Characteristics of Water

Phase diagram

Water substance exists in all three phases in abundant quantities in the range of temperatures occurring near the earth's surface. The accompanying phase diagram (Figure 1) for water substance gives the set of pressures, temperatures, and volumes that define the conditions separating solid from liquid, solid from vapor, and liquid from vapor. The relation between pressure (e_s) and temperature (T) of Figure 2 can be used to explain many commonly observed features of water substance. The convergence point of the three lines on this graph gives the triple point of water (T = 273.1675 K or 0.0075 ° C and e_s = 611.21 Pa or 6.1121 mb) at which vapor, solid, and liquid all exist in equilibrium. The critical temperature for H₂O of 647 K (compared, for instance, to 126 K for nitrogen) is quite high compared to earth surface temperatures and allows for abundant water and ice on earth. The negative slope of the line separating the solid and liquid phases in the plot of pressure vs. temperature means that raising the pressure on ice at constant temperature will cause it to melt. This is the reason that glaciers can slip down a mountain: high pressure from the glacier at the earth’s surface melts a thin layer that reduces friction and allows downward movement. Also, volume increases as water goes isothermally from liquid to solid. This means that ice is less dense than water, explaining the why ice floats on water rather than sinking to the bottom. This seemingly insignificant fact has a profound influence on the behavior of water bodies in cold climates. What would happen in a Minnesota lake if ice formed at the surface and sank to the bottom?

1. Latent heats

   When substances change phase there usually is a release or absorption of energy (known as latent heat) in the process. Latent heats for the phase changes of water have the following values:

   Latent heat of condensation (vapor to liquid, L₃₂) or vaporization (liquid to vapor, L₂₃):

   L₃₂ = L₂₃ = 2.500 x 10⁶ J kg ^—1 at 0°C

   = 2.25 x 10⁶ J kg ^—1 at 100°C

   Latent heat of fusion (liquid to solid , L₂₁) or melting (solid to liquid, L₁₂):

   L₂₁ = L₁₂ = 3.34 x 10⁵ J kg ^—1 0°C

   Latent heat of deposition (vapor to solid, L₃₁) or sublimation (solid to vapor, L₁₃):

   L₃₁ = L₁₃ = 2.834 x 10⁶ J kg ^—1 at 0°C

   Latents heats are needed to calculate the equilibrium (saturation) pressures shown in the previous figure, as is the specific gas constant for water vapor, R_v = 4.61 J/(K kg).

2. Water vapor as a greenhouse gas

   Water vapor belongs to a class of gases whose molecular structure (Figure 3) is such that it absorbs infrared radiation emitted by the earth. Details of this topic are covered in the unit on the Global Energy Balance. Water vapor is the most abundant greenhouse gas, and its global abundance in the atmosphere is considered to be constant. However, the prospect of global warming raises the possibility that water vapor amounts in the atmosphere might increase, thereby further accelerating the global warming (this is called positive feedback, and will be discussed in the unit on Climate Models.

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