Joan Pringle of Oamaru asks :-

I once read that oxygen is blue because clear skies and lakes are blue. Is this so?

Henrik Kjaergaard, an atmospheric chemist at the University of Otago, responded.

Oxygen is blue, but not for the same reason the sky or water are blue.

The reason for the blue sky is because of the way light is scattered by small particles and fluctuations in density of air in the atmosphere. This Rayleigh scattering depends on the wavelength (colour) of the light. Blue light is scattered much more than red light. Of the light coming from the Sun, which has all colours, the blue is scattered most and hence the sky appears blue. On the Moon, which has no atmosphere, the sky is black.

During sunset, when the sunlight goes through a longer path of the atmosphere, even more of the blue light is scattered. So at sunset the Sun appears red. The lower on the horizon you see a sunset, the redder it looks.

Molecular oxygen, (O2), has a faint blue colour, which is most easily observed when it is liquefied. The reason for this colour is that the electrons in the oxygen molecule can absorb red light by going from one orbit (electron cloud) to another. Absorbing red coloured light means it will appear blue, whereas liquid nitrogen is colourless.

As for water (H2O), the atoms of hydrogen and oxygen in the water molecule vibrate. These vibrations depend on the masses of the atoms involved. These atoms can go from vibrating a little to vibrating more by absorbing light (energy). It just so happens that the vibrations in the water molecule will weakly absorb red light. A glass-full of water appear colourless, but in large quantities, such as swimming pools or lakes or seas, appears blue.

In heavy water (D2O) where the hydrogen is replaced by deuterium atoms, which are twice as massive as the hydrogen atom, the vibrations are very different and liquid heavy water is colourless.