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A flame test is one of the most regularly used identification procedures in chemistry for detecting the presence of various elements, most commonly metal ions in the compound. It is based on each element’s unique emission spectrum and thus each element will emit a distinct flame colour when subjected to strong heating. It is one of the important tools that can be used in small laboratories to identify different metal ions. Flame colours for common ions questions usually arise on metals alkali and alkaline earth metals like sodium, potassium, calcium, etc. These are the best examples of flame colours for common ions. Flame tests are generally employed for group 1 elements that comprise alkali and alkaline earth metals.
Flame Test:
The flame test is an analytical method used to identify a metal that is not known in a sample. The flame test produces a characteristic colour when the salt is being heated on a Bunsen burner. For different elements, the flame colour is different. The flame colours for common ions, such as sodium and potassium, are yellow and lilac. Flame colours for common ions questions usually arise to identify an element from a compound.
Principle of Flame Test:
When we excite an atom or an ion with the help of strong heating, electrons gain energy and thus can be promoted from their normal ground state into higher orbitals and become excited. As they fall back down to lower levels from higher ones, energy is released as light. Each of these jumps that an electron does involves a specific amount of energy being released in the form of light energy. This light energy will then correspond to a particular wavelength. Due to these jumps, a spectrum of lines will be produced, some of which can be seen in the visible part of the spectrum. Every element emits a characteristic spectrum and this can be used to differentiate one element from another. Many ‘flame colours for common ions questions’ involve this principle that remains the same for different elements.
The procedure involved in Flame Test
The samples are to be carried on a platinum or nichrome wire, which is repeatedly cleaned using hydrochloric acid (HCl) to remove any contamination.
The compounds used for the analysis are usually made into a paste using concentrated hydrochloric acid since it is volatile and gives good quality results. The wire is placed back in the flame.
If the resultant flame colour is weak, it is often advised to dip the wire back into the acid and put it back in the flame. This, in turn, produces a very short but intense flash of colour and we are able to identify the element.
Flame colours for common ions examples
Flame tests are generally employed for alkali and alkaline earth metals like Sodium, Potassium, Lithium, Cesium, Calcium, Strontium, Rubidium, etc., since it is by far the easiest way to identify which metal you have got. Thus, these are considered to be the common ions for whom the flame test is employed. However, there are other elements also that produce a characteristic colour when subjected to strong heating like copper, zinc, phosphorus, nickel, etc.
Flame colours for common ions examples are given in the following table:
Limitations of Flame test
Generally, in topics like flame colours for common ions, questions arise regarding its limitation: can it be used to differentiate all the elements? Even though the flame test is widely used to identify a substance, it cannot give a concrete and absolute result that the elements present in the compound are the same as interpreted by the flame test. It can be due to the following reasons:
The test is unable to detect ions of low concentrations.
The intensity of the flame varies depending on the sample. Flame colours for common ions examples like sodium and lithium are that the yellow coloured flame emitted by sodium appears more intense as compared to that of the lithium-ion even though its concentration is the same as that of sodium.
Test results may get affected by the impurities present in the sample. Sodium, for example, is an impurity in most compounds and will colour the flame. Blue glass is used as a filter to avoid it.
The test is incapable of distinguishing between all of the constituents. A variety of metals can produce the same flame colour. Some substances don’t even cause the flame to change colour.
Conclusion:
The flame test is a technique to identify an element in a compound by subjecting it to strong heating. It results in the excitation and de-excitation of electrons, which in turn results in the emission of light energy having a particular wavelength lying in the visible spectrum. Thus, different elements produce different flame colours. Examples of flame colours of common ions like sodium, potassium, calcium are yellow, lilac, orange-red, respectively. However, all the elements cannot be differentiated on the basis of the flame test; thus, it is used to rule out the identity of an element in a sample rather than definitively identify it.
