Water is the liquid form of the molecule H₂O, which is liquid at room temperature. At a pressure of 1 atm, the maximum density of the liquid phase is

(1)

which occurs at a temperature of 3.98° C. The density of liquid water is tabulated in Weast (1981, p. F-5). At 1 atm, water boils at 100 (373 K, 212° F), turning into steam, and freezes at 0° C (273 K, 32° F). It is an unusual substance because the solid form (ice) is less dense than the liquid. Explaining the phase diagram for water has proven difficult, but some success has been recently obtained by Cho et al. (1996) by including second neighbors of water molecules in their models.

The specific heat of water at constant atmospheric pressure is

(2)

i.e., 1 calorie is needed per degree Kelvin (or Celsius) of temperature change for 1 gram of liquid water. In fact, the definition of (one of the several types of) the calorie is the amount of heat needed to change the temperature of 1 g of water by 1 at its temperature of maximum density.

The latent heat of fusion of water is

(3)

and the latent heat of vaporization is

(4)

Absolute Humidity, Ice, Relative Humidity, Water Wave