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What is the Flame Test?


The flame test is one of the most widely used analytical procedures in Chemistry. It is primarily used to observe and analyze the presence of certain elements in the given compound or salt. Generally, the flame test observes the occurrence of metal ions in a  compound. The flame test for every element is different as ions of each element have a specific feature based on their emission spectrum.

 

This difference is observed by the color of flames given out when the salt including the metal ion is burnt. It should be observed here that the emission spectrum of each element that discovers the flame color includes atoms rather than ions. The transformation of electrons in the ions has a tendency to produce the visible color lines which are observed in flame tests.

 

Read the article below to know flame test definition in Chemistry, how to do a flame test, flame test color and limitation of the flame test.

 

Flame Test Definition

The flame test is a method used by scientists to observe the occurrence of specific metals in a compound by the color they give to a flame. For example, the presence of Sodium turns the flame color to yellow.

 

How to do a Flame Test?

After knowing what a flame test is? You must be wondering, how to do a flame test? Let us now look at the methods of performing flame tests.

 

There are two methods to perform the flame test. These are:

  • Classic Wire Loop Method.

  • Wooden Split or Cotton Swab Method.

 

Here you will need a clean wire loop. You can use Platinum or nickel-chromium loops. Always clean these loops with hydrochloric acid or nitric acid by dipping in it . After that, rinse with distilled water. Also, test the cleanliness of the loop by inserting it into a gas flame.Remember that if prescribed colors are not produced then, it means loops are not sufficiently cleaned.The loop must be cleaned before the test.

 

The clean loop is dipped in either a powder or solution of a metallic salt. Place the loop with a sample in the blue part of the flame.After that note the observed color.

 

Let us now discuss each method in detail:

Classic Wire Loop Method: In Classic Wire Loop Method, you will require a wire loop. The most commonly used loops are platinum or nickel-chromium loops. These can be cleaned by immersing in nitric or hydrochloric acid, followed by washing with distilled or deionized water. Test the cleanliness of the loop by placing it into a gas flame. The loop may not be adequately cleaned if the burst is produced. The loop should be properly cleaned between tests.

 

The cleaned loop is immersed in either a powder or solution of an ionic (metal) salt. The loop along with a sample is placed in the blue or clear part of the flame and the resulting color is observed.

 

Wooden Split or Cotton Swab Method: Wooden or Cotton Swab’s method of conducting flame text provides a competitive alternative to wire loops. To use wooden splints, immerse them overnight in distilled water. Discharge the water and wash out the splints with clear water. Be cautious to prevent polluting the water with sodium. Hold a cotton swab or splint that has been soaked in water, immerse it in the sample that has to be tested and flush the splint or swab through the flame. Avoid holding the sample in the flame as this would cause the split or swab to catch fire. Each time use a new splint or swab for a new flame test.

 

Here you will need a clean wire loop. You can use Platinum or nickel-chromium loops. Always clean these loops with hydrochloric acid or nitric acid by dipping in it . After that, rinse with distilled water. Also, test the cleanliness of the loop by inserting it into a gas flame.Remember that if prescribed colors are not produced then, it means loops are not sufficiently cleaned.The loop must be cleaned before the test.

 

The clean loop is dipped in either a powder or solution of a metallic salt. Place the loop with a sample in the blue part of the flame.After that note the observed color.

 

How to Interpret Flame Test Results?

The sample is identified by the distinct observed color with the help of known values from a table or chart.

 

  • Red

Carmine to Magenta: Lithium compounds. Masked by barium or sodium.

 

Scarlet or Crimson: Strontium compounds. Masked by barium.

 

Red: Rubidium (unfiltered flame)

 

Yellow-Red: Calcium compounds. Masked by barium.

 

  • Yellow

Gold: Iron

 

Intense Yellow: Sodium compounds, even in trace amounts.Here the yellow flame is not the indicator of sodium.It can be indicative of it when 1% of NaCl is added to the dry compound.White

 

Bright White: Magnesium

 

White-Green: Zinc

 

  • Green

Emerald: Copper compounds, other than halides. Thallium.

 

Bright Green: Boron

 

Blue Green : Moistened phosphate with H2SO4 and B2O3

 

Faint Green: Antimony and NH4 compounds.

 

Yellow-Green: Barium, manganese(II), molybdenum.

 

  • Blue

Azure : Lead, selenium, bismuth, caesium, copper(I), CuCl2 and copper compounds. These are moistened with hydrochloric acid, indium and lead . 

 

Light Blue : Arsenic and its compounds

 

Greenish Blue: CuBr2, antimony

 

  • Purple

Violet : Generally Potassium compounds except for borates, phosphates and silicates.

 

Masked by sodium or lithium.

