Do Flame Tests Give Continuous Spectrum
Flame Tests: Meaning, Process, Interpretation of Results
Flame Tests: Do you know why the outer zone of a candle flame is light blue? The colour inside a candle flame varies from blue, yellow, to black. The colours in a flame do not correspond only to colour temperature. Gas excitations also play a major role in imparting colours to the flame. Soot is one of the major constituents in a burning flame with a diverse composition of carbon compounds. The variety of these compounds creates a continuous range of possible quantum states to which electrons can be excited. The colour of light emitted is the energy emitted by each electron returning to its original state. Ionic compounds too impart a specific colour when subjected to a flame. Let's understand how this test is carried out and what are the various flame colours of metals.
What is the Flame Test?
It is a qualitative test used in chemistry to help determine an unknown metal or metalloid ion of an ionic compound. In a flame test, when the compound is placed in the flame of a gas burner, a characteristic colour is given off by the compound visible to the naked eye.
How does Flame Tests Work?
When a particular salt is heated, atoms of an element get excited and the electrons corresponding to that particular element move from their ground state to higher energy levels. As they return to their ground state, they emit photons of very specific energy. This energy corresponds to particular wavelengths of light, and the colour of its flame reveals the hiding element.
For example, lithium and strontium produce a red flame; copper produces a blue flame, calcium brick red flame, and barium an apple green flame.
Similarly, when sodium chloride is subjected to a bunsen burner, some sodium ions regain their electrons to form neutral sodium atoms. The sodium atom in an unexcited state has the structure \(1{{\rm{s}}^2}2{{\rm{s}}^2}2{{\rm{p}}^2}3{{\rm{s}}^1}\). But the electrons get excited and jump over to \(3{{\rm{p}}^1}\) level within the flame. Sodium's familiar bright orange-yellow flame colour results from promoted electrons falling back from the \(3{{\rm{p}}^1}\) level to their normal \(3{{\rm{s}}^1}\) level.
How to Do the Flame Test?
Classic Wire Loop Method
In this method, platinum or nickel-chromium wires are made into loops. These loops are cleaned by dipping in nitric acid or hydrochloric acid, followed by rinsing with distilled or deionized water. The cleanliness of the loop is tested by inserting it into a gas flame. If coloured flames are produced, the loop is not sufficiently clean. The cleanliness step is repeated until there are no coloured flames produced.
Once the loop is clean, it is dipped in either a powder or solution of an ionic (metal) salt. The loop with the sample is placed in the blue part of the flame, and the resulting colour is observed.
Wooden Splint or Cotton Swab Method
The wooden splints or cotton swabs are an inexpensive alternative to the expensive wire loops. The wooden splints are soaked in water overnight. The splints are rinsed with clean water without contaminating it with sweat. A damp splint or cotton swab moistened in water is dipped in the sample to be tested. The splint or swab is waved through the flame. A new splint or swab is used for each test.
How to Interpret Flame Test Results?
The table below lists the expected colours for elements in the flame test.
| Symbol | Element | Color |
| \({\rm{As}}\) | Arsenic | Blue |
| \({\rm{B}}\) | Boron | Bright green |
| \({\rm{Ba}}\) | Barium | Apple green |
| \({\rm{Ca}}\) | Calcium | Brick red |
| \({\rm{Cs}}\) | Caesium | Blue |
| \({\rm{Cu}}\left( {\rm{I}} \right)\) | Copper(I) | Blue |
| \({\rm{Cu}}\left( {\rm{II}} \right)\) | Copper(II) non-halide | Green |
| \({\rm{Cu}}\left( {\rm{II}} \right)\) | Copper(II) halide | Blue-green |
| \({\rm{Fe}}\) | Iron | Gold |
| \({\rm{In}}\) | Indium | Blue |
| \({\rm{K}}\) | Potassium | Violet |
| \({\rm{Li}}\) | Lithium | Crimson red |
| \({\rm{Mn}}\left( {\rm{II}} \right)\) | Manganese(II) | Yellowish green |
| \({\rm{Mo}}\) | Molybdenum | Yellowish green |
| \({\rm{Na}}\) | Sodium | Yellow |
| \({\rm{P}}\) | Phosphorus | Pale bluish green |
| \({\rm{Pb}}\) | Lead | Blue |
| \({\rm{Rb}}\) | Rubidium | Red violet |
| \({\rm{Sb}}\) | Antimony | Pale green |
| \({\rm{Se}}\) | Selenium | Azure blue |
| \({\rm{Sr}}\) | Strontium | Crimson |
| \({\rm{Te}}\) | Tellurium | Pale green |
| \({\rm{Tl}}\) | Thallium | Pure green |
| \({\rm{Zn}}\) | Zinc | Bluish-green to Whitish green |
Limitations of the Flame Test
- The test fails to detect ions if present in low concentrations.
- The brightness of the coloured flame varies from one sample to another. For example, the red emission from lithium is less bright than the yellow emission for the same amount of sodium.
- Some elements such as Be, Mg doesn't give the flame test. This is because the electrons in beryllium and magnesium are too strongly bound to get excited by flame. Hence, these elements do not impart any colour to the flame.
- The presence of contaminants or impurities affects the test results. Sodium, present in sweat, is present in most compounds and will mask the actual colour of the flame. Blue glass is used to filter out the yellow sodium.
- The test fails to differentiate between all elements. This is because several metals impart the same colour to the flame. There are certain compounds that do not change the colour of the flame at all.
Because of these limitations, the flame test cannot be used to identify an element in a sample definitively. Other analytical procedures should be conducted in addition to this test.
Summary
The flame test is a diagnostic test used to identify the ions, mostly cations in an ionic compound. The cationic part of an ionic compound is the metal part; hence the flame test helps us identify the metals present in an ionic compound. The metal atoms on being excited, impart a definite colour to the Bunsen flame when the excited electrons return to their lower energy state. In this article, we learned the flame test concept, the principle behind it, and the different colours exhibited by the metal ions.
FAQs on Flame Tests
Q.1. How does the flame test work?
Ans: In a flame test, a sample of the element or compound is introduced to a hot, non-luminous flame, and the colour of the resulting flame is observed. When a particular salt is heated, atoms of an element get excited and the electrons corresponding to that particular element move from their ground state to higher energy levels. As they return to their ground state, they emit photons of very specific energy. This energy corresponds to particular wavelengths of light, and the colour of its flame reveals the hiding element.
Q.2. What is the main limitation of flame tests?
Ans: The test fails to detect ions if present in low concentrations. The brightness of the coloured flame varies from one sample to another. The test fails to differentiate between all elements. This is because several metals impart the same colour to the flame. Some elements such as Be, Mg doesn't give the flame test. This is because the electrons in beryllium and magnesium are too strongly bound to get excited by flame. Hence, these elements do not impart any colour to the flame.
Q.3. Why is hydrochloric acid used in the flame test?
Ans: In a flame test, concentrated \({\rm{HCl}}\) is used to convert the compounds into their metallic chlorides. This is because the metallic chlorides are very much volatile and can easily impart colour to the flame.
Q.4. Why is platinum used in the flame test?
Ans: A platinum wire is used in the flame test because it is unreactive and does not produce a colour in the flame, which will mask the presence of other metals.
Q.5. Do anions affect flame colour?
Ans: The anion can affect the result of the flame test. For example, a non-halide copper\(\left( {\rm{II}} \right)\) compound produces a green flame, while a copper\(\left( {\rm{II}} \right)\) halide yields a blue-green flame.
Study The Structure Of Flame Here
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