Color of water

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.

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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. 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.^[1] 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.^[2] 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).

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.

Names

Other cultures divide the semantic field of colors differently than the English language 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 colors have given name to the red tide and the black tide.

References