When metals are heated to high temperatures, the color of the flames they produce can vary widely depending on the type of metal. Some metals burn with the typical orange-yellow flames we expect, while others burn with more unusual colored flames. One of the more striking flame colors some metals can produce is a vivid pink.
What Causes the Different Flame Colors in Metals?
The color of a flame is dependent on which wavelengths of light are emitted by the heated metal atoms. When metals are heated, their electrons become excited and jump to higher energy levels. As the electrons drop back down to lower energy levels, photons are released in the form of visible light. The specific wavelengths of these photons determine the color we see.
For example, sodium emits a strong yellow light as it burns, caused by electrons transitioning between specific energy levels. Copper emits both blue and green light as different transitions occur. Each metal has its own unique set of energy levels, leading to signature flame colors.
Why Do Some Metals Burn Pink?
In metals that burn with pink flames, the dominant wavelengths of light emitted are in the red-orange and violet range. The combination of these wavelengths is perceived by our eyes as pink. The metals that exhibit strong pink flames are those that emit light strongly in the 600-630 nm range (red-orange) and 400-450 nm range (violet).
Metals with Pink Flames
Let’s take a look at some of the most prominent metallic elements that can burn with pink or purple-hued flames:
Potassium
Potassium is an alkali metal that reacts vigorously when heated in air. It ignites easily and burns with a neon pink or lilac flame. The purple color arises from electrons transitioning between potassium’s two lowest energy levels, which emit wavelengths around 404 nm and 770 nm.
Rubidium
Rubidium is another reactive alkali metal in the same column of the periodic table as potassium. When burned, it emits a deep crimson red flame. Its flame spectrum has strong emission lines around 420 nm and 680 nm, combining to look dark pink.
Copper
Copper is a transition metal known for its blue-green flame color. However, when burned in the vapor state or as nanoparticles, copper can also produce purple, violet, or pink flames. This is caused by transitions from its 3d energy levels, which emit wavelengths between 500-580 nm.
Indium
Indium is a post-transition metal that burns with a bright lavender or pink color. This is caused by indium’s dominant wavelength emissions between 410-450 nm in the violet range and 600-650 nm in the red-orange range.
Thallium
Thallium is a toxic post-transition metal that emits green spectral lines near 535 nm. But it also has emission lines in the red and violet that combined look pink to our eyes. When thallium burns, it produces vibrant pink and magenta flames.
Calcium
Calcium is a reactive alkaline earth metal that burns with an intense flame color ranging from orange-red to pink or purple on the edges. This is caused by calcium’s line emissions in the red at 612 nm and violet at 423 nm. Adding calcium to other metal flames can turn them pink.
Why These Metals Burn Pink
Looking at these metallic elements that exhibit pink flames, we see some commonalities. Alkali metals like potassium and rubidium tend to burn with particularly strong, vivid pink or purple colors. Their electron configurations lead to dominant emissions in the key red and violet wavelengths.
The post-transition metals indium and thallium also follow this trend, with electrons in s and p orbitals leading to strong spectral lines in the pink and purple ranges.
For the transition metal copper, pink flames are only achieved under special conditions where the energy level transitions responsible for the red and blue-green colors are not present.
The alkaline earth metal calcium bridges the transition metals and alkali metals, with similar flame color chemistry. Adding calcium is a common technique for turning flames pink.
Why are emission spectra so important?
Emission spectra are important because they allow us to identify elements based on the specific wavelengths of light they emit. This is how we know that the pink color comes from key wavelengths around 400 nm and 600 nm. Analyzing flame emission spectra using spectroscopes led to the discoveries of rubidium and cesium in the 1860s. Emission spectra continue to be a vital tool in chemistry today.
How Different Chemical Forms Affect Flame Color
While the intrinsic electronic structure of an element determines its emission spectra, the exact color we perceive can vary slightly depending on the chemical form of the sample. Here are some ways the chemical state influences flame color:
Metal Salts vs Elemental Metals
Metallic salts containing pink-burning metals often burn with more vivid and intense pink hues than the pure elemental metals. For example, potassium chloride burns brighter purple than elemental potassium metal. Salts are easier to ignite and their ions may promote more energetic transitions.
Chlorides vs Other Salts
Chloride salts of purple-burning metals tend to enhance pink flames more than other anions like sulfates or nitrates. Alkali metal chlorides like potassium chloride and rubidium chloride burn with particularly brilliant purple and magenta flames. The chloride may promote energy transitions ideal for pink colors.
