Hess's Law
Hess's Law : The heat given out in the making of a compound is the same whether the compound is made at one, or slowly in aseries of changes, e. g. calcium chloride can be made (i) by the dissolving of quicklime or (ii) by dissolving the quicklime in water and then adding the acid. The heat given out in (i) is the same as the sum of the heat given out in the two steps of (ii)