The entropy of a pure substance has a true zero, that of its pure crystalline solid form at zero (Kelvin) temperature, according to the third law of thermodynamics. The entropy, and the free energies that involve it, are the least obvious of the thermodynamic functions, and textbooks of general thermodynamics should be consulted for a full explanation. In the thermodynamics of mixed liquid nonelectrolytes, the usual choice is the pure liquid component at the same temperature and pressure as the solution. In dilute solutions of electrolytes, the activity, and even the activity coefficient of the solvent are very little different from unity, so that the reporting of solvent properties in these terms requires a large number of significant figures. The osmotic pressure of electrolyte solutions is of great biological importance, but its measurement is seldom used as a direct source of thermodynamic data on simple electrolytes owing to the difficulty of making genuinely semipermeable membranes for small ions.