Complete combustion occurs when all carbon atoms in a substance are consumed by a reaction with oxygen and heat, resulting in carbon dioxide and water. Incomplete combustion occurs when there is not enough oxygen, resulting in the release of gas and potentially dangerous carbon monoxide. Different materials have different ignition temperatures and burn thresholds, and the ratio of fuel to oxygen can affect the completeness of combustion. Propane is an example of a hydrocarbon commonly burned in households, and incomplete combustion can result in yellow flames and the release of carbon monoxide.
Complete combustion is a chemical reaction in which all carbon atoms in a particular substance are entirely consumed. “Combustion” is generally understood to mean “to burn”, although the chemical definition is usually much broader than just burning with flames or fire. Fires are certainly a form of combustion and can lead to the complete burning of wood and other materials. There are a number of other possibilities as well, though. From a scientific point of view, this type of reaction occurs whenever oxygen, heat and any type of carbon-containing fuel are present together. The carbon atoms bond to the oxygen atoms in such a way that they are exactly matched, and the heat triggers a conversion, usually to carbon dioxide and water, but this can depend on the chemical composition of the elements at the starting point.
If there isn’t enough oxygen in the atmosphere to match every carbon atom, combustion is usually incomplete, meaning that the conversion turns some of the material into gas, but not all of it. The amount of heat required for the reaction in either event usually depends on the fuel, since the temperature must be at or above that material’s burning threshold for the reaction to take place.
Understand combustion in general
Many materials, and anything that sustains life, are considered “carbon-based.” Carbon is an almost ubiquitous element, just like oxygen. Every carbon-containing compound has what’s called an “ignition temperature,” which is the temperature at which it will burn. Different compounds have different thresholds, but heat is always the first requirement. Sometimes this heat can be very low, such as that produced by friction when a match strikes a rough surface; in many cases it must be much higher, though.
Once the material is exposed to heat beyond its ignition temperature, the carbon atoms begin to rearrange themselves. They match the oxygen in the atmosphere and a small atomic-level reaction takes place which, when viewed from the outside, can often be quite dramatic. Sometimes the whole thing goes up in flames, or it might appear to melt or dissolve quickly; it may go up in smoke and sometimes it makes a popping or popping noise. A lot depends on the substances involved, as well as what else the oxidizing compound contains besides just carbon. These sub-elements are usually involved rather by default.
What makes a burn “complete”
When scientists talk about “complete” combustion, they are usually talking about an event where everything is consumed by the reaction. For this to happen, there must be enough oxygen available for each carbon atom in the compound to find a match or pair in the ambient air. Most of the time this isn’t a problem; the atmosphere usually contains a lot of oxygen. There is usually only a deficiency when the reaction occurs in a more unusual place; underground in a cave, for example, at a very high altitude, or in a laboratory where conditions are artificially controlled.
Alternative incomplete
The perfect ratio of carbon to oxygen is also referred to as stoichiometric or zero excess air burning. Combustion can still occur when the ratio is out of balance, but under these circumstances there is usually something left, which is that the entire compound does not convert to a gas or otherwise change shape. In most cases this is known as incomplete combustion. The process is the same up to the final point; the carbon and oxygen fit together as closely as possible, then leave the rest more or less untouched.
Propane as a model
Burning propane serves as an example of a hydrocarbon commonly burned in household use. Usually, propane combustion occurs when the gas in the air mixture is between 2.2% and 9.6%. This range is referred to as the “flammable limits” of propane. A properly functioning propane appliance that produces ideal combustion usually emits a blue flame.
Incomplete combustion of propane occurs when the mixing ratio is higher or lower than the ideal ratio, but still occurs within flammability limits. If the ratio of propane to air is less than the ideal ratio, a “lean burn” will occur, as evidenced by flames appearing to leap off the burner or go out. Alternatively, a “rich burn” occurs when the ratio of propane to air is greater than the ideal ratio and can be recognized by larger flames that are yellow rather than blue. Incomplete combustion of propane or other hydrocarbons typically releases carbon monoxide, an extremely serious environmental and health risk to humans and most animals. Carbon monoxide is odorless and cannot be detected without special sensors, but it is often lethal if inhaled for prolonged periods.
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