What Is The Flame Test?

A flame test is a qualitative analysis used by the chemist to identify the metal and metalloid ion in the sample. Not all metal ions emit colour when heated in the gas burner. A flame test is the simplest way of identifying the presence of group 1 metal ions in the compound. For other metals, there are plenty of reliable techniques, but a flame test will give a better hint on where to look.

There are some safety techniques to perform the flame test in the laboratory.

  • Use chemical splash/impact goggles
  • Perform the flame test under the direction or supervision of chemistry teachers.
A flame test is a qualitative process for determining the particular metal ion, depending on the colour of the produced flame.

When the sample is heated, metal ions gain energy and shift from a lower energy level to a higher energy level. Ions are not stable at a high energy level, and they return to the ground with energy-release. The energy is released in the form of light and it varies from one metal ion to another. Thus, each metal ion gives a characteristic change of colour when it is heated.

Practical details to carry out the flame test

Platinum or nickel-chromium alloy (nichrome) wire.

  • Cleaning the wire is achieved by dipping it in the concentrated solution of HCl (hydrochloric acid) and burning it in the hot bunsen burner flame until the wire shows no colour in the flame.
  • The clean wire is either dipped into the powder or in the ionic metal salt solution, then the wire is heated in the bunsen burner flame.
  • Observe and record the flaming colour.

Flame-Test

Flame colours for common ions

Ions Flame colour
Lithium Red
Sodium strong, persistent orange (yellow)
Potassium lilac (pink)
Calcium orange-red
Rubidium red (red-violet)
Barium pale green
Lead grey-white
Copper blue-green (often with white flashes)
Strontium red
Barium pale green
Caesium blue/violet
Boron Bright green
Arsenic Blue
Iron Gold
Magnesium Bright white
Manganese (II) Yellow-green
Molybdenum Yellow-green
Phosphorous Pale blue-green
Antimony Pale green
Strontium Crimson
Selenium Bright blue
Thallium Bright green
Tellurium Pale green
Zinc Blue-green to pale green
Bismuth Azure
Cadmium Brick red
Cerium Yellow
Cobalt Silver-white
Chromium Silver-white
Mercury Red
Molybdenum Yellowish green
Tin Blue-white
Zirconium Mild red

Note:

  • A wooden Splint or Cotton Swab Method can also be used for the flame test.
  • Several elements like gold, silver, platinum and palladium do not produce characteristic colour during the flame test.

Limitations of the Flame Test

1. If the ion concentration is very low, then the ions are not detected by the flame test.

2. The light intensity varies from one sample to another. Think of the same amount of sodium and lithium, for example. Yellow sodium emissions during the flame test are much more intense than the red lithium emissions.

3. The test results will be influenced by the presence of impurities, especially sodium. It is present in most of the compounds and gives the yellow colour flame. To prevent that, cobalt blue glass is used. The yellow colour is removed and the flame colour associated with the other substance is visible.

BOOK

Free Class