We present some physical and chemical properties of the water molecule. The Helmholtz free energy formalism is used to study some of those properties as thermal ones of the single-phase region and of the vapor–liquid phase boundary, including the phase-equilibrium condition (Maxwell criterion), the caloric properties specific isochoric heat capacity, compressibility and the thermal expansion, specific isobaric heat capacity, speed of sound and differences in the specific enthalpy and in the specific internal energy. By the other hand, water and ice coexist at a thermodynamic equilibrium at all temperatures in the 230-272 K range. The temperature dependence of the equilibrium constant of the water <> ice interconversion in a macromolecular-rich milieu (an important condition for astrobiology) does not obey the Gibbs-Helmholtz equation, and this indicates a strong interaction of macromolecules with water. Among the contributions to the enthalpy and entropy change on cooling and annealing at subfreezing temperatures, the largest contribution remains that from water's crystallization in hydrated macromolecules. We point out that the effect of impurities overcomes the importance of H-bond interaction with macromolecules. The thermodynamic parameters of liquid water are derived in the frame of the Frank and Wen “flickering cluster” model. Various models proposed for the structure of liquid water are reviewed, and the advantages of the Frank—Wen model are stressed. Consequences for astrobiology of some of the results are pointed out, as specific water absorption bandshifts due to organics in the vicinity.