The dissolution of quicklime in water is an exothermic reaction. Why?

• A chemical reaction in which reactants react together to form products along with the heat is called an exothermic reaction.
• When water is added to quicklime$\left(\mathrm{CaO}\right)$, a lot of heat energy is produced along with calcium hydroxide$\left[\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\right]$.

$$\begin{gathered} \mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)+\mathrm{heat} \\ \mathrm{Calcium}\mathrm{oxide}\mathrm{Water}\mathrm{Calcium}\mathrm{hydroxide} \end{gathered}$$