Color of water
While relatively small quantities of water appear to be colorless, water's tint 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 the selective absorption and scattering of white light. Impurities dissolved or suspended in water may give water different colored appearances.
The intrinsic color of liquid water may be demonstrated by looking at a white light source through a long pipe that is filled with purified water and 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.
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. Nevertheless, the water molecule has three fundamental modes of vibration, including two stretching vibrations of the O-H bonds 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 meter or more and the water must be purified by microfiltration to remove any particles that could produce Mie scattering.
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