The phenomenon of metal flame tests provides a direct answer to the question of what metal burns purple. When certain metal compounds are introduced into a high-temperature flame, they emit light at specific wavelengths, a process that results in a distinct color visible to the human eye. The metal responsible for producing a purple or lilac-colored flame is Potassium, which is one of the alkali metals. This colorful display is a simple yet effective method used in chemistry to help identify the presence of this particular element in a sample.
The Element Responsible for Purple
Potassium (K) is the element that yields the characteristic purple flame, often described more specifically as lilac or violet. This alkali metal is typically observed in flame tests as a compound, such as potassium chloride (KCl). The color is produced by the potassium atoms themselves, meaning the negative ion does not affect the color observed.
Potassium is highly reactive, which is why it is usually handled as an ionic salt for this type of chemical analysis. The lilac color it generates is distinctive among the Group 1 metals, which include elements like Lithium (crimson red) and Sodium (intense yellow-orange). However, the pale purple color of potassium can be easily masked by the much brighter yellow emission of trace amounts of sodium contamination, making the purple difficult to see clearly without using a specialized filter.
How Metal Atoms Create Light
The vivid colors observed during a flame test are a result of atomic emission spectroscopy, a process involving the movement of electrons within the metal atoms. When a metal compound is subjected to the high heat of a flame, the energy absorbed causes their electrons to jump from their lowest-energy position (the ground state) to a higher-energy orbital (the excited state).
Since the excited state is unstable, the electron quickly falls back down to its original, lower-energy ground state. As the electron transitions back, it releases the excess energy it absorbed in the form of a photon, which is a particle of light. The specific amount of energy released corresponds directly to a unique wavelength of light. For potassium, this energy drop releases photons in the violet and red ends of the visible light spectrum, which combine to produce the lilac color.
Other Distinct Metal Colors
This principle of electron excitation allows chemists to use flame coloration as a qualitative test to identify unknown metal substances. The different colors are essentially a fingerprint for each element, and they are also the basis for the brilliant colors seen in fireworks displays. For instance, Strontium compounds produce a deep red flame, while Barium salts yield a pale green color.
Copper compounds can produce a blue-green flame, and the common element Sodium creates an extremely strong, persistent yellow-orange light. The range of colors, from the crimson red of Lithium to the brick red of Calcium, demonstrates the variety of energy transitions possible across the periodic table. Observing these distinct colors provides a direct, visible confirmation of the specific metal atoms present in a sample.