Color of water
Encyclopedia
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 colorless
, pure water has a slight blue
color
that becomes a deeper blue as the thickness of the observed sample increases. The blue hue of water is an intrinsic property and is caused by selective absorption and scattering
of white light. Impurities dissolved or suspended in water may give water different colored appearances.
blue color is caused by weak absorption
in the red part of the visible spectrum
.
For most substances, absorptions in the visible spectrum are usually attributed to excitations of electronic
energy states. However, water is a simple 3-atom molecule
, H2O, and all its electronic absorptions occur in the ultraviolet
region of the electromagnetic spectrum
and are therefore not responsible for the color of water in the visible region of the spectrum.
The water molecule has three fundamental modes of vibration, including two stretching vibrations of the O-H bond
s which occur at v1 = 3650 cm−1 and v3 = 3755 cm−1. Absorption due to these vibrations occurs in the infrared
region of the spectrum. The absorption in the visible spectrum is due mainly to the harmonic
v1 + 3v3 = 14,318 cm−1, which is equivalent to a wavelength of 698 nm.
Absorption intensity decreases markedly with each successive overtone, resulting in very weak absorption for the third overtone. For this reason, the pipe needs to have a length of a metre or more and the water must be purified by microfiltration
to remove any particles that could produce Rayleigh scattering
.
It is a common misconception that in large bodies, such as the ocean
s, the water's color is blue
due to the reflections from the sky on its surface.
The opportunity to visibly observe the blue color of water from land or airplanes is provided by the optical scattering of unabsorbed light from water molecules, from white sandy ocean bottoms, as well as from suspended particles in the water. The back-scattering from water molecules alone is very small and only observable in highly purified water.
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. Water in swimming pools (which may also contain various chemicals) with white-painted sides and bottom will appear as a turquoise blue.
Clean water appears blue in white-tiled swimming pools as well as in indoor pools where there is no blue sky to be reflected. The deeper the pool, the bluer the water.
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.
The surfaces 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.
s are large bodies of ice
and snow
formed during very cold climate
s by processes involving the compaction of fallen snow. While snowy glaciers appear white from a distance, up close and when shielded from direct ambient light, glaciers usually appear a deep blue due to the long path lengths of the internal reflected light.
Dissolved and particulate material in water can cause discoloration. Slight discoloration is measured in Hazen units (HU). Impurities can be deeply colored as well, for instance dissolved organic compound
s 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:
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 color can reveal physical, chemical and bacteriological conditions that give them the colors. In public and domestic drinking water they can be as such: Green can represent copper leaching from copper plumbing and, can also, represent algae growth. Blue can represent represent copper also and can represent a syphoning of industrial cleaners in the tank of commodes, commonly known as backflowing. Reds can be both signs of rust from iron pipes and airborne bacteria's from lakes etc. Black water can indicate sulfur reducing bacteria growth inside of a hot water tank set at less than 120 degrees temperature. This is normally present with a strong sulfur or rotten egg odor and is easily corrected by draining the water heater and increasing the temperature to 120 or higher. Caution should be given if children or elderly will be using the hot water. The presence of a rotten egg odor will always be in the hot water side if sulfate reducing bacteria is the cause and never in the cold water side. The color spectrum with water indicators is wide and, if learned, can make solving cosmetic, bacteriological and chemical problems easier to identify.
Color is not removed by typical water filters; however, slow sand filter
s 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:
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.
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 Greek
s classifying colors primarily by luminosity rather than hue.
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
are observed by humans to be colorless
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...
, pure water has a slight blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
that becomes a deeper blue as the thickness of the observed sample increases. The blue hue of water is an intrinsic property and is caused by selective absorption and scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
of white light. Impurities dissolved or suspended in water may give water different colored appearances.
Intrinsic color
The intrinsic natural color of liquid water 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 turquoiseTurquoise
Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium, with the chemical formula CuAl648·4. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue...
blue color is caused by weak absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
in the red part of the visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
.
For most substances, absorptions in the visible spectrum are usually attributed to excitations of electronic
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
energy states. However, water is a simple 3-atom molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
, H2O, and all its electronic absorptions occur in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
region of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
and are therefore not responsible for the color of water in the visible region of the spectrum.
The water molecule has three fundamental modes of vibration, including two stretching vibrations of the O-H bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
s which occur at v1 = 3650 cm−1 and v3 = 3755 cm−1. Absorption due to these vibrations occurs in the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
region of the spectrum. The absorption in the visible spectrum is due mainly to the harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
v1 + 3v3 = 14,318 cm−1, which is equivalent to a wavelength of 698 nm.
Absorption intensity decreases markedly with each successive overtone, resulting in very weak absorption for the third overtone. For this reason, the pipe needs to have a length of a metre or more and the water must be purified by microfiltration
Microfiltration
Microfiltration is a membrane technical filtration process which removes contaminants from a fluid by passage through a microporous membrane. A typical microfiltration membrane pore size range is 0.1 to 10 micrometres...
to remove any particles that could produce Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
.
Color of lakes and oceans
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
s, the water's color is blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
due to the reflections from the sky on its surface.
