Question

When quicklime is reacted with water, the reaction is:
A. an endothermic reaction
B. a reaction involving reduction
C. an exothermic reaction
D. a reaction involving oxidation

Answer 1

Hint: We know that chemical reactions are the chemical changes in which formation of new substances of different physical and chemical properties occurs. They are accompanied by some observable changes such as, evolution of heat, formation of precipitate, colour change, evolution of gas etc.

Complete step by step answer:
Let's understand the exothermic reaction first. It is the reaction in which heat is released along with formation of product. The release of heat in the reaction is known by temperature change of the reaction mixture.

Let's understand the reaction of quicklime and water. The chemical formula of quicklime is CaO. So, the reaction is,

${\rm{CaO}} + {{\rm{H}}_2}{\rm{O}} \to {\rm{Ca}}{\left( {{\rm{OH}}} \right)_2}$

The reaction releases heat and produces slaked lime $\left( {{\rm{Ca}}{{\left( {{\rm{OH}}} \right)}_2}} \right)$. We know the heat producing reactions are termed as exothermic reactions. So, reaction of water and quicklime is an example of exothermic reaction.

So, the correct answer is Option C.

Additional Information:
Chemical reactions are classified in four types, combination, decomposition, displacement and double displacement reaction.

In combination reactions, two or more reactants combine to result in a product.
${\rm{CaO}} + {{\rm{H}}_2}{\rm{O}} \to {\rm{Ca}}{\left( {{\rm{OH}}} \right)_2}$

In decomposition reaction, decomposition of one reactant to more than one reactant takes place.

${\rm{A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}} \to {\rm{Al}} + {{\rm{O}}_{\rm{2}}}$

In displacement reaction, displacement of a less reactive metal from its compound by a more reactive metal takes place.

${\rm{Zn}} + {\rm{CuS}}{{\rm{O}}_4} \to {\rm{ZnS}}{{\rm{O}}_{\rm{4}}} + {\rm{Cu}}$

Here, zinc is more reactive than copper. So, the displacement reaction takes place.
In a double displacement reaction, exchange of ions of two compounds takes place.

${\rm{AgN}}{{\rm{O}}_{\rm{3}}} + {\rm{NaCl}} \to {\rm{AgCl}} + {\rm{NaN}}{{\rm{O}}_3}$


Note: Endothermic reaction is the reaction heat is consumed from surroundings to undergo reaction. In this reaction, decrease of temperature takes place with the progress of the reaction. The change in enthalpy is positive for endothermic reaction.