Archives - Chemistry: Page 3
Author: paul carson (Sat May 12, 2007 12:38 pm)
Title: CHEMISTRY
Chemistry
From earliest childhood, people are fascinated with the flickering yellow glow of candle flames and burning logs. However, few of us realize, even in adulthood, that soot, a material that epitomizes blackness, is behind that warm light. The 19th century physicist Michel Faraday put it well when he said, “You would hardly think that all those substance which fly about London for instance, in the form of soot and blacks, are the very beauty and life of the flame.” Indeed, if it weren’t for the presence of cloud of tiny soot particles within these fires, they would appear blue, like those on a well-operating gas stove, which give off considerably less visible and thermal radiation.
We can vividly illustrate this phenomenon in the laboratory. Ethylene, for example, burning in air yields plenty of soot and a bright yellow flame, whereas the same fuel diluted with nitrogen to suppress the formation of soot gives a much dimmer blue fame. The cerulean color arises from the highly excited products of the combustion reaction. These molecules emit light in discrete spectral bands that correspond to the excitation levels of their electrons. One of the primary molecules is of a class called radicals because they have unpaired electrons in their outer shells. This radical is made from a single carbon and a single hydrogen, denoted CH, which emits light at a wavelength of about 431 nanometers, squarely at the blue end of the visible spectrum.
In contrast, the light emitted from soot extends across the visible wavelength range and into the near-infrared. Cool soot looks black, but while it’s in flame, the heat liberated from the surrounding combustion reactions makes these particles incandesce like so many tiny light-bulb filaments. Gas molecules cannot absorb or emit such large amounts of energy across a range of wavelengths. As Faraday correctly surmised, it is the solid nature of soot that gives most flames their distinctive luminescent qualities.
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