What is the flame test? 

The flame test is a qualitative analysis which is the most widely used analytical procedure in Chemistry. The main use of flame tests is to observe and identify the presence of certain elements in the given compound or salt. When the salts are heated or placed on the gas burner, all metal ions will not emit colour. Generally, the flame test involves the occurrence of metal ions in a given compound.  

When salts are heated in the presence of a gas burner, metal ions containing electrons gain energy and excite from a lower energy level to a higher energy level. The metal ions are less stable at high energy levels, and ions will return to the ground state by releasing energy. This is also known as light energy. Hence every metal ion produces different colours in the form of light.  The shifting of electrons in the ions at energy levels tends to produce the visible colour lines observed in flame tests.

Diagram of flame test experiment

In the given diagram, a gas burner is used for heating purposes, and platinum wire with a sample is used to test the colour of the metal cations. 

How to do a flame Test?

To carry out the flame test, there are two different methods to perform the flame test. These are

1. Classic Wire Loop Method
2. Wooden Split or Cotton Swab Method

Let us discuss these methods in detail.

Classic Wire Loop Method: 

First, let us take an apparatus, a clean wire loop, which is required to perform this method. The most common loops are made up of platinum or nickel-chromium. 

Dipping wire loops must clean these in acids such as hydrochloric or nitric acid. And. then rinse with distilled water. Now, test the cleanliness of the loop by inserting it into a gas flame. If a burst of colour is produced, it indicates that the loop is not sufficiently clean. The loop must be cleaned before proceeding to the flame test.

The clean loop needs to be dipped in powder form or a metallic salt solution. The loop should be placed with a sample in the blue part of the flame and observe the colours of the flame.

Wooden Splint or Cotton Swab Method

Wooden splints or cotton swabs are used as an inexpensive alternative source to wire loops. To perform this method,  wooden splints are taken in a beaker and soaked overnight in distilled water. Remove or pour out the water from the beaker and again rinse the splints with clean water. Be cautious to prevent polluting the water with sodium. 

Now, take a damp splint or cotton swab that has been moistened in water, and dip it in the sample or metallic salt to be tested.  Flush the splint or swab through the flame. Do not hold the sample in the flame, as this would cause the splint or swab to ignite. Use a new splint or swab for each test.

What are the flame test colours?

When a sample is placed on flame, it gives distinct colours. The colours are produced depending upon the type of salt taken.  The flame test colour table below describes each flame’s colour as precisely as possible.

Here is the table list of the common elements observed in the flame test. Each element has different emission spectrums that permit them to show a particular coloured flame in a flame test. The names of the colours are subjective, so the best way to learn to recognise close-coloured elements is to test known solutions so you know what to expect.

Flame test colour table: 

Symbol	Element	Color
As	Arsenic	Blue
B	Boron	Bright green
Ba	Barium	Pale/Yellowish Green
Ca	Calcium	Orange to red
Cs	Cesium	Blue
Cu(I	Copper(I)	Blue
Cu(II)	Copper(II) non-halide	Green
Cu(II)	Copper(II) halide	Blue-green
Fe	Iron	Gold
In	Indium	Blue
K	Potassium	Lilac to red
Li	Lithium	Magenta to carmine
Mg	Magnesium	Bright white
Mn(II)	Manganese(II)	Yellowish green
Mo	Molybdenum	Yellowish green
Na	Sodium	Intense yellow
P	Phosphorus	Pale bluish green
Pb	Lead	Pale blue
Rb	Rubidium	Red to purple-red
Sb	Antimony	Pale green
Se	Selenium	Azure blue
Sr	Strontium	Crimson
Te	Tellurium	Pale green
Tl	Thallium	Pure green
Zn	Zinc	Bluish green to whitish green

Source: Lange’s Handbook of Chemistry, 8th Edition, Handbook Publishers Inc., 1952.

What are the limitations of the flame test?

Some of the limitations of the flame test are given below:

• The flame test is always useful for identifying highly concentrated ions and cannot detect low concentrations of ions.

• The brightness or colour intensity will vary from one sample to another. For example, the yellow emission from sodium is much brighter than the red emission from the same amount of lithium.

• Platinum wire must always be cleaned to perform the flame test. If Impurities or contaminants are present on the surface of platinum wire, it will affect the test results. Sodium, in particular, is present in most compounds and will colour the flame. Sometimes a blue glass is used to filter out the yellow sodium.

• The flame test cannot distinguish between all elements. Several metal ions produce the same flame colour. Some metal ions do not change the color of the flame at all.

Flame Test Experiment

Experiment Title –  Systematic Analysis of Cations – Flame Test

Experiment Description – 

Flame test: In this virtual lab experiment, let us learn the use of flame tests to identify the presence of certain cations in a salt. 

Aim of Experiment – To perform flame tests and identify the presence of certain cations

Apparatus & Material Required – 

Sample salts 
Conc. HCl
Platinum wire
Pencil 
Watch glass
Glass rod
Beaker
Burner
Dropper 
Cobalt glass 

Procedure – 

• Make a circular loop at the end of the platinum wire using the pencil.
• Dip the platinum wire in concentrated hydrochloric acid (HCl), taken in a  beaker. 
• Hold it over the flame until the loop emits a colourless flame.
• To the salt sample taken in a watch glass, add 2-3 drops of concentrated hydrochloric acid (HCl) using a dropper.
• Make a paste by mixing the salt and concentrated hydrochloric acid (HCl).
• Dip the cleaned platinum wire into the sample paste and hold it to the flame.
• Observe the colour of the flame.
• Hold the blue glass (cobalt glass) in front of the flame and observe the colour of the flame.
• Refer to the inference table and report the possible cation present.
• Repeat the experiment for each of the given samples. 

Precautions – 

• Make sure to clean the platinum wire every time before use.
• Wear appropriate protective gear like a lab coat, goggles, etc., while performing a flame test.
• Handle conc. HCl with gloves, as it is corrosive

FAQs on Flame Test Experiment