Question

The energy of coloured light increases in order: red, yellow, green, blue, violet. What are the metallic elements used in the flame test in increasing order of energy of the light emitted?

Answer 1

Hint: A flame test is a common qualitative test in chemistry that is used to identify a metallic cation. The elements whose valence electrons can be excited and de-excited using the energy of the flame can show characteristic colours in the flame. This will be helpful in distinguishing the elements in their cationic form.

Complete answer:
We can see the spectrum and say that red has the lowest frequency and thus the lowest energy whereas violet has the highest frequency and hence the largest energy.
The metallic elements used in the flame test in increasing order of energy of the light emitted are given below:
Red colour is emitted by Lithium, zirconium, strontium, mercury, Rubidium (red-violet)
Brick-red colour flame is shown by calcium
Yellow coloured flame is imparted by sodium and iron (brownish yellow)
Green coloured flame is observed when Barium cation is introduced to the flame
Blue colour flame is shown by caesium, Arsenic, copper, tantalum, indium and lead
Potassium emits a violet colour to the flame. It is best described as a lilac colour.

Note:
Remember that flame test is not always the final say in distinguishing metal ions. Impurities or contaminants present in the sample might affect the test results in the flame test. This might lead to wrong colours being imparted in the flame. Thus the test cannot differentiate between all elements reliably.