Even When Pure, Water Is Blue
    By Enrico Uva | July 4th 2011 01:34 PM | 11 comments | Print | E-mail | Track Comments
    About Enrico

    I majored in chemistry, worked briefly in the food industry and at Fisheries and Oceans. I then obtained a degree in education. Since then I have...

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    We have all seen supposedly colorless water in a glass and in raindrops on a windshield. If there's little air in it, water even looks colorless when it forms ice cubes. The fixed but incorrect idea that water is intrinsically without color survives in many of our minds even though we all observe various shades of blue in snow or glaciers or in large bodies of water.

    If you put a bit of shampoo in your hand, you will notice that it looks a little ligher in color than when it sits in a bottle. This makes us aware that light's path length (as light moves across a transparent object) influences the intensity of color. Could it be that water only appears colorless when there are not enough molecules in light's path to remove some of white light's components?

    I like to demonstrate this by simply using a large 8 inch white mixing bowl. If it's only 1/4 filled, the water still appears like it does when coming out of the tap. But when about 3/4 full, a pale blue colour becomes obvious. The bowl's white background helps because light bounces back and forth within it, allowing even more vibrating water molecules to absorb a red portion of the spectrum. Deprived of a bit of red, water in most of its forms consequently transmit a pale blue light.

    One can argue that the tap water I used is not pure. Admittedly, various ions, algae and even suspended silt and mud can definitely introduce all sorts of green and brown hues. If you fill a mixing bowl with deionized water from the lab, which is created by forcing water through an impurity-removing column, the water is still colored.
    Here's another picture that looks less greyish. It was obtained by placing the same deionized water in a narrower but taller white container.

    According to the Journal of Chem Ed authors, a better way to demonstrate this is to use a 3 m long by 4 cm diameter length of aluminum tubing with a Plexiglass window epoxied to one end of the tube. I tried it with a 2.5 m by 4 cm plastic pipe, sealed with a glass stopper. When I photographed it with a flash, I did see a beautiful blue color, but my control (a picture of the pipe without the water) appeared just as blue!

    In order to eliminate the scattering of blue light from suspended particles in water, it's not enough to use deionized water. Microfiltration is also necessary. Once that variable is removed, the persistent blue is sure to come from the 3rd overtone of the oxygen-hydrogen stretching vibration of the water molecule.

    In fact, the only way that water will leave white light alone is if its hydrogen atoms are replaced with a heavier form called deuterium. The key absorption peak then shifts into the invisible infrared. The following spectra are from the J of Chem Ed reference cited below, and I've pointed out the differences between purified and deuterated water.

     color wavelength range(nm)
     violet  380–450 
     blue  450–475 
     cyanide  475–495 
     green  495–570 
     yellow  570–590 
     orange  590–620
     red  620–750 

    If you have the artist's color wheel in mind, you may be confused by the absorption of red leading to the transmission of blue. Although red's opposite is green, red's wide range of wavelengths is actually complementary to all hues of green, cyanide and blue. I've drawn a color wheel which is proportional to perceived wavelengths to illustrate this point.

    Don't feel bad if you were unaware of the color of water. You have company. When Kurt Nassau published his book on colour in 1983, and even a decade later when an excellent article in the Journal of Chemical Education appeared on the subject, the authors pointed out that many scientists were under the illusion that water was intrinsically without color. I looked through my own books and found in The Flying Circus of Physics a completely incorrect explanation for why a lake can seem blue, attributing it mainly to reflections from the surface.


    Charles L. Braun and Sergei N. Smirnov. Why is Water Blue? J. Chem. Edu., 1993, 70(8), 612

    Kurt Nassau. The Physics and Chemistry of Color: the 15 Causes of Color. Wiley-Interscience. 1983

    Jearl Walker. The Flying Circus of Physics. Wiley and Sons. 1977


    The Flying Circus of Physics gets it wrong?  We need to rewrite the Standard Model!

    I imagine that similar overtone bands in methane might be responsible for the green and blue of Uranus and Neptune, and of ammonia for the orange and yellow of Jupiter and Saturn.
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    The Flying Circus of Physics gets it wrong? We need to rewrite the Standard Model!
    Except for the odd error, it's actually a gem of a book. In it, Jearl Walker asks all sorts of probing questions about the science of everyday occurrences. He even has a web site with updates, but as far as I can tell, he has not yet corrected the error in his color of water answer. His book was inspired by a student who asked, "“What has anything of this(physics) to do with my life?"
    Hi Enrico,

    Sabine also wrote on this, with a different slant, but worth mentioning.

    Thanks Tomasso for providing the relevant link.
    Actually you see the blue very well when scuba diving, the reds fade first and at around 120 feet the world is very blue (this is in the clear water around the Cayman Islands). Of course that is sea water but. This is why old fashioned film underwater photography always used a flash (not needed now the computer can fix the color balance for you).

    "green, cyanide and blue"

    Cyan, Sir, Cyan...

    Also, in lab tests with a Nessler tube and Eye-ball Mk_1, we'd check for 'Water White and Crystal Clear'...

    Photometers --And Spectro-photometers-- came later...

    Photographing your water-filled containers (and the long tube), I think you have been fooled by the white-balance of your digital camera.
    You can witness what I described without photographing it, and if you fill a white bathtub with the curtains shut with a grey sky, you will still see that water is not colourless. The fact that the sky can indeed change the colour of water through reflections does not alter the fact that water does indeed intrinsically have colour. For example, a certain amount of melanin makes eyes brown. But they can also reflect the color of clothes--that does not change the fact they are intrinsically brown.