The decomposition reactions are endothermic reactions. The reactions ${\mathrm{I}}_2( \mathrm{g})\to 2\mathrm{I}\left(\mathrm{g}\right)$ and $\mathrm{HCl}\left(\mathrm{g}\right)\to \mathrm{H}\left(\mathrm{g}\right)+\mathrm{Cl}\left(\mathrm{g}\right)$ are decomposition reactions. The heat vaporisation is an endothermic reaction. Hence, the conversion of ${\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{}{\to }{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$ is endothermic process. The heat of combustion is an exothermic process. The heat involved in $\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}_2( \mathrm{g})⟶{\mathrm{CO}}_2( \mathrm{g})$ is heat of combustion. The dissolution of ammonium chloride in water is an endothermic process. ${\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{s}\right)\to {\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right); \mathrm{\Delta H}=+15. 1\mathrm{kJ}/\mathrm{mol}.$