Q.When water is added to quick lime the reaction is:

Exothermic and Endothermic Reactions

Chemical reactions can be classified by whether they release or absorb heat, measured by the enthalpy change $\Delta H = H_{products} - H_{reactants}$.

An exothermic reaction releases heat to the surroundings. The products have lower enthalpy than the reactants, so $\Delta H$ is negative and the surroundings warm up. Examples include combustion, respiration, neutralisation, and the slaking of quick lime:

which evolves so much heat that the water can hiss and boil.

An endothermic reaction absorbs heat from the surroundings. The products have higher enthalpy, so $\Delta H$ is positive and the surroundings cool. Examples include the thermal decomposition of limestone, photosynthesis, and dissolving $NH_4NO_3$ in water (used in instant cold-packs):

$CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g), \quad \Delta H = +178\ \text{kJ mol}^{-1}$ …