 

Lilac to Purple Red : Potassium , Rubidium and caesium when seen through blue glass in the presence of sodium.

 

Flame Test Color

The Flame test color table given below describes the color of each flame as precisely as possible. Generally, most of the metals produce green colors and you can also see various shades of red and blue. The feasible method to observe metal ions is to compare it to the set of standards (known as composition) to determine what color can be expected when using the fuel in a laboratory.

 

The flame test is not distinctive as multiple variables are included in it. Generally, only a single tool is available to observe the elements present in a compound. While performing a flame test, be cautious of any contamination of the fuel or loop with sodium which has a bright yellow color and hides other colors. Many fuels include sodium contamination. You may observe the flame test color with the help of the blue filter to take off any yellow color.

 

Here is the list of the common elements that are observed through the flame test. They have different emission spectrums that permit them to show a particular coloured flame in a flame test. However, the color given below of different elements is only guidance as different colors are described differently by different people performing a flame test.

 

Flame Test Color Table

S.No.

Element

Symbol

Color

1

Sodium

Na

Intense Yellow

2

Potassium

K

Intense Yellow Orange

3

Calcium

Ca

Orange Red

4

Lithium

Li

Red

5

Strontium

Sr

Red

6

Barium

Ba

Pale- Green

7

Copper

Cu

Blue- Green

8

Caesium

Cs

Blue

9

Iron

Fe

Gold 

10

Germanium

Ge

Pale Blue

11

Mercury

Hg

Red

12

Nickel

Ni

Silver - White

13

Hafnium

Hf

White

14

Phosphorus

P

Pale Bluish Green

15

Molybdenum 

Mo

Yellowish Green

16

Manganese

Mn

White

17

Tin

Sn

Blue - White

18

Zinc

Zn

colorless or Bluish Green

19

Lead

Pb

Bluish White

20

Zirconium

Zr

Light Red

21

Radium

Ra

Crimson

22

Chromium

Ch

Silver - White

23

Cadmium

Cd

Brick red

24

Arsenic

As

Blue

 

Limitation of the Flame Test

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

  • The ions will not be observed during the flame test as long as the concentration ions are minimum.

  • The intensity of the light changes from one sample to another. For example, yellow sodium emissions are much more intense during the flame test in comparison to the red litmus emission.

  • The flame test will be affected by the presence of contaminants or impurities. For example, sodium is generally present in most of the compounds and gives the yellow color to the flame. To avoid that, cobalt blue gas is used to filter out the yellow color of sodium.

  • The flame test cannot make a distinction between all elements. Most of the metals produce similar colors whereas some of the compounds do not change the color of the flame at all.

 

Flame Test Colors

This table lists the expected colors for elements in the flame test.

 

Here the name of colors are subjective. The best way to recognise coloured elements is to test known solutions so that you know what to expect.

Symbol

Element

Color

As

Arsenic

Blue

B

Boron

Bright green

Ba

Barium

Pale/Yellowish Green

Ca

Calcium pic

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

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

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FAQs (Frequently Asked Questions)

1. What is the Flame Test?

The flame test is used to identify any metal or metalloid in any of the compounds. It is based on the characteristic color produced when burning. The salt changes the color of the flame of a burner. The main reason for the color changing by the flame is that the heat of the flame excites the electrons of the metal ions. Every element has a specific emission spectrum that differentiates one element from another.

2. What are the limitations or disadvantages  of the flame test ? 

The test is not effective to detect low concentrations.

 

The brightness of the flame varies from one sample to another. As yellow light emission from sodium element is much greater than the red emission by the same amount of lithium.

 

Impurities affect the reports of the test. Particularly, sodium is present in many of the compounds and so will change the color of the flame.So sometimes a blue glass is also used to filter the yellow sodium.It is not very effective so other tests should be done to confirm the identity.

3. For what type of element flame test is not effective?

The test is unable to differentiate between all types of elements. Many metals do not produce a different flame color. Also, Some of the compounds do not change the color of the flame.

 

So due to these limitations, this flame test is generally used to identify in a sample a single element.

4. What is the hottest part of a Bunsen burner flame?

It depends on the factor that is fueling the bunsen burner. For natural gas flames, the hottest temperature will be at the boundary which is between a reducing flame and an oxidizing flame. Generally, that is the point where the pure blue flame is shifting to the yellow fully oxidized fuel flame.

5. What are the characteristics of a flame test ?

The flame test is a qualitative test of the analytical chemistry used to identify the presence of some specific elements in the compound.

 

During the flame, test heat provides the energy to the electrons causing them to emit light at a characteristic color which is also called the emission spectrum.

 

The flame test is a quick method to identify the presence of any element in a particular method. But also other tests should be done to confirm the composition of the elements.

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