Question

Which cation is detected by the flame test?
A. $N{H_4}^ + $
B. ${K^ + }$
C. $M{g^{2 + }}$
D. $A{l^{3 + }}$

Answer 1

Hint: The ions of group 1 metals such as $L{i^ + },N{a^ + },{K^ + }$ form soluble hydroxides. Therefore, they are identified using flame tests and not identified by the addition of sodium hydroxide solution.

Complete step by step answer:
There are various different tests to detect and identify the ions in the compounds. It is very important that the test for an identification of ions is unique. The results of a test should let us determine which ion is present, rather than being uncertain about which ion it is.
The flame test is used in chemistry to help determine the identity or possible identity of a metal or metalloid ion found in an ionic compound. It is a qualitative test. If the compound is placed in the flame of a gas burner, then there may be a characteristic colour given off which is visible to our naked eye. Different metal ions produce different flame colours when we heat them strongly. This is the basis of the flame tests.
For testing of positive ions i.e. cations we conduct a flame test and potassium ion is a cation. Ever ion has a specific colour which they liberate when exposed to a flame. The ions liberate a lilac colour when it is exposed to a flame while copper ions liberate a blue flame.
To carry out a flame test, we should dip a clean nichrome wire loop into the beaker of HCl and we should place the loop into the edge of the blue flame from a Bunsen burner and observe the flame colour produced. If the flame produced is of a lilac colour then it consists of potassium ions. The colours produced are due to the movement of the electrons from one shell to another. As the electrons gain energy they move to a higher shell and when they return to their original shell, they release energy in the form of a specific wavelength of light which we see.

Therefore, the correct answer is option (B).

Note: The flame test is used to visually determine the identity of an unknown metal of an ionic salt based on the characteristic colour the salt turns the flame of a Bunsen burner. Corn-starch, glucose, sucrose, magnesium sulphate will give a negative flame test.