The opportunity to visibly observe the blue color of water from land or airplanes is provided by the optical scattering of unabsorbed light from water molecules, from white sandy ocean bottoms, as well as from suspended particles in the water. The back-scattering from water molecules alone is very small and only observable in highly purified water.
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. Water in swimming pools (which may also contain various chemicals) with white-painted sides and bottom will appear as a turquoise blue.
Clean water appears blue in white-tiled swimming pools as well as in indoor pools where there is no blue sky to be reflected. The deeper the pool, the bluer the water.
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
Colored dissolved organic matter
Colored dissolved organic matter is the optically measurable component of the dissolved organic matter in water. Also known as chromophoric dissolved organic matter, yellow substance, and gelbstoff, CDOM occurs naturally in aquatic environments primarily as a result of tannins released from...
(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.
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
in the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
along one's line of sight: the horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...
is typically 4–5 km distant and the air (being just above sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
in the case of the ocean) is at its densest
Density of air
The density of air, ρ , is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity...
. 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.
The surfaces 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 of glaciers
GlacierGlacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s are large bodies of ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
and snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...
formed during very cold climate
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
s by processes involving the compaction of fallen snow. While snowy glaciers appear white from a distance, up close and when shielded from direct ambient light, glaciers usually appear a deep blue due to the long path lengths of the internal reflected light.
Color of water samples
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s called tannins can result in dark brown colors, or algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
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 dissolvedSolutionIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
and suspendedSuspension (chemistry)In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
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 color can reveal physical, chemical and bacteriological conditions that give them the colors. In public and domestic drinking water they can be as such: Green can represent copper leaching from copper plumbing and, can also, represent algae growth. Blue can represent represent copper also and can represent a syphoning of industrial cleaners in the tank of commodes, commonly known as backflowing. Reds can be both signs of rust from iron pipes and airborne bacteria's from lakes etc. Black water can indicate sulfur reducing bacteria growth inside of a hot water tank set at less than 120 degrees temperature. This is normally present with a strong sulfur or rotten egg odor and is easily corrected by draining the water heater and increasing the temperature to 120 or higher. Caution should be given if children or elderly will be using the hot water. The presence of a rotten egg odor will always be in the hot water side if sulfate reducing bacteria is the cause and never in the cold water side. The color spectrum with water indicators is wide and, if learned, can make solving cosmetic, bacteriological and chemical problems easier to identify.
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 filter
Slow sand filter
Slow sand filters are used in water purification for treating raw water to produce a potable product. They are typically 1 to 2 metres deep, can be rectangular or cylindrical in cross section and are used primarily to treat surface water...
s can remove color, and the use of coagulants
Coagulation
Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...
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 erythraeumTrichodesmiumTrichodesmium, also called sea sawdust, is a genus of filamentous cyanobacteria. They are found in nutrient poor tropical and subtropical ocean waters . Trichodesmium fixes atmospheric nitrogen into ammonium, usable also for other organisms...
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 flourRock flourRock flour, or glacial flour, consists of fine-grained, silt-sized particles of rock, generated by mechanical grinding of bedrock by glacial erosion or by artificial grinding to a similar size...
, 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.
Color names
Semantic field
A semantic field is a technical term in the discipline of linguistics to describe a set of words grouped by meaning in a certain way. The term is also used in other academic disciplines, such as anthropology and computational semiotics.-Definition and usage:...
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.
Other color names assigned to bodies of water are sea green and ultramarine
Ultramarine
Ultramarine is a blue pigment consisting primarily of a double silicate of aluminium and sodium with some sulfides or sulfates, and occurring in nature as a proximate component of lapis lazuli...
blue. Unusual oceanic colorings have given rise to the terms red tide
Red tide
Red tide is a common name for a phenomenon also known as an algal bloom , an event in which estuarine, marine, or fresh water algae accumulate rapidly in the water column and results in discoloration of the surface water. It is usually found in coastal areas...
and black tide
Oil spill
An oil spill is the release of a liquid petroleum hydrocarbon into the environment, especially marine areas, due to human activity, and is a form of pollution. The term is mostly used to describe marine oil spills, where oil is released into the ocean or coastal waters...
.
Furthermore, the Ancient Greek poet Homer
Homer
In the Western classical tradition Homer , is the author of the Iliad and the Odyssey, and is revered as the greatest ancient Greek epic poet. These epics lie at the beginning of the Western canon of literature, and have had an enormous influence on the history of literature.When he lived is...
uses the epithet
Epithet
An epithet or byname is a descriptive term accompanying or occurring in place of a name and having entered common usage. It has various shades of meaning when applied to seemingly real or fictitious people, divinities, objects, and binomial nomenclature. It is also a descriptive title...
"wine-dark sea"; in addition, he also describes the sea as "grey". Some have suggested that this is due to the Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
s classifying colors primarily by luminosity rather than hue.
External links
- What color is water?
- Why is Water Blue?
- Is water blue? The absorption spectrum of water in the visible range
- Why is the snow blue?
- The Color of the Ocean from science@nasaNASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...