Why is water blue? (vibrations & rotations)

Water has an intrinsic color, and this color has a unique origin. This intrinsic color is easy to see, as can been seen in the Caribbean and Mediterranean Seas and in Colorado mountain lakes. Pure water and ice have a pale blue color, best seen at tropical white-sand beaches and in ice caves in glaciers (green colors are usually derived from algae). It is neither due to light scattering (like the sky), nor dissolved impurities (e.g., Cu²⁺). Because the absorption which gives water its color is in the red end of the visible spectrum, one sees blue, the complementary color of orange, when observing light that has passed through several meters of water. This color of water can also be seen in snow and ice as an intense blue color scattered back from deep holes in fresh snow.

The faint blue color of water is seen in this photo. Here, you look upwards through 3 meter long sealed aluminum tubes filled with purified water. On the left, the faintly bluish tube contains regular (light) water, and at right, the clear tube contains heavy water.

Water owes its intrinsic blueness to selective absorption in the red part of its visible spectrum. The absorbed photons promote transitions to high overtone and combination states of the nuclear motions of the molecule, i.e. to highly excited vibrations. To our knowledge the intrinsic blueness of water is the only example from nature in which color originates from vibrational transitions. Other materials owe their colors to the interaction of visible light with the electrons of the substances. Their colors may originate from resonant interactions between photons and matter such as absorption, emission, and selective reflection or from non-resonant processes such as Rayleigh scattering, interference, diffraction, or refraction, but in each case, the photons interact primarily or exclusively with electrons. The details of the mechanism by which water is vibrationally colored will be discussed in the paragraphs which follow.

Laboratory observation of the vibrational transitions that give water its color requires only simple equipment. The graph at right gives the visible and near IR spectrum of H₂O at room temperature. The absorption below 700 nm in wavelength contributes to the color of water. This absorption consists of the short wavelength tail of a band centered at 760 nm and two weaker bands at 660 and 605 nm. The vibrational origin of this visible absorption of H₂O is demonstrated by the spectrum of heavy water, D₂O. Heavy water is chemically the same as regular (light) water, but with the two hydrogen atoms (as in H₂O) replaced with deuterium atoms (deuterium is an isotope of hydrogen with one extra neutron -- the extra neutron that makes heavy water "heavy," about 10% heavier). Heavy water is colorless because all of its corresponding vibrational transitions are shifted to lower energy by the increase in isotope mass. For example the H₂O band at 760 nm is shifted to approximately 1000 nm in D₂O.


	Hydrogen bonding (purple) occurs when an atom of hydrogen is attracted by rather strong forces to two atoms instead of only one, so that it may be considered to be acting as a bond between them. In water the hydrogen atom (white) is covalently attached to the oxygen (red) of a water molecule (about 470 kJ/mol) but has an additional attraction (about 22 kJ/mol) to a neighboring oxygen atom of another water molecule.

Hydrogen bonding makes liquid water more colorless than gas. Water is unique among the molecules of nature in its high concentration of OH bonds and in its plentiful supply. Most important, the OH symmetric (v₁) and antisymmetric (v₃) vibrational stretching fundamentals are at high enough energy so that a four quantum overtone transition (v₁+ 3v₃) occurs just at the red edge of the visible spectrum. These gas phase transition energies are all shifted to slightly lower energy by the hydrogen bonding of liquid water.

The role of hydrogen bonding can be determined by comparing gas and liquid phase water. When comparing the vibrational transitions of gaseous and liquid water, the liquid phase OH stretching band centered at about 3400 cm^{- 1} is red-shifted from the gas phase values of v₁ and v₃ by several hundred wavenumbers. This shift is primarily the result of hydrogen bonding in the liquid. The shift to lower energy induced by hydrogen bonding is seen most clearly in a comparison of the stretching frequencies observed for monomeric and dimeric water in a solid nitrogen matrix. While the monomer OH stretching bands v₁ and v₃are slightly red shifted (ca. 25 cm^-1) from gas phase energies by the matrix, the frequency of the dimer (with hydrogen bonding) OH stretch involved in a hydrogen bond was red shifted to 3547 cm^{- 1}. That is 105 and 209 cm^-1 below the gas phase values of v₁ and v₃, respectively. The near IR absorption bands of ice are the most red-shifted of all.

As the temperature is increased, hydrogen bonding decreases in importance to the point that at 375º C the liquid peaks are only an average of 80 cm^-1 below the gas phase transitions. The red absorption which gives rise to the blue color of liquid water can be plausibly ascribed to high energy vibrational overtone and combination bands which, like the other vibrational transitions, have been shifted to lower energy by hydrogen bonding. Also, as the temperature of H₂O is lowered, the band near 760 nm shifts to lower energy but also broadens enough to slightly increase the intensity near 700 nm. However, the changes are small enough that the color of water should not vary significantly with temperature between 0 and 50º C.

Hydrogen bonding in water causes the stretching frequencies of H₂O to shift to lower values, making water more clear. If water did not have hydrogen bonds, it would still be colored, perhaps even more intensely than is actual water.

One might assume that some other hydrogen-containing liquids and solids beside water, such as liquid ammonia, could possess traces of bluish color because of similar effects. However, water and ice are the only two chemical substances we normally have the opportunity to observe in pure form in sufficiently large bulk so that a weak coloration becomes detectable.

What color is water?