- A flame test is performed by introducing a sample into the blue flame of a bunsen burner and noting any change in the colour of the flame.
- Flame tests can be used to detect the presence of some metallic elements in salts.
- The heat of the bunsen burner's flame causes electrons in the element to move from their lower energy ground state to a higher energy excited state.
When these excited electrons fall back to their lower energy levels, they emit energy in the form of light of a characteristic wavelength which is the colour of the flame we observe.
- Flame tests are an example of a qualitative test, that is, they can detect the presence of certain elements.
Flame tests are not quantitative tests, they can not tell us how much of the element is present in the sample.
Performing a Flame Test
- Clean a platinum or nickel-chromium wire loop by dipping it into concentrated acid (hydrochloric acid or nitric acid are often used).
- Rinse the acid washed wire loop with deionized water.
- Test the cleanliness of the wire loop by placing it into the flame of a bunsen burner.
- If the bunsen burner's flame changes colour, then the wire loop is not clean and you will need to repeat steps 1 to 3.
If the bunsen burner's flame does not change colour, then the wire is clean and you can proceed to step 5.
- Dip the clean wire loop into the sample (powder or solution).
- Place the wire loop containing the sample into the blue part of the bunsen burner's flame.
- Observe the change in the colour of the bunsen burner's flame.
- To perform another test, repeat steps 1 to 7.
Using Flame Tests
The element present in an unknown salt is identified by comparing the colour of the flame obtained during the flame test with the colour of the flames known to be produced by a range of elements.
The flame colours produced by a number of elements are given in the table below:
|Flame tests are usually used to detect the presence of metals, but some semi-metals (metalloids) and non-metals (such as phosphorus) can also be detected.
There are a number of limitations of flame tests:
- Flame tests cannot detect all elements.
Some elements produce similar flame colours.
Many elements produce no change in flame colour.
- Flame tests cannot usually detect low concentrations.
- Impurities can mask the flame colour.
Sodium, with its intense yellow flame, is capable of masking the colour produced by other elements if it is present as an impurity.
Sometimes, viewing the flame through blue glass can filter out the yellow of sodium's flame.
||orange to red
||green or blue
||pink to red
||lilac to violet
||red to violet
||crimson to red