Question

A flame test of a particular element gives a persistent yellow color. This element is most likely which of the following?
A. ${\text{Pb}}$
B. ${\text{Na}}$
C. ${\text{Cu}}$
D. ${\text{Sr}}$

Answer 1

In chemistry, you use flame test as an analytical method to find out if certain elements are present in the sample or not. These elements are alkali metals and alkaline earth metals. The elements are mainly present as metal cations.

Complete answer:

Each element has a characteristic colour. When you observe a particular colour in the flame test, you can say that particular element is present.
When a metal cation is heated in a flame, it results in the excitation of electrons from lower energy level to higher energy level by absorbing the energy in the form of heat. To return to the ground state, the electrons in the excited state emit visible light.
Lilac colour is obtained from potassium cations.
Gray white colour is obtained from lead cations.
Blue or bluish green colour is obtained by halides of copper.
Golden yellow colour is obtained by sodium cation.
Carmine colour is obtained by Lithium cation. You cannot observe carmine colour through green glass.
Red colour is obtained from strontium cations.
Calcium compound gives brick red color in flame test.

Hence, the correct option is the option (C).

Additional information:
If the concentration of metal cation is very low, it cannot be detected by the flame test. Different samples have different light intensity. Yellow colour of sodium is more intense than the red colour of lithium. Presence of impurities, particularly sodium, will affect the results of the flame test. In such situations, you should use cobalt blue glass to filter out the yellow colour of sodium.

Note: You can use the flame test to detect the presence of a small range of metal cations but it cannot be used to detect the presence of all the metal cations as it is just a preliminary test and needs a confirmatory test to detect the ion present.