The color of a flame is a result of combustion, the rapid oxidation of a fuel that releases energy as heat and light. To produce fire, a fuel source must be heated to its ignition temperature in the presence of an oxidizer, usually oxygen, initiating a self-sustaining exothermic reaction. The color visible to the human eye acts as an indicator, influenced both by the temperature of the reaction and the specific composition of the substances being burned.
The Standard Colors of Fire
The yellow and orange glow of a campfire or candle is not a result of the combustion reaction itself, but rather a physical phenomenon called blackbody radiation. This light originates from tiny, incandescent solid particles of unburnt carbon (soot) heated within the flame. Since combustion in these fires is often incomplete due to limited oxygen, these particles glow according to their temperature. They emit a continuous spectrum of light that peaks in the yellow-orange range, typically around 1,000 to 1,200 degrees Celsius.
A blue flame, such as one seen on a gas stove or Bunsen burner, indicates a hotter and more efficient reaction. This color results from complete combustion, where the fuel and oxygen are thoroughly mixed, minimizing soot formation. The blue light is produced by the spectral emission of excited molecules and molecular fragments, like the methylidyne radical (CH\), which emit light at specific, short wavelengths. These efficient flames can reach temperatures between 1,500 and 2,000 degrees Celsius.
The Specific Elements That Create Pink
A pink or crimson fire is created not through temperature variation or the burning of standard hydrocarbon fuels, but by introducing specific trace metallic elements. The most common element used to achieve this color is Lithium, typically in the form of a lithium salt like lithium chloride. Lithium’s unique atomic structure causes it to emit light in the deep red to crimson range, which is often perceived as pink or magenta when viewed against a less intense background flame.
Strontium also contributes to the red-pink spectrum, producing a bright crimson or scarlet flame. While strontium is commonly associated with the intense reds found in emergency flares and fireworks, its color can appear pinkish depending on its concentration and the presence of other elements. These elements are not naturally abundant in wood or common fuels, meaning a pink flame almost always signals the presence of a contaminant, such as treated wood, pyrotechnic compounds, or lithium from discarded battery components.
How Atomic Structure Determines Fire Color
The mechanism behind these unusual colors is distinct from the temperature-dependent glow of soot particles. When a metal-containing compound is introduced into a flame, the heat provides energy to the metal atoms. This thermal energy is absorbed by the atom’s electrons, causing them to temporarily jump to a higher energy level, an unstable state referred to as the excited state.
The electron immediately seeks to return to its original, lower-energy orbit, known as the ground state. To do this, the electron must release the excess energy it absorbed by emitting a tiny packet of light energy called a photon. The amount of energy released is quantified precisely by the distance the electron falls between energy levels.
Because every element possesses a unique arrangement of electron shells, the energy gap for an electron returning to the ground state is specific to that element. This unique energy value determines the wavelength of the photon emitted, which the human eye perceives as a distinct color. For example, specific electron transitions in the Lithium atom release photons with a wavelength of approximately 670 nanometers, creating the crimson-red light. This principle, known as atomic emission spectroscopy, means the color is an elemental fingerprint, allowing scientists to identify the atom by the exact light it emits.