Monday, 24 March 2008

Physics in Our Every Day Life Part II (WHY IS WATER BLUE?)


Charles L. Braun and Sergei N. Smirnov
Department of Chemistry
Dartmouth College, Hanover, NH 03755

Added and Edited By:Arif Nurahman
Department of Physics
Faculty of Sciences and Mathematics, Indonesia University of Education


Follower Open Course Ware at Massachusetts Institute of Technology
Cambridge, USA
Department of Physics
Aeronautics and Astronautics Engineering


Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam.

About 1.460 petatonnes (Pt) (1021kilograms) of water covers 71% of the Earth's surface, mostly in oceans and other large water bodies, with 1.6% of water below ground in aquifers and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation. Saltwater oceans hold 97% of surface water, glaciers and polar ice caps 2.4%, and other land surface water such as rivers, lakes and ponds 0.6%.

A very small amount of the Earth's water is contained within water towers, biological bodies, manufactured products, and food stores. Other water is trapped in ice caps, glaciers, aquifers, or in lakes, sometimes providing fresh water for life on land.
Water moves continually through a cycle of evaporation or transpiration (evapotranspiration), precipitation, and runoff, usually reaching the sea. Winds carry water vapor over land at the same rate as runoff into the sea, about 36 Tt (1012kilograms) per year. Over land, evaporation and transpiration contribute another 71 Tt per year to the precipitation of 107 Tt per year over land. Clean, fresh drinking water is essential to human and other life. However, in many parts of the world—especially developing countries—there is a water crisis, and it is estimated that by 2025 more than half of the world population will be facing water-based vulnerability.

Water plays an important role in the world economy, as it functions as a solvent for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately 70% of freshwater is consumed by agriculture.
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The color of water is a subject of both scientific study and popular misconception. Pure water has a light blue color which becomes a deeper blue as the thickness of the observed sample increases. The blue color is caused by selective absorption and scattering of the light spectrum. Impurities dissolved or suspended in water may give water different colored appearances.

Color of lakes and oceans


Large bodies of water such as oceans manifest water's inherent slightly blue color, not a reflection of the blue sky, as was once believed.


It is a common misconception that in large bodies, such as the oceans, the water's color is blue due to the reflections from the sky on its surface. This is not true, but was believed to be so decades ago. The main reason the ocean is blue is because water itself is a blue-colored chemical. Optical scattering from water molecules provides a second source of the blue color, but colored light caused by scattering only becomes significant with extremely pure water.

According to the frequency spectra for pure liquid water, a short water column has a very light shade of turquoise blue. Thicker layers (many meters) appear much darker blue. It is only when collected in a large body that water's blue color becomes apparent.
If the oceans owed their color to the sky, they would be a lighter shade of blue and would be white on cloudy days. Some constituents of sea water can influence the shade of blue you see in the ocean. This is why it can look greener or bluer in different areas. 

A swimming pool with a white painted bottom should look white, yet the water appears turquoise blue, even as it is observed in indoor pools where there’s no sky to be reflected. 

The reason why water is a blue colored substance involves the theory of radiative transfer (absorption and scattering), and material electromagnetic spectra.

 Note that heavy water, D2O, is transparent and not blue as is H2O.
Scattering from suspended particles also plays an important role in the color of lakes and oceans. A few tens of meters of water will absorb all light, so without scattering, all bodies of water would appear black. Because most lakes and oceans contain suspended living matter and mineral particles, known as colored dissolved organic matter (CDOM) light from above is reflected upwards. 

Scattering from suspended particles would normally give a white color, as with snow, but because the light first passes through many meters of blue-colored liquid, the scattered light appears blue. In extremely pure water as is found in mountain lakes, where scattering from white colored particles is missing, the scattering from water molecules themselves also contributes a blue color.

Color of water samples

The color of a small water sample is caused by both dissolved and particulate material in water, and is measured in Hazen Units (HU). Either of these components can be deeply colored, for instance dissolved organic molecules called tannins can result in dark brown colors, or algae floating in the water ("particles") can impart a green color. But in a lot of cases water is a clear to neutral color due to a lack of pigments in the water. (e.g. the sea)
The color of a water sample can be reported as:
  1. Apparent color
  2. True color
Apparent color is the color of the whole water sample, and consists of color due to both dissolved and suspended components.
True color is measured by filtering the water sample to remove all suspended material, and measuring the color of the filtered water, which represents color due to dissolved components.
Testing for color can be a quick and easy test which often reflects the amount of organic material in the water (although certain inorganic components like iron or manganese can also impart color).

