Question

The hissing sound that is heard when water is poured over quicklime is due to the evolution of:

• A. ${\mathrm{H}}_2$ gas
• B. ${\mathrm{O}}_2$ gas
• C. ${\mathrm{CO}}_2$ gas
• D. Steam

Answer 1

The correct option is D

Steam

Explanation for the correct option:

1. Exothermic reaction: a chemical reaction in which evolution of heat energy is experienced.
2. When quick lime is added to water a hissing sound is produced and the reaction is as follows : $\underset{(\mathrm{Quick}\mathrm{lime}/\mathrm{Calcium}\mathrm{oxide})}{\underbrace{\mathrm{CaO}\left(\mathrm{s}\right)}}+\underset{\left(\mathrm{Water}\right)}{\underbrace{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}}\to \underset{(\mathrm{Slaked}\mathrm{lime}/\mathrm{Calcium}\mathrm{hydroxide})}{\underbrace{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}}+\mathrm{Heat}\mathrm{energy}$.
3. It is an exothermic reaction because a lot of heat is produced in this reaction and the hissing sound arises due to evolution of heat.

Explanation for the incorrect options:

1. Since, ${\mathrm{H}}_2$(Hydrogen),gas is not evolved during the reaction, so option (A) is incorrect.
2. Since ${\mathrm{O}}_2$(Oxygen) gas is not involved during the reaction, so option (B) is incorrect.
3. And also , ${\mathrm{CO}}_2$(Carbon dioxide) is again not involved in this reaction, hence option (C) is also incorrect.

Therefore, the hissing sound that is heard when water is poured over quicklime is due to the evolution of heat or steam. Hence, the option (D) steam is correct.