Fire is combustion, a rapid chemical reaction involving a fuel and an oxidizer, typically oxygen. This process releases significant amounts of energy in the forms of heat and light. Fire’s color is not uniform and varies considerably, providing insights into combustion conditions and materials.
The Role of Heat and Soot in Fire Color
The most common colors observed in everyday fires, such as red, orange, and yellow, primarily result from incandescence. This describes the light emitted by hot, glowing solid particles within the flame. As these particles heat up, they begin to radiate light, with the specific color emitted directly correlated to their temperature.
Cooler parts of a flame, often found at the outer edges or near the base where oxygen supply is more restricted, appear red. These red regions indicate lower temperatures, generally around 600 to 850 °C (1,112 to 1,562 °F). As the temperature within the flame increases, the incandescent particles emit light at shorter wavelengths, causing the color to progressively shift from red to orange, then to yellow, and eventually to white or pale blue at significantly higher temperatures.
The familiar yellow and orange hues of many common fires, like those from wood or candles, are largely due to unburnt carbon particles, known as soot. Soot forms when hydrocarbons within the fuel undergo incomplete combustion due to an insufficient supply of oxygen. These soot particles are heated to incandescence by the surrounding combustion reactions, causing them to glow brightly and produce the characteristic warm colors.
As these soot masses travel upward within the flame, they continue to react with oxygen, enhancing the flame’s yellow glow. If the soot is not fully consumed, it is released from the flame as visible smoke.
In contrast, a blue flame signifies complete combustion, where the fuel burns efficiently with an ample supply of oxygen. This efficient oxidation means there is less unburnt carbon available to form incandescent soot particles, resulting in a cleaner burn. Blue flames are hotter than yellow or red flames, with temperatures for hydrocarbon gases like LPG or natural gas reaching approximately 1,980°C (3,600°F). The blue color in these hotter, soot-less flames arises from the emission of excited molecular radicals, which emit light in the blue and green regions of the spectrum.
How Chemical Elements Create Distinct Fire Hues
Beyond the effects of temperature and soot, specific chemical elements within the burning material alter fire’s color. This phenomenon is distinct from incandescence and relies on the principle of atomic emission. When atoms of certain elements are heated to high temperatures within a flame, their electrons absorb energy and jump to higher energy levels.
Excited electrons are unstable and quickly fall back to their original, lower energy levels. As they return, they release absorbed energy as light at very specific wavelengths. Since each chemical element has a unique atomic structure, the energy differences between its electron shells are distinct, leading to a characteristic pattern of emitted light wavelengths and a unique flame color.
This principle is widely used in pyrotechnics to create the array of colors seen in fireworks. For example, compounds containing copper often produce green or blue hues in a flame. Copper(II) compounds can yield a green flame, while copper(I) may emit blue light, or a combination can result in blue-green.
Sodium, a common impurity in many fuels, is responsible for an orange or yellow flame, which can sometimes overpower other colors. Strontium salts impart a deep red or scarlet color, frequently seen in emergency flares and red fireworks. Lithium compounds produce a deep red flame.
Barium compounds produce a pale green flame. Potassium yields a lilac or light purple flame. This interplay of electron transitions allows chemists and pyrotechnicians to predict and control the colors of flames.