Flame test

A flame test is a simple way to check if certain elements are present in a sample. Even though it is not very reliable by today’s standards, it used to be an important part of studying chemicals. When we look at flames, we can see different colors depending on what elements are burning. This happens because the energy from the fire makes the electrons in the atoms move, and when they settle back down, they give off light of specific colors.

The flame test carried out on a copper halide. The characteristic bluish-green color of the flame is due to the copper.

This idea is connected to how fireworks work and to a more advanced science called atomic emission spectroscopy. By watching the color of the flame, scientists can often guess which elements are in the sample. It’s a quick and easy test that helps us understand what’s inside different materials.

History

Robert Bunsen created the famous Bunsen burner in 1855. This tool was great for flame tests because its flame did not change the colors shown by the materials being tested. With the Bunsen burner and a special tool called a prism, scientists could look at the colors of different elements. In 1860, Bunsen and Gustav Kirchhoff saw new colors—sky-blue and dark red—which helped them find two new metals: caesium and rubidium. Today, people still use this simple method to teach students how to find metals in different samples.

Process

A flame test showing the presence of lithium

A flame test is a way to check for some elements by heating a sample and watching the color of the flame. The sample can be mixed with special liquid to make it easier to see. Different tools like wooden splints or cotton swabs can be used to hold the sample. It is important to be careful because some substances can catch fire or be harmful.

The color of the flame can change depending on the heat and air. Sometimes, a special glass is used to help see the colors better. However, this test has limits. Not all elements show a clear color, and some colors can be hard to see. The results can also depend on how the test is done.

Principle

Electron excitation

In flame tests, tiny particles called ions get heated up. When they heat up, they glow and then calm down, sending out tiny bits of light called photons. The amount of energy in these glowing particles and the light they send out is special to each element. This special glowing and light is what helps us know which element is present. This same glowing idea is used in other tests like flame emission spectroscopy, atomic emission spectroscopy, and flame photometry.

Common elements

Coloured flames of methanol solutions of different compounds, burning on cotton wool. From left to right: lithium chloride, strontium chloride, calcium chloride, sodium chloride, barium chloride, trimethyl borate, copper chloride, cesium chloride and potassium chloride.

Some common elements and their corresponding colors are:

Symbol	Name	Color
Al	Aluminium	Silver-white, in very high temperatures such as an electric arc, light blue
As	Arsenic	Blue
B	Boron	Bright green
Ba	Barium	Light apple green
Be	Beryllium	White
Bi	Bismuth	Azure blue
Ca	Calcium	Brick/orange red; light green as seen through blue glass.
Cd	Cadmium	Brick red
Ce	Cerium	Yellow
Co	Cobalt	Silvery white
Cr	Chromium	Silvery white
Cs	Caesium	Blue-violet
Cu(I)	Copper(I)	Blue-green
Cu(II)	Copper(II) (non-halide)	Green
Cu(II)	Copper(II) (halide)	Blue-green
Fe(II)	Iron(II)	Gold, when very hot such as an electric arc, bright blue, or green turning to orange-brown
Fe(III)	Iron(III)	Orange-brown
Ge	Germanium	Pale blue
H	Hydrogen	Pale blue
Hf	Hafnium	White
Hg	Mercury	Red
In	Indium	Indigo blue
K	Potassium	Lilac invisible through cobalt blue glass (purple)
Li	Lithium	Carmine red; invisible through green glass
Mg	Magnesium	Colorless due to magnesium oxide layer, but burning Mg metal gives an intense white
Mn(II)	Manganese(II)	Yellowish green
Mo	Molybdenum	Yellowish green
Na	Sodium	Bright yellow; Golden; invisible through cobalt blue glass. See also Sodium-vapor lamp
Nb	Niobium	Green or blue
Ni	Nickel	Colorless to silver-white
P	Phosphorus	Pale blue-green
Pb	Lead	Blue-white
Ra	Radium	Crimson red
Rb	Rubidium	Violet red
S	Sulfur	Blue
Sb	Antimony	Pale green
Sc	Scandium	Orange
Se	Selenium	Azure blue
Sn	Tin	Blue-white
Sr	Strontium	Crimson to scarlet red; yellowish through green glass and violet through blue cobalt glass
Ta	Tantalum	Blue
Te	Tellurium	Pale green
Ti	Titanium	Silver-white
Tl	Thallium	Pure green
V	Vanadium	Yellowish green
W	Tungsten	Green
Y	Yttrium	Carmine, crimson, or scarlet red
Zn	Zinc	Colorless to blue-green
Zr	Zirconium	Mild/dull red

History

Process

Principle

Common elements

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