of the fire tetrahedron. Consider a natural gas flame, such as from a stovetop burner. 


Let's explain what fire is... 

Fire is a chemical process. Three things are needed for this process: oxygen, heat and fuel. Without one of these elements a fire cannot start or continue.  In a chemical process, the molecules rearrange themselves. Energy is either released or absorbed. The process in a fire is called oxidation, where oxygen atoms combine with hydrogen and carbon to form water and carbon dioxide. Oxidation is the same chemical process that turns iron into rust. But with iron, the reaction is VERY slow. So, the heat energy that is released is VERY low. With certain things, like paper or wood, the oxidation rate of the molecules can be very fast. If the heat cannot be released faster than it is created, then combustion happens.Besides heat, there must also be flames or smoldering present during the chemical process for it to be called fire. Exhaust gases also are produced. If the burning process is very clean, you don't see the exhaust gases. If some of the particles of the fuel are not completely burned, you see smoke. Smoke is made up of evaporated water, carbon dioxide and unburnt particles of the fuel.