The conceptually simplest approach to experimental estimation of rH is a direct
measurement of heat evolved or absorbed when Ca(OH)2 solid is mixed with an appropriate amount
of 1m H2SO4 solution. However, the reaction involves two solid phase components, Ca(OH)2 and
CaSO42H2O. It is a practical fact that reactions of this type tend to be sluggish and may require
considerable time for complete reaction. (The solids may cake and remain unreacted for some
time.) We need to find another way to measure the enthalpy of our reaction of interest.