Nanoparticles vs Bulk Metals
When ignited as nanoparticles, many metals exhibit different flame colors than their bulk forms. Copper nanoparticles burn purple-pink whereas bulk copper burns its signature green-blue. At the nano-scale, the arrangement of energy levels changes, altering the emission spectra.
Organometallic Compounds
Complexes where metals are bonded to organic ligands can also burn with distinctive colors. Indium and thallium bonded to acetylacetonates create vivid pink combustions. The ligands influence which metal orbitals are involved in photon emission, changing the spectra.
Metal | Flame Color – Bulk Metal | Flame Color – Salt/Compound |
---|---|---|
Potassium | Lilac | Vivid magenta (KCl) |
Rubidium | Red | Deep crimson (RbCl) |
Copper | Green-blue | Pink-purple (nanoparticles) |
How Flame Color Depends on Temperature
The color emitted by burning metals also depends greatly on the temperature. Hotter temperatures promote electrons into higher energy states before they drop down, emitting higher energy photons.
For example, copper burns red, orange, green, and finally blue as the temperature increases. Lower temperatures only allow transitions from lower levels.
Most pink-burning metals will transition through red and orange before reaching the critical violet wavelengths needed for pink. Rubidium transitions from red to purple as temperature rises.
This is why the inner cone of a hot Bunsen burner flame needs to be used to observe pink flames. The outer edges are not hot enough. It also explains why pink colors are often enhanced by adding chlorides or nanoparticles, which increase flame temperatures.
Why are higher temperatures needed for purple flames?
Higher temperatures are critical for pink flames because reaching the Violet end of the visible spectrum requires exciting electrons into very high energy levels. The photons emitted as the electrons drop down from these levels are extremely high frequency and high energy.
The key ~400 nm violet wavelengths can only be achieved if metals are heated to temperatures of 700°C or above. Many alkali metals don’t emit violet wavelengths until around 1000°C. High temperatures provide the energy needed to promote the essential electronic transitions.
Using Pink Flame Tests to Identify Metals
The pink and purple colors produced by certain heated metals can be used as a simple analytical test to detect their presence. This forms the basis of the flame test.
Here are some key tips for identifying pink-burning metals using flame tests:
Sample Preparation
– For solid metals, create a small wire loop out of clean platinum or stainless steel wire. Dip the loop into solid powdered sample.
– For aqueous solutions, dip a platinum loop into the solution, or suck a small amount into a dropper and allow the liquid to dry on the loop.
– Chloride salts will produce the most vivid pink colors. To test unknown salts, first dissolve in water and test the solution.
Flame Test Procedure
– Heat the loop with sample in the hot, blue inner cone of a Bunsen burner flame.
– Observe the color in the regions where the sample meets the flame. Pink colors will often occur at the edges or in bursts.
– If needed, use cobalt glass to filter out yellow and view only the pink emission color.
– Compare results to known standards to identify the metal ions present.
– A spectroscope can also be used to more precisely analyze the emission wavelengths.
Metals that Produce Pink Flame Test Results
– Alkali metals potassium, rubidium, and cesium
– Alkaline earth metals calcium and strontium
– Post-transition metals indium and thallium
– Transition metals copper (sometimes) and manganese
Photographing Metals with Pink Flames
Capturing images of the brilliant pink and purple flames requires special photography techniques. Here are some tips:
Camera Settings
– Use a DSLR in manual mode for full control over exposure.
– Select daylight white balance to preserve flame colors accurately.
– Use a fast shutter speed (1/200 s or higher) to freeze motion.
– Open the aperture wide (f/3.5 or wider) for brightness.
– Increase the ISO to maintain fast shutter speed. 800-1600 ISO range is often needed.
Set Up
– Shoot in a very dark room or at night outdoors to maximize flame brightness.
– Position the burning metal diagonal in the frame against a black background.
– Backlight droplets with a flashlight to bring out pink colors.
Capture the Moment
– Focus precisely on the inner cone of the flame where colors will emerge.
– Shoot in burst mode to capture the perfect moment. Colors are fleeting.
– Catch droplets of burning metal in the air for dramatic pink flare effects.
Conclusion
The rare sight of vivid pink and purple flames can be produced by heating certain reactive metals to high temperatures. The pink color arises from strong spectral lines emitted in the red-orange and violet wavelength ranges. Alkali metals like potassium and rubidium are prized for their brilliant fuchsia flames when burned as chlorides. More unusual metals like indium and thallium also burn pink when heated. Photographing these striking pink colors requires patience and the right camera settings to catch the transient moments when the metals flare the elusive pink hues. These pink flame tests allow us to identify metals based on their unique emission spectra.