How Flames Get Their Color
Flames exhibit a spectrum of colors due to the physics of light emission. One reason for flame color is blackbody radiation, where hot objects like tiny soot particles emit light. As these particles heat up, they glow, and the peak wavelength of the light they emit shifts from red to orange, then yellow, and eventually toward bluer hues at extremely high temperatures. This process is similar to how a heated metal bar changes color.
Another mechanism involves the excitation and subsequent de-excitation of atoms and molecules. When specific atoms or molecular radicals, like those containing carbon (C2) or hydrogen (CH), absorb energy within the flame, their electrons jump to higher energy levels. As these excited electrons fall back to their original states, they release this energy as photons of light. This emitted light has specific wavelengths, corresponding to distinct colors like the blue in a gas burner flame.
The Link Between Color and Heat
The color of a flame indicates its temperature. Blue flames signify more complete combustion and higher temperatures than yellow or orange flames. In these hotter blue flames, fuel burns efficiently, producing minimal soot particles. The light comes from excited molecular species, like carbon monoxide or C2 radicals, which glow blue at these elevated temperatures.
Conversely, yellow, orange, or red flames indicate incomplete combustion and lower temperatures. These flames contain many unburnt soot particles. The visible light is primarily due to blackbody radiation from glowing soot particles. For instance, the yellow tip of a candle flame is cooler than the blue base, demonstrating this.
What Else Affects Flame Appearance
Beyond temperature, other factors influence a flame’s visual appearance. The type of fuel being burned plays a role, as different chemical compositions introduce unique colors. For example, burning wood might yield orange and yellow flames primarily due to soot, while the presence of certain metal ions, like sodium, can produce an orange color.
The supply of oxygen is another important factor for flame color and efficiency. Ample oxygen promotes complete combustion, leading to hotter, bluer flames with less soot production. In contrast, a restricted oxygen supply results in incomplete combustion, generating more soot and producing cooler, yellower or orange flames. This is why a well-adjusted gas stove burner shows a blue flame, indicating efficient burning.
Impurities or trace elements present in the fuel or surrounding air can also alter a flame’s color. These contaminants, even in small amounts, can emit light at distinct wavelengths when heated, overriding typical combustion colors. This principle is utilized in fireworks, where specific metal salts are added to create displays of green, red, or purple flames.