Quicklime is made by heating limestone: CaCO₃(s) → CaO(s) + CO₂(g).

The standard state is the pure substance at 1 bar. $\Delta_f H^\ominus$ is the enthalpy change for making one mole of a compound from its elements, and is zero for an element in its standard state. $\Delta_r H^\ominus = \sum a_i \Delta_f H^\ominus(\text{products}) - \sum b_i \Delta_f H^\ominus(\text{reactants})$. $\Delta_{sub}H = \Delta_{fus}H + \Delta_{vap}H$. Combustion enthalpies are negative and atomisation enthalpies positive.