fire is the result of a chemical reaction
Answers
Answer:
Fire is the result of a chemical reaction called combustion. At a certain point in the combustion reaction, called the ignition point, flames are produced. Flames consist primarily of carbon dioxide, water vapour, oxygen, and nitrogen.
Explanation:
please follow me
Answer and Explanation:
The Chemistry and Physics of Fire
Fire is a manifestation of uncontrolled combustion. It involves combustible materials which are found around us in the buildings in which we live, work and play, as well as a wide range of gases, liquids and solids which are encountered in industry and commerce. They are commonly carbon-based, and may be referred to collectively as fuels in the context of this discussion. Despite the wide variety of these fuels in both their chemical and physical states, in fire they share features that are common to them all. Differences are encountered in the ease with which fire can be initiated (ignition), the rate with which fire can develop (flame spread), and the power that can be generated (rate of heat release), but as our understanding of the science of fire improves, we become better able to quantify and predict fire behaviour and apply our knowledge to fire safety in general. The purpose of this section is to review some of the underlying principles and provide guidance to an understanding of fire processes.
Basic Concepts
Combustible materials are all around us. Given the appropriate circumstances, they can be made to burn by subjecting them to an ignition source which is capable of initiating a self-sustaining reaction. In this process, the “fuel” reacts with oxygen from the air to release energy (heat), while being converted to products of combustion, some of which may be harmful. The mechanisms of ignition and burning need to be clearly understood.
Most everyday fires involve solid materials (e.g., wood, wood products and synthetic polymers), although gaseous and liquid fuels are not uncommon. A brief review of the combustion of gases and liquids is desirable before some of the basic concepts are discussed.
Diffusion and premixed flames
A flammable gas (e.g., propane, C3H8) can be burned in two ways: a stream or jet of gas from a pipe (cf. the simple Bunsen burner with the air inlet closed) can be ignited and will burn as a diffusion flame in which burning occurs in those regions where gaseous fuel and air mix by diffusive processes. Such a flame has a characteristic yellow luminosity, indicating the presence of minute soot particles formed as a result of incomplete combustion. Some of these will burn in the flame, but others will emerge from the flame tip to form smoke.
If the gas and air are intimately mixed before ignition, then premixed combustion will occur, provided that the gas/air mixture lies within a range of concentrations bounded by the lower and upper flammability limits.
Outside these limits, the mixture is non-flammable. (Note that a premixed flame is stabilized at the mouth of a Bunsen burner when the air inlet is open.) If a mixture is flammable, then it can be ignited by a small ignition source, such as an electrical spark. The stoichiometric mixture is the most readily ignited, in which the amount of oxygen present is in the correct proportion to burn all the fuel to carbon dioxide and water (see accompanying equation, below, in which nitrogen can be seen to be present in the same proportion as in air but does not take part in the reaction). Propane (C3H8) is the combustible material in this reaction:
C3H8 + 5O2 + 18.8N2 = 3CO2 + 4H2O + 18.8N2
Table 41.1 Lower and upper flammability limits in air
Lower flammability limit (% by volume)
Upper flammability limit (% by volume)
Carbon monoxide
12.5
74
Methane
5.0
15
Propane
2.1
9.5
n-Hexane
1.2
7.4
n-Decane
0.75
5.6
Methanol
6.7
36
Ethanol
3.3
19
Acetone
2.6
13
Benzene
1.3
7.9
An electrical discharge as small as 0.3 mJ is sufficient to ignite a stoichiometric propane/air mixture in the reaction illustrated. This represents a barely perceptible static spark, as experienced by someone who has walked across a synthetic carpet and touched a grounded object. Even smaller amounts of energy are required for certain reactive gases such as hydrogen, ethylene and ethyne. In pure oxygen (as in the reaction above, but with no nitrogen present as a diluent), even lower energies are sufficient.
The diffusion flame associated with a flow of gaseous fuel exemplifies the mode of burning that is observed when a liquid or solid fuel is undergoing flaming combustion. However, in this case, the flame is fed by fuel vapours generated at the surface of the condensed phase. The rate of supply of these vapours is coupled to their rate of burning in the diffusion flame. Energy is transferred from the flame to the surface, thus providing the energy necessary to produce the vapours. This is a simple evaporative process for liquid fuels, but for solids, enough energy must be provided to cause chemical decomposition of the fuel, breaking large polymeric molecules into smaller fragments which can vaporize and escape from the surface.