Furu ike ya!
Kawazu tobikomu
Mizu no oto

Matsuo Basho
[ see Matsuo Basho (1644-1694) ]

Topics

•  

  

  Why is Water Blue?

   

  In an article first published in the Journal of Chemical Education (1993, 70(8), 612), Charles L Braun and Sergei N Smirnov explain:

  "Over the years, we have often asked scientific colleagues why it is that water is blue. Common responses have included light scattering—after all the sky is blue—and coloration by dissolved impurities—Cu₂⁺ has been a popular suggestion. However, the work described below demonstrates that water has an intrinsic color, and that this color has a unique origin. This intrinsic color is easy to see, and has been seen by the authors in the Caribbean and Mediterranean Seas and in Colorado mountain lakes. Because the absorption which gives water its color is in the red end of the visible spectrum, one sees blue, the complementary color of red, when observing light that has passed through several meters of water. This color of water can also be seen in snow and ice as an intense blue color scattered back from deep holes in fresh snow. Blue to bluegreen hues are also scattered back when light deeply penetrates frozen waterfalls and glaciers." [ from Why is Water Blue? ]

  Water is probably the most common substance on Earth. Water constitutes about 50 to 70% of the mass of the human body. In fact water is essential to life as we know it: it transports chemicals throughout the organism, it acts as a lubricant, it provides protection to sensitive organs, it participates in most biochemical reactions, it helps regulate body temperature, etc. See, for example, L Boeckner's good article Water: The Nutrient or the more in-depth summary in   Water Biology. Indeed water was crucial in the very development of life on Earth. See for example D J Eernisse's very comprehensive course Biology 404.

   

  

  The Water Cycle

   

  More generally, water is considered the universal solvent: "an extraordinary property of water is its ability to dissolve other substances. There is hardly a substance known which has not been identified in solution in the earth's waters. Were it not for the solvent property of water, life could not exist because water transfers nutrients vital to life in animals and plants." [ from Environment Canada's Properties of Water ] Don't forget that water plays a primary role in the regulation of Earth's climate, even if life were absent.
  Water is often bound up with other compounds. For example, large amounts of water are present in methane hydrates, "These interesting formations are made up of a lattice of frozen water, which forms a sort of 'cage' around molecules of methane. These hydrates look like melting snow and were first discovered in permafrost regions of the Arctic."
  [ from Unconventional Natural Gas Resources ]

   

  

  A Molecule of Methane Hydrate

   

  Note the hexagonal structure of the water 'cage.'
• Not surprisingly, and as you can guess from the brief introduction above, water is such a pervasive and unique substance that an entire course devoted to it would not be sufficient to cover its properties and its functions. I will therefore focus on some intriguing peculiarities of water, but not before having asked the obvious question: how, where does water originate? Half of the answer is obvious: one of the ingredients of water (H₂O), is hydrogen, the most abundant and most primordial element in the universe. Oxygen, on the other hand, like most heavier elements, is synthesized from hydrogen, via nuclear fusion, by stars. Its abundance in the universe is estimated to be about 1/1,000 (by atoms) ( see WebElements Periodic Table ). It should not therefore come as a surprise that water itself is rather common in the universe. Read for example Astronomers Track Origin of Solar System Water, and also Water in the Universe: Abundant? Yes, But Not Where We Thought it Would Be. Here is a simplified description of the so-called carbon-nitrogen-oxygen cycle or CNO cycle ( see e.g. CNO Cycle ).

  ¹²C + ¹H → ¹³N + γ
  ¹³N         → ¹³C + e⁺ + ν_e
  ¹³C + ¹H → ¹⁴N + γ
  14N + ¹H → ¹⁵O + γ
  ¹⁵O         → ¹⁵N + e⁺ + ν_e
  ¹⁵N + ¹H → ¹²C + ⁴He

  which is thought to be one of the main ways in which oxgen is synthesized in stars. As to the symbols used above, the chemical symbols refer to atomic nuclei (C is carbon, H is hydrogen, N is nitrogen, O is oxygen, and He is helium), while γ stands for gamma ray (high-energy photon), e⁺ for positive electron or positron, and ν_e for the electron neutrino.
• In which way is water peculiar? Most substances contract when they are cooled. Water, however, above about 4°C, contracts, normally, but below that temperature it expands, abnormally. The density of water reaches a maximum at about 4°C. Incidentally, that's why ice floats in liquid water. This almost unique behavior has been partially understood only recently. The main reason seems to be that water molecules have a tendency to aggregate, to form clusters. See Water: A Gentle Introduction to Water and its Structure for pictures and further explanations.

   

  

  The Phase Diagram for Water

   

  Water behaves strangely also at high temperatures. From what we know about molecular structure in general, we would expect water to boil at a far lower temperature than 100°C: at around –90°C ! The main reason seems to be that the oxygen atom in one molecule of water forms a secondary bond with one of the hydrogen atoms in a nearby molecule of water. This type of bond is very important in ice, but does not completely disappear in the liquid form.

