As water is so common-place, it is often regarded as a ‘typical’ liquid. In reality water is most atypical as a liquid, with its properties at low temperatures quite different from its properties when hot. It has often been stated that life depends on these anomalous properties of water. In particular, the large heat capacity, high thermal conductivity and high water content in organisms contribute to thermal regulation and prevent local temperature fluctuations, thus allowing us to more easily control our body temperature. The high latent heat of evaporation gives resistance to dehydration and considerable evaporative cooling. Water is an excellent solvent due to its polarity, high dielectric constant and small size, particularly for polar and ionic compounds and salts. It has unique hydration properties towards biological macromolecules (particularly proteins and nucleic acids) that determine their three-dimensional structures, and hence their functions, in solution. This hydration forms gels that can reversibly undergo the gel-sol phase transitions that underlie many cellular mechanisms. Water ionizes and allows easy proton exchange between molecules, so contributing to the richness of the ionic interactions in biology.

At 4°C water expands on heating or cooling. This density maximum together with the low ice density results in

  1. the necessity that all of a body of fresh water (not just its surface) is close to 4°C before any freezing can occur,
  2. the freezing of rivers, lakes and oceans is from the top down, so permitting survival of the bottom ecology, insulating the water from further freezing, reflecting back sunlight into space and allowing rapid thawing, and
  3. density driven thermal convection causing seasonal mixing in deeper temperate waters carrying life-providing oxygen into the depths. The large heat capacity of the oceans and seas allows them to act as heat reservoirs such that sea temperatures vary only a third as much as land temperatures and so moderate our climate (e.g. the Gulf stream carries tropical warmth to northwestern Europe). The compressibility of water reduces the sea level by about 40 m giving us 5% more land. Water’s high surface tension plus its expansion on freezing encourages the erosion of rocks to give soil for our agriculture.

Notable amongst the anomalies of water are the opposite properties of hot and cold water, with the anomalous behavior more accentuated at low temperatures where the properties of supercooled water often diverge from those of hexagonal ice. As cold liquid water is heated it shrinks, it becomes less easy to compress, its refractive index increases, the speed of sound within it increases, gasses become less soluble and it is easier to heat and conducts heat better. In contrast as hot liquid water is heated it expands, it becomes easier to compress, its refractive index reduces, the speed of sound within it decreases, gasses become more soluble and it is harder to heat and a poorer conductor of heat. With increasing pressure, cold water molecules move faster but hot water molecules move slower. Hot water freezes faster than cold water and ice melts when compressed except at high pressures when liquid water freezes when compressed. No other material is commonly found as solid, liquid and gas.

The anomalies

  1. Water has unusually high melting point.
  2. Water has unusually high boiling point.
  3. Water has unusually high critical point.
  4. Water has unusually high surface tension and can bounce.
  5. Water has unusually high viscosity.
  6. Water has unusually high heat of vaporization.
  7. Water shrinks on melting.
  8. Water has a high density that increases on heating (up to 3.984°C).
  9. The number of nearest neighbors increases on melting.
  10. The number of nearest neighbors increases with temperature.
  11. Pressure reduces its melting point (13.35 MPa gives a melting point of -1°C)
  12. Pressure reduces the temperature of maximum density.
  13. D2O and T2O differ from H2O in their physical properties much more than might be expected from their increased mass; e.g. they have increasing temperatures of maximum density (11.185°C and 13.4°C respectively).
  14. Water shows an unusually large viscosity increase but diffusion decrease as the temperature is lowered.
  15. Water’s viscosity decreases with pressure (at temperatures below 33°C).
  16. Water has unusually low compressibility.
  17. The compressibility drops as temperature increases down to a minimum at about 46.5°C. Below this temperature, water is easier to compress as the temperature is lowered.
  18. Water has a low coefficient of expansion (thermal expansivity).
  19. Water’s thermal expansivity reduces increasingly (becoming negative) at low temperatures.
  20. The speed of sound increases with temperature (up to a maximum at 74°C).
  21. Water has over twice the specific heat capacity of ice or steam.
  22. The specific heat capacity (CP and CV) is unusually high.
  23. Specific heat capacity; CP has a minimum.
  24. NMR spin-lattice relaxation time is very small at low temperatures.
  25. Solutes have varying effects on properties such as density and viscosity.
  26. None of its solutions even approach thermodynamic ideality; even D2O in H2O is not ideal.
  27. X-ray diffraction shows an unusually detailed structure.
  28. Supercooled water has two phases and a second critical point at about -91°C.
  29. Liquid water may be supercooled, in tiny droplets, down to about -70°C. It may also be produced from glassy amorphous ice between -123°C and – 149°C [74] and may coexist with cubic ice up to -63°C [137].
  30. Solid water exists in a wider variety of stable (and metastable) crystal and amorphous structures than other materials.
  31. Hot water may freeze faster than cold water; the Mpemba effect.
  32. The refractive index of water has a maximum value at just below 0°C.
  33. The solubilities of non-polar gases in water decrease with temperature to a minimum and then rise.
  34. At low temperatures, the self-diffusion of water increases as the density and pressure increase.
  35. The thermal conductivity of water is high and rises to a maximum at about 130°C.
  36. Proton and hydroxide ion mobilities are anomalously fast in an electric field
  37. The heat of fusion of water with temperature exhibits a maximum at -17°C [15].
  38. The dielectric constant is high and behaves anomalously with temperature.
  39. Under high pressure water molecules move further away from each other with increasing pressure.
  40. The electrical conductivity of water rises to a maximum at about 230°C and then falls.
  41. Warm water vibrates longer than cold water.
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