Question

A substance which gives brick red colour in flame test and breaks down on heating to give oxygen and a brown gas is:
A. magnesium nitrate
B. calcium nitrate
C. barium nitrate
D. strontium nitrate

Answer 1

The colour actually arises from electronic transitions in short lived species which are formed momentarily in the flame. The flame is rich in electrons. The flame colours depend on wavelength and wavenumber as well.

Complete step by step solution:
In the flame test electrons may also be quite readily excited to a higher energy level. To perform any flame test, a sample of the metal or any salt of the metal moistened with concentrated HCl, is heated on a platinum wire or nichrome wire in a Bunsen burner flame. The heat from the burner excites one of the orbital electrons to a higher energy level. When the excited electron drops back to its original energy level it gives out the extra energy obtained. The energy E is related to the wavenumber v by the Einstein relationship,
E=hv
For group1 metals, the energy emitted appears as visible light, thus giving the characteristic flame colorations,

Li	Crimson
Na	Yellow
K	Lilac
Rb	Red violet
Cs	Blue

In the question we are given that the brick red colour is given in flame test therefore it has calcium ions in it as we can see from the table. Therefore the reaction goes like,
$2Ca{(N{O_3})_2}\xrightarrow{\Delta }2CaO + {O_2} + 4N{O_2}$
The nitrate ion gives reddish brown vapours of nitrogen dioxide.

**Hence the correct option is B.

Note: **
The flame test should be avoided in case of As, Sb, Sn, Pb and Bi salts since they corrode the platinum wire. An asbestos fibre can be safely used in place of platinum wire for performing flame tests. Glass rod should never be used as it gives a golden yellow persistent colour due to sodium present in it.