Question

A soft metal (A) gives the violet flame test. When burnt in ${O_2}$ (A) gives chrome yellow powder (B) which on reaction with water gives alkaline solution (C) and ${O_2}$ Identify the metal?
(A) Na
(B) Mg
(C) K
(D) Ca

Answer 1

Hint: Alkali metals are silver white in color and are generally soft and light metals. Alkali metals and their salts are heated in the oxidizing flame of a Bunsen burner, they impart characteristic colour to the flame. Alkali metals have weak metallic bonding.

Complete step by step answer:
Alkali metal when burnt in air forms different kinds of oxides, for example, the alkali metals on reaction with limited quantities of oxygen form normal oxides of the formula ${M_2}O$ . However, when heated with excess of air, lithium forms normal oxide, and some peroxide. Sodium forms peroxide and some superoxide whereas potassium, rubidium and cesium form superoxide having general formula $M{O_2}$
The oxides and the peroxides are colorless when pure but super oxides are yellow or orange in color. Peroxides are diamagnetic whereas superoxides are paramagnetic in nature.
A soft metal A is potassium (K) gives the violet flame test.
Potassium (K) burnt in oxygen to give yellow powder (B)
\[K\left( {potassium} \right) + {O_2}\left( {excess} \right) \to K{O_2}\left( B \right)\left( {inorganic \,compound} \right)\]
The potassium superoxide is in yellow color powder.
Then potassium superoxide reacts with water to give an alkaline solution.
\[2K{O_2}\left( {potassium\,superoxide} \right) + 2{H_2}O\left( {water} \right) \to 2KOH\left( {potassium\, hydroxide} \right) + {O_2} + {H_2}{O_2}\left( {hydrogen\,peroxide} \right)\]
The higher oxides generally act as good oxidizing agents, sodium peroxide is widely used as an oxidizing agent in inorganic chemistry.

So, the correct answer is “Option C”.

Note:
Potassium is an alkali metal, it belongs to the first group of periodic-table. It is a soft metal which gives the violet flame test. Potassium superoxide is used to supply oxygen and to absorb carbon dioxide and water in self-contained lung actuated breathing apparatus. Potassium is softer than sodium.