  "Liquid water can be thought of as a seething mass of water molecules in which hydrogen-bonded clusters are continually forming, breaking apart, and re-forming. Theoretical models suggest that the average cluster may encompass as many as 90 H₂O molecules at 0°C, so that very cold water can be thought of as a collection of ever-changing ice-like structures. At 70°C, the average cluster size is probably no greater than about 25." [ from Water: A Gentle Introduction to Water and its Structure ]

  If these properties of water seem a bit esoteric, keep in mind that in living organisms they play an absolutely critical role. Most biochemical reactions would not take place if water behaved like a 'normal' substance. Once again, please, browse   Water: A Gentle Introduction to Water and its Structure. You should also read (in the Library) Structured Water Is Changing Models: Large Water-Molecule Clusters May Be Crucial to Cellular Processes, from which the image below is taken

   

  

  Water Molecules Clusters

   

  In the same article there is a link to another excellent resource on water: Martin Chaplin's Water Structure and Behavior.
• Very recent studies suggest that the picture is even more interesting:

  "In 1992, computer simulations suggested that supercooled water might come in two distinct phases, a high density phase in which the molecules pack together more tightly, and a low density phase in which they snuggle together more loosely. At high pressure and very low temperatures, supercooled water should pass abruptly from the low density phase to the high density phase, just as steam suddenly condenses into water. The transition would be extremely difficult to observe, as experimenters would have to prevent the supercooled water from freezing for a very long time. Nonetheless, theorists realized that the mere existence of a temperature and pressure at which such transitions become possible--a so-called critical point--would exert a subtle influence that could neatly explain all of water's weirdness. Now a theoretical analysis predicts which liquids ought to have two phases. Any liquid that eventually expands as it cools must have a liquid-liquid critical point." [ from A Tale of Two Liquids ]

  In other words, liquid water has at least two forms.
• I want to conclude this lecture with a few words on snowflakes and snow crystals. "Snowflakes and snow crystals are made of ice, nothing more. A snow crystal, as the name implies, is a single crystal of ice. A snowflake is a more general term, which can mean an individual snow crystal, a few snow crystals stuck together, all the way up to the large 'puff-balls' of agglomorated snow crystals that often fall in warmer weather." This definition is from   Snow Crystals, a marvelous resource created by Kenneth Libbrecht at Caltech. You have probably already noticed that snow crystals exhibit hexagonal symmetry. The reason is that the shape of a water molecule is such that it can only link up with other water molecules in a hexagonal lattice. You have also probably noticed that, with notable exceptions. most ice crystals, while retaining their hexagonal symmetry, show an incredible variety of complex shapes. Why?

   

  

  Snowflake from The Rasmussen & Libbrecht Collection

   

  Here is a very concise and lucid explanation by Libbrecht:

  "The answers to these questions lie in just how water molecules travel through the air to condense onto a growing snow crystal. The water molecules have to diffuse through the air to reach the crystal, and this diffusion slows their growth. The farther water molecules have to diffuse through the air, the longer it takes them to reach the growing crystal. So consider a flat ice surface that is growing in the air. If a small bump happens to appear on the surface, then the bump sticks out a bit farther than the rest of the crystal. This means water molecules from afar can reach the bump a bit quicker than they can reach the rest of the crystal, because they don't have to diffuse quite as far. With more water molecules reaching the bump, the bump grows faster. In a short time, the bump sticks out even farther than it did before, and so it grows even faster. We call this a branching instability—small bumps develop into large branches, and bumps on the branches become sidebranches. Complexity is born. This instability is a major player in producing the complex shapes of snow crystals." [ from Snow Crystals ]

  Branching instabilities do not occur only in the growth of ice crystals, but are a fundamental feature of the behavior of many physical systems. They are important, for example, in the processes underlying the propagation of cracks and fissures in materials. You can find other fascinating examples in M Buchanan's Ubiquity ( see Syllabus ).

 
Readings, Resources and Questions

• 2003 was the International Year of Freshwater. Perhaps we should celebrate and learn to appreciate water every year!
• Water is such a ubiquitous substance that we have barely scratched the surface here. Think of other phenomena where water plays an important role.
• Other excellent resources:
  • Earth's Water
  • Water and Nature
  • Water Biology
  • Water Vapor, Possible Comets, Found Orbiting Star
  • The Bentley Collection of Snow Crystal Photomicrographs
  A unique resource is John Gregory Brown's seminar on H₂O, The Mystery, Art, and Science of Water: Literature and Water
• Another, very extensive and very beautiful collection of snowflake images is at the Bentley Snow Crystal Collection of the Buffalo Museum of Science. Each image is fully documented (e.g. weather data at the time the photograph was taken). Yet another excellent site is Electron Microscopy Snow Page.