Color of Water - Intrinsic Color

Intrinsic Color

For more details on this topic, see Electromagnetic absorption by water.

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 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.

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 Rayleigh scattering.