Color of water
The color of water is a subject of both scientific study and popular misconception. While relatively small quantities of water are observed by humans to be transparent, 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.
Intrinsic color
Liquid water's intrinsic natural color may be demonstrated by looking at a white light source through a long pipe, filled with purified water, that is closed at both ends with a transparent window. The light turquoise blue color is caused by weak absorption in the red part of the visible spectrum. Absorptions in the visible spectrum are usually attributed to excitations of the electronic energy states in matter. Water is a simple 3-atom molecule, H2O, and all its electronic absorptions occur only in the invisible ultraviolet region of the electromagnetic spectrum and therefore, cannot be responsible for the intrinsic water color. The fundamental vibrational modes of water, having only three such vibrations, on the other hand, fall into the microwave region, far too low in energy to be visibly observed. The observed absorption in the visible spectrum is a result of the fourth harmonic frequency of the bond-stretching vibrations and some other overtone bands.[1] It is a rare example of such occurrences in nature and only naturally visible because of the large bodies of water on Earth.
Color of lakes and oceans
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.[citation needed] Reflection of light off the surface of water only contributes significantly when the water surface is extremely still,[citation needed] i.e., mirror-like, and the angle of incidence is high, as water's reflectivity rapidly approaches near total reflection under these circumstances, as governed by the Fresnel equations.
Optical scattering of unabsorbed light from water molecules as well as from impurities in the water of oceans or lakes back into the atmosphere provides the opportunity to visibly observe the blue color of water from land or airplanes. The back-scattering from water molecules alone is very small and only observable in highly purified water.[2]
If the oceans owed their color to the sky, they would be a lighter shade of blue and would be colorless[clarification needed] on cloudy days.[citation needed] Some constituents of sea water can influence the shade of blue of 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.
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.
Another phenomenon that occurs is Rayleigh scattering in the atmosphere along one's line of sight: the horizon is typically 4-5 km distant and the air (being just above sea level in the case of the ocean) is at its densest. This mechanism would add a blue tinge to any distant object (not just the sea) because blue light would be scattered into one's line of sight.
Color of water samples
Dissolved and particulate material in water can cause discoloration. Slight discoloration is measured in Hazen Units (HU).[3] Impurities can be deeply colored as well, 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.
The color of a water sample can be reported as:
- Apparent color is the color of the whole water sample, and consists of color from both dissolved and suspended components.
- True color is measured after filtering the water sample to remove all suspended material.
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.
Water quality and color
The presence of color in water does not necessarily indicate that the water is not potable. Color-causing substances such as tannins may be harmless.
Color is not removed by typical water filters; however, slow sand filters can remove color, and the use of coagulants may also succeed in trapping the color-causing compounds within the resulting precipitate.
Other factors can affect the color we see:
- Particles and solutes can absorb light, as in tea or coffee. Green algae in rivers and streams often lend a blue-green color. The red sea has occasional blooms of red Trichodesmium erythraeum algae.
- Particles in water can scatter light. The Colorado river is often muddy red because of suspended reddish silt in the water. Some mountain lakes and streams with finely ground rock, such as glacial flour, are turquoise. Light scattering by suspended matter is required in order that the blue light produced by water's absorption can return to the surface and be observed. Such scattering can also shift the spectrum of the emerging photons toward the green, a color often seen when water laden with suspended particles is observed.
- The surface of seas and lakes often reflect blue skylight, making them appear bluer. The relative contribution of reflected skylight and the light scattered back from the depths is strongly dependent on observation angle.
Color names
Various cultures divide the semantic field of colors differently than the English language usage and do not make the blue-green distinction in the same way. An example is Welsh where glas is the color of the sea and also that of grass. Sometimes the word grue ("green + blue") is used to translate this nuance into English.
Other color names assigned to bodies of water are sea green and ultramarine blue.
Unusual oceanic colorings have given rise to the terms red tide and black tide.
Furthermore, the Ancient Greek poet Homer uses the epithet "wine-dark sea"; in addition, he also describes the sea as "grey". Some have suggested that this is due to the Ancient Greeks classifying colors primarily by darkness rather than hue.
References
- ^ Braun CL, Smirnov SN (1993). "Why is water blue?" (PDF). J. Chem. Edu. 70 (8): 612.
- ^ Pettit E (1936). "On the Color of Crater Lake Water". Proc. Natl. Acad. Sci. U.S.A. 22 (2): 139–46. doi:10.1073/pnas.22.2.139. PMC 1076722. PMID 16588059.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ International Organization for Standardization, ISO 2211:1973, Measurement of colour in Hazen units (platinum-cobalt scale) of Liquid Chemical Products