Question

When quicklime is immersed in water, the reaction is:
A. Exothermic
B. Endothermic
C. Explosive
D. None of these

Answer 1

Hint:
We can categorize the reactions on the basis of whether energy is absorbed or released as endothermic and exothermic reactions respectively.

Complete step by step solution
The chemical reactions that we might encounter all around us can be categorized on the basis of how the reaction is taking place as:
- Combination reactions: Here, multiple reactants combine to give a single product. One general representation for such reaction can be shown as:
${\rm{A + B}} \to {\rm{C}}$
- Decomposition reactions: Here, a single reactant gets broken down into multiple products. One general representation for such reaction can be shown as:
${\rm{A}} \to {\rm{B + C}}$
- Displacement reactions: Here, an element/ion gets displaced by another. One general representation for such reaction can be shown as:
${\rm{AB + C}} \to {\rm{AC + B}}$
- Double displacement reactions: Here, ions get exchanged between the reactants. One general representation for such reaction can be shown as:
${\rm{AB + CD}} \to {\rm{AD + CB}}$
- Redox reactions: Here, one reactant gets oxidized and another gets reduced. One general representation for such reaction can be shown as:
${\rm{A + }}{{\rm{O}}_2} \to {\rm{A}}{{\rm{O}}_2}$

These reactions can further be classified on the basis of whether energy is being used up or released during the reaction:
- Endothermic reactions: These reactions use energy for being carried out. For example:
${\rm{CaC}}{{\rm{O}}_3}\left( s \right) + {\rm{heat}} \to {\rm{CaO}}\left( s \right) + {\rm{C}}{{\rm{O}}_{\rm{2}}}\left( g \right)$
It is an endothermic, decomposition reaction of limestone to give quicklime and carbon dioxide.
- Exothermic reactions: These reactions release energy for being carried out. For example:
${\rm{C}}{{\rm{H}}_{\rm{4}}}\left( g \right) + 2{{\rm{O}}_{\rm{2}}}\left( g \right) \to {\rm{C}}{{\rm{O}}_{\rm{2}}}\left( g \right) + 2{{\rm{H}}_2}{\rm{O}}\left( g \right) + {\rm{heat}}$
It is exothermic, burning of natural gas to give carbon dioxide and water.
Here, we are given the reaction of quick lime with water which can be represented by the following chemical equation:
${\rm{CaO}}\left( s \right) + {{\rm{H}}_2}{\rm{O}}\left( l \right) \to {\rm{Ca}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\left( {aq} \right) + {\rm{heat}}$

As we can see that heat is evolved during the reaction making it an exothermic one.

Hence, the correct option is A.

Note:
We are given the common name of the reactant which might get confused with that of other compounds such as slaked lime or lime water, so we have to be careful about this.