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.Water owes its intrinsic blueness to selective absorption in the red part of its visible spectrum. The absorbed photons promote transitions to high overtone and combination states of the nuclear motions of the molecule, i.e. to highly excited vibrations. To our knowledge the intrinsic blueness of water is the only example from nature in which color originates from vibrational transitions. Other materials owe their colors to the interaction of visible light with the electrons of the substances. Their colors may originate from resonant interactions between photons and matter such as absorption, emission, and selective reflection or from non-resonant processes such as Rayleigh scattering, interference, diffraction, or refraction, but in each case, the photons interact primarily or exclusively with electrons. The details of the mechanism by which water is vibrationally colored will be discussed in the paragraphs which follow.Laboratory observation of the vibrational transitions that give water its color requires only simple equipment. Figure 1 gives the visible and near IR spectrum of H2O at room temperature recorded using a Perkin Elmer Lambda 9 Spectrophotometer and a 10 cm quartz cell filled with "nanopure" water from an ion exchange apparatus manufactured by Barnstead. Lower purity, distilled water gave an almost identical spectrum. The absorption below 700 nm in wavelength contributes to the color of water. This absorption consists of the short wavelength tail of a band centered at 760 nm and two weaker bands at 660 and 605 nm. The vibrational origin of this visible absorption of H2O is demonstrated in Figure 1 by the spectrum of D2O recorded in the same 10 cm cell. D2O is colorless because all of its corresponding vibrational transitions are shifted to lower energy by the increase in isotope mass. For example the H2O band at 760 nm is shifted to approximately 1000 nm in D2O (See Figure 1 Below )

Figure 1.
The upper spectrum is that for liquid H2O in a 10 cm cell at room temperature; the lower spectrum is for D2O under the same conditions. No cell was used in the reference beam of the spectrophotometer so that the "baseline" absorbance of approximately 0.04 originates in cell reflections.

The blue color of water may be easily seen with the naked eye by looking through a long tube filled with purified water. We used a 3 m long by 4 cm diameter length of aluminum tubing with a Plexiglass window epoxied to one end of the tube. Ten or more observers each reported seeing a blue color when they looked through the tube and observed a sunlight-illuminated white paper placed below the vertically-suspended tube (see for yourself in Fig. on the right: H2O- on the left and blue, D2O-on the right and transparent).

This observation is in accord with the spectrum of H2O recorded in Fig. 1. For example, from the measured absorbance at 660 nm, the calculated transmission of a 3 m water-filled tube is 44% -- a loss of red intensity that should be perceptible. Light transmitted through the empty cell was white. The large tube volume and a limited budget precluded checking to see if light transmitted through a D2O filled tube was indeed white, as expected.
 Water is unique among the molecules of nature in its high concentration of OH bonds and in its plentiful supply. Most important, the OH symmetric (v1) and antisymmetric (v3) vibrational stretching fundamentals are at high enough energy [3650 cm-1 and 3755 cm-1, respectively] (1) so that a four quantum overtone transition (v1+ 3v3) occurs at 14,318.77 cm-1 (698 nm), just at the red edge of the visible spectrum. As we shall see, these gas phase transition energies are all shifted to lower energy by the hydrogen bonding of liquid water.
 Because the absorption strength of the successive overtone transitions of water falls by a factor of 10 to 20 as the number of stretching quanta increases by unity (2), overtone transitions with more than five stretching quanta do not significantly contribute to the color of water. Of course if the OH stretch were perfectly harmonic, the strength of all overtones and combination bands would be zero. 

Thus it is critical both that the OH potential is anharmonic and that the fundamental stretching frequencies are high enough that overtones with only four and five stretching quanta can contribute to absorption intensity in the visible. D2O is colorless, at least for path lengths on the order of meters, because a minimum of 6 stretching quanta would be required for any transition in the visible region.

To Be Continued

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