The isobaric heat capacity of liquid H2O, C-p, as a function of temper
ature, decreases between 0 degrees and about 35 degrees C and then inc
reases up to 100 degrees C. Analogous behaviour is shown by liquid D2O
. A statistical thermodynamic model has been applied to the experiment
al heat capacity data. The behaviour is explained by assuming that an
equilibrium A + B = AB is established between clusters A and AB of wat
er of different composition. The total heat capacity is considered as
the sum of three terms C-p = (1-alpha)C-p,C-0,C-AB + alpha C-p,C-0,C-A
+ Delta C-p,C-app. The term Delta C-p,C-app depends explicitly on the
reaction enthalpy. In H2O, the enthalpy Delta H = -1.84 kJ mol(-1) fo
r the dissociation reaction and the heat capacity C-p,C-B = 47.8 J K-1
mol(-1) for free water molecules are calculated. Analogous calculatio
ns performed for D2O yield the enthalpy, Delta H = -1.64 kJ mol(-1) an
d the heat capacity, C-p,C-B = 49.18 J K-1 mol(-1).