Question

Which of the following compounds imparts red color to the flame and decomposes on heating to give oxygen and a brown gas?
A. $CaC{O_3}$
B. $MgN{O_3}$
C. $MgC{O_3}$
D. $Ca{(N{O_3})_2}$

Answer 1

Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues strontium and barium. Calcium, when placed over a Bunsen burner, imparts red color to the flame. Nitrogen dioxide is a gas which has a brown color.

Complete answer:
During the thermal decomposition of calcium nitrate, the following reaction takes place:
$2Ca{(N{O_3})_2} \to 2CaO + 4N{O_2} + {O_2}$
As per the above balanced chemical equation, the presence of calcium atom in the compound imparts a red color to the flame because of the excitation of the electrons in the calcium atom which raises the electron to the higher energy level. The hat absorption is the reason for the raise in the energy level of the electron. When this electron comes back to the ground state, it releases some energy in the form of a photon whose wavelength lies in the visible region of the electromagnetic spectrum. Due to the release of nitrogen dioxide ($N{O_2}$ ), there is also a reddish brown appearance of the gas released from the thermal decomposition of calcium nitrate.

Thus, the correct option is D. $Ca{(N{O_3})_2}$ .

Note:
Some compounds break down when heated, forming two or more products from one reactant. This type of reaction is called thermal decomposition. Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition. If decomposition is sufficiently exothermic, a positive feedback loop is created producing thermal runaway and possibly an explosion or other chemical reactions.