Question

Take about $1.0g$ of $CaC{O_3}$ in a test tube. Heat it over a flame, a colourless gas comes out. The reaction is called a
A. decomposition reaction
B. displacement reaction
C. double decomposition reaction
D. double displacement reaction

Answer 1

$CaC{O_3}$ is represented by the name calcium carbonate. The carbonate of calcium undergoes decomposition on heating.

Complete step by step answer:
The carbonate of alkaline earth metal is represented by the formula $MC{O_3}$. The central metal atom is in $+ 2$ oxidation state. Calcium carbonate is an example of alkaline earth metal carbonate composed of $C{a^{2 + }}$ and$C{O_3}^{2 - }$ .
Calcium is an element in the periodic table with atomic number $20$. Its electronic configuration is
$Ca:1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}$
In $C{a^{2 + }}$ two electrons present in the $4s$ orbital are lost in the formation of $CaC{O_3}$. On heating the calcium carbonate produces colourless gas carbon dioxide and a residue calcium oxide.
The calcium oxide upon reaction with water forms calcium hydroxide. Calcium hydroxide thus reacts with excess carbon dioxide to form calcium hydrogen carbonate.
Thus heating gives calcium carbonate undergoes a decomposition reaction and forms calcium oxide and carbon dioxide.
Displacement reaction: Displacement reaction is the substitution reaction of an ion or atom in a molecule to form a new molecule. For example, when iron is added to the solution of $CuS{O_4}$, then the iron will replace copper from the solution.
Double decomposition reaction: It is the reaction formed between two reactants by interchanging the positive and negative ions between the reactants to form new compounds.
Double displacement reaction: In this reaction also the two reactants react with each other to form new compounds by interchanging their ions and resulting in formation of precipitate. This is also known as precipitation reaction.

Hence option A is the correct answer, i.e. decomposition reaction.

Note:
The thermal stability of the substrates is considered for determining the decomposition of it under heating. Decomposition always leads to formation of new molecules with lower molecular weights.