Colour of Water

We have learnt in schools that pure water is colourless, tasteless and odorless. Is pure water that very clear in the natural setting? The water we drink from the tap contains dissolved minerals and suspended materials. When we fill it in a glass it may appear colourless. But the fact is that pure water is not at all colourless. Water appears to have a tint of blue when we look through a long column of water.

Water in a glass may appear colourless but the blue tint becomes deeper when the thickness of water increases. The colour of water is blue because of the molecular behavior, structure and hydrogen bonding.

Water is differently coloured due to the colour of the suspended particles or dissolved impurities. For example, muddy water looks brown in colour because mud is suspended in it. Green algae in rivers and lakes lend blue green colour to water. A pinch of pink food colour added to water makes the water look pinkish as food colour is the dissolved impurity of water.

Water comprises of particles. Water is transparent. Therefore, light can penetrate the transparent particles of water. The particles in water can absorb light, scatter light and reflect light.

Lakes and oceans appear blue for several reasons. One is that the surface of the water reflects the colour of the sky. Sea water is rich in several dissolved salts. So the sea appears either green or blue. Water in the swimming pools also contains chemicals with white-painted sides and bottom will appear turquoise blue. So water in the swimming pool appears turquoise blue in colour. If the swimming pool is deep, then water appears bluer.

Lakes and oceans contain suspended living matter and mineral particles, known as coloured dissolved organic matter. Light from above is reflected upwards. Scattering of suspended particles would normally give a white colour, but because the light first passes through many meters of blue-coloured liquid, the scattered light appears blue. 

The blue hue of water is an intrinsic property of water. It is caused by the selective absorption and scattering of white light. When sun light falls on water all the colours of rainbow are mixed with water. Water absorbs red, yellow, green light. Blue light is reflected back. The blue colour is caused by the weak absorption in the red part of the visible spectrum as blue is the complementary colour of orange. The intrinsic blueness of water originates from vibrational transitions.

Colour is caused by the interaction of photons of light with electrons. Other materials owe their colours to the interaction of visible light with the electrons of the substances. Their colours may originate from resonant interactions between photons and matter. The mechanism includes absorption, emission and selective reflection. Other mechanisms are Rayleigh scattering, interference, diffraction and refraction. But in each case, the photons interact primarily or exclusively with electrons. 

                                                            

A molecule of water has totally three atoms. The water molecule has three fundamental nodes. Water is unique among the nature because it has high concentration of OH bonds. The OH bonds vibrate and stretch symmetrically and anti-symmetrically at the red edge of the visible spectrum. Hydrogen bonding in water causes the stretching frequencies of water to lower values. Suppose water did not have hydrogen bonds, the colour of water will be intense blue.

The colour of water can be demonstrated by a simple experiment: Take a long pipe filled with purified water so that the water doesn’t flood out when we look through it. Cover each end with a transparent window. Take a piece of paper. Focus it towards the sunlight. Look down through any one of these windows through the piece of paper lit by natural sunlight. We will see the pale blue colour of water.