Standard enthalpies of formation

 The enthalpies, heat capacities, and other thermodynamic properties of water in its different physical states are all different and each must be used over that temperature range in which that state exists. This is true for all substances; the thermodynamic properties of different physical states of the same substance are not the same. In addition, these properties depend upon the conditions under which a substance exists, such as temperature and pressure. A kilogram of iron at 25oC has a considerably lower heat content than the same kilogram of iron at 500oC, as your fingers will quickly tell you. In general, then: the values of the thermodynamic properties of substances depend upon the conditions under which the substance exists. To make meaningful quantitiative calculations, therefore, we must clearly understand the conditions to which those calculations will apply. Chemists use the concepts of state functions and standard conditions to help understand the effect of conditions.