Colour of Water

Relatively small quantities of waters observed by humans to be colourless.  The fact is, pure water has a light blue colour which become deeper blue as the thickness of the observed samples increases.   Impurities dissolved or suspended in water may give water different coloured appearance.

Liquid water’s intrinsic natural colour 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 is caused by weak absorption in the red part of the visible spectrum.  Absorptions in the visible spectrum are attributed to excitations of the electronic energy states in matter.

The water molecule has 3 fundamental modes of vibration.  There are two O-H stretching vibrations which occure at ca. 3500 cm^-1 and an H-O-H bending vibration at ca. 1640 cm^-1.   Absorption due to these vibrations occurs in the infrared region of the spectrum.

The observed absorption in the visible spectrum is due mainly to 4^th harmonic frequency ( 3^rd overtone ) of O-H bond-stretching vibrations. 4 x 3500 = 14000 cm cm^-1  is equivalent to a wavelength of ca. 715 nm.  The actual absorption maximum occurs at 698 nm. 

Because absorption intensity decreased significantly with each successive overtone, absorption due to the 4^th overtone is very weak, so to see the blue colour the pipe needs to be a meter or more in length and the water be removed any particles that would otherwise cause color to be generated by Rayleigh scattering.

It is a common misconception that in large bodies, such as the oceans, the water’s colour is blue due to the reflection from the sky on its surface.  It is true only when the water surface is extremely still, 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. 

If the oceans owed their colour to the sky, they would be a lighter shade of blue and would be colourless on cloudy days.

Particles and solutes can absorb light.  Green algae in rivers often lend a blue-green colour.  The red sea has occasional bloom of red Trichodesmium erythraeum algae.  Tannins caused rivers to be dark brown.   High concentration of dissolved lime give the water a turquoise colour. 

Slight discoloration is measured in Hazen Units ( HU ). 

The colour of a water sample can be reported as :

1. Apparent colour, colour of the whole water sample, and consists of colour from both dissolved and suspended components

2. True colour, is measured after filtering the water sample to remove all suspended material.