A flame test is a qualitative test applied to determine the identity of metals and their ions in compounds. This can be done by observing the colors of metal ions based on the emission spectrum. On heating, every metal ion has its unique color at a specific wavelength.
When metals are heated on flame, their electrons jump to higher energy levels. On returning to their original energy level, they emit energy that can be seen in the form of colors by the naked eye. Based on their colors, metal ions can be easily identified in a compound. Some of them do not show colors because they are colorless.
How to perform a flame test?
These are the techniques to perform a flame test:
Classical technique
In the classical technique, a wire is used to test the solution of the sample. The wire is made up of platinum or nickel-chromium known as nichrome wire. There is a circle or loop at the end of the nichrome wire. This loop is used to hold the sample in it.
The first step is the cleaning of nichrome wire which is achieved by using hydrochloric acid. The wire is dipped in the acid and then rinse with the distilled water. To check whether the wire is cleaned or not, it can be placed on the bunsen burner. If the wire show colors, this means it has impurities of metals which can be removed by washing again. If there is no color, then it is ready for the next steps.
In the next step, the clean loop of wire is dipped in the solution or powder of the sample. Then it is placed on the blue flame of a bunsen burner. The color of the sample is observed which is an indication of the presence of a specific metal ion.
For better results, it is good practice to use different nichrome wires for the next test. This avoids the contamination of other metals.
Alternative techniques
These are the alternative methods for performing flame tests:
Wooden splints
They are used to perform the flame test. They are about 6 inches long strips made up of wood. These are easily available and are inexpensive.
Before using the wooden splints for the test, they are soaked in distilled water. This process continues for 12 hours minimum. Later, splints are removed from the distilled water and rinsed. Sometimes, sweat from hands contaminates the splints. That’s why it should be used carefully for the flame tests.
Cotton Swabs
Cotton swab is another way to do the flame test. It is very cheap and easy to perform in practical laboratories. It can be moistened by using distilled water. After that, it is dipped in the sample and then placed on the flame. When using the cotton swab for the flame test, it should be cleaned.
Contaminants in the flame test
When performing the flame test to determine the presence of metal ions in the sample. There are substances responsible for the contamination of flame tests.
These are the sources of contaminants:
- Sodium metal ions (mix the yellow color with other metals)
- Sweat from hands (contain several metals)
- Dust impurities
- Organic materials
- Unwashed nichrome wire
The major contaminant is sodium metal. It gives a yellow color flame that mask the colors of other metals. Cobalt blue glass is used to minimize the yellow color of sodium. In this way, minimization of the yellow color of the flame allows us to see other colors of the metal.
Flame test colors
These are the metals that give the flame test and show colors.
First list of flame test colors
| Element | Symbol | Color |
| Arsenic | As | Blue |
| Aluminium | Al | Silver white |
| Barium | Ba | Pale green |
| Boron | B | Bright green |
| Beryllium | Be | White |
| Bismuth | Bi | Azure |
| Carbon | C | Light orange |
| Calcium | Ca | Light green |
| Cadmium | Cd | Brick red |
| Cerium | Ce | Yellow |
| Cobalt | Co | Silver white |
| Chromium | Cr | Silver white |
| Cesium | Cs | Blue violet |
| Copper (l) | Cu (l) | Bluish green |
| Copper (ll) | Cu (ll) halide | Blue green |
| Copper (ll) | Non-halide | Green |
| Germanium | Ge | Pale blue |
| Iron (ll) | Fe (ll) | Gold, blue, green, orange-brown |
| Iron (lll) | Fe (lll) | Orange-brown |
| Rubidium | Rb | Red-violet |
| Scandium | Sc | Orange |
| Tin | Sn | Blue-white |
Second List of flame test colors
| Element | Symbol | Color |
| Hydrogen | H | Pale blue |
| Hafnium | Hf | White |
| Mercury | Hg | Red |
| Indium | In | Indigo/blue |
| Potassium | K | Lilac |
| Lithium | Li | Carmine red |
| Magnesium | Mg | White |
| Manganese (ll) | Mn (ll) | Yellowish green |
| Molybdenum | Mo | Yellowish green |
| Sodium | Na | Yellow |
| Niobium | Nb | Green, blue |
| Nickel | Ni | Silver-white |
| Phosphorus | P | Pale green |
| Lead | Pb | Blue, white |
| Radium | Ra | Crimson |
| Vanadium | V | Yellowish green |
| Tungsten | W | Green |
| Zinc | Zn | Colorless or bluish green |
| Zirconium | Zr | Mile red |
| Antimony | Sb | Pale green |
| Selenium | Se | Azure |
| Yttrium | Y | Carmine |
Compounds show competitive colors and negative flame tests
There are some compounds that are similar in colors on flame called competitive colors compounds. They are very hard to identify. Some compounds do not give a flame test known as a negative flame test.
Competitive color compounds
- Calcium nitrate (Ca(NO3)2)
- Calcium sulfate (CaSO4)
- Calcium carbonate (CaCO3)
- Ammonium chloride (NH4Cl)
- Sodium acetate (NaCH3COO)
- Sodium chloride (NaCl)
- Sodium hydrogen carbonate (NaHCO3 )
- Sodium carbonate (Na2CO3)
- Lithium chloride (LiCl)
- Potassium chloride (KCl)
- Boric acid (H3BO3)
Negative flame test compounds
- Magnesium sulfate (MgSO4)
- Glucose
- Sucrose
- Cornstarch
Limitations of the flame test
It is the best method to identify the metal and their ions but there are some limitations:
- Flame test doesn’t detect the low concentration metal ions in the sample.
- Sodium and impurities mask the colors of other metals that cannot be seen without cobalt blue glass.
- Some metals give the same colors that why they cannot be distinguished.
- Different metal shows different color brightness. Some metals have less brightness and others have more brightness.
- Bulk compounds of metals don’t give better results.
Key takeaways
Concepts Berg
How to do the flame test?
It can be done by using nichrome wire. The loop of the wire dip into the solution of sample or powder then placed on the flame of a bunsen burner. This results in a color of metal ion which can be observed and identify the type of metal.
How to interpret flame test results?
The results of the flame test can be tested and verified by using the colors of the metal ions. Different metal ions have different colors.
What is the purpose of the flame test?
The purpose of the flame test is to determine the type of metal ions in the given sample by heating.
What is a flame test in chemistry?
When metals are heated, the electrons jump to a higher energy level. When they come back to their original energy level, they emit energy which can be seen in the form of colors.
Would a flame test be useful for detecting?
Yes, it is a useful method for the detection of metal ions in the given sample.
What are some applications of the flame test?
It is applicable for the determination of metal and metalloid ions in ionic compounds.
How do make an experiment using flame tests more accurate?
When performing the flame test, it should be free from contamination. It can be achieved by washing the wire with hydrochloric acid. The removal of sodium ions from the wire.
Why is HCl used in a flame test?
It is a strong acid and is used to clean the nichrome wire from contaminants like sodium ions.
Reference Books
- Chemical Reactions By Denise Walker
