What colour is water? Apparently it is a simple question, and the answer is usually that water is colourless, but is this completely true? As kids we are taught that water is colourless, odourless and tasteless. But if we only take into account the first of those, colour ─ the subject of this post ─ when we look at a lake or walk along the bank of a river, we see water bodies of different colours, sometimes water is greenish, other times blueish, often dark blue... Depending on the sky, on the time of day, on the amount of dissolved material in the water, whether the river bed is clean or dirty, water has different hues.
The truth is water does have a colour, but the human eye cannot see it in small amounts of water. To notice the colour of water we have to look at a large volume of water.
Water does have a colour, but the human eye cannot see it in small amounts of water
We know that the white light from sun, which is electromagnetic waves, is composed of almost all frequencies in the visible spectrum, that is, it contains all the colours in the rainbow from violet to red, through blue, green, yellow and orange. When white light hits an object, usually the object absorbs only a certain set of colours (or, in other words, of frequencies). The rest of colours that are not absorbed are transmitted, reflected, or scattered. The colour of water is due to the selective absorption of light. When light passes through water, water absorbs from the electromagnetic spectrum the wavelengths that correspond to red light, and therefore, the transmitted light that we see is blue (which is the complementary colour of red). In other words, water absorbs mostly the lower frequencies, associated to red and orange, whereas blue and green are completely transmitted. Therefore, a beam of light travelling through a large water body will lose its red hue, and will appear blue.
Also, the larger the volume of water, the more the absorption as a light beam goes through it, and so it appears more blue. To test it, you can do something as simple as filling a glass with seawater: we will see it transparent. However, if we throw it again into the sea we would see it blue again. This is because there is very weak absorption of the red wavelength by water, so for our eyes to be able to notice it, light needs to travel across water for quite a distance.
A popular belief maintains that the colour of water is caused by the reflection of the sky from a large surface, such as the sea. This is a myth, but not entirely devoid of truth. The water surface reflects a small part of the sky's light, on top of the blue colour of water itself. Establishing what is the contribution of the sky in any situation in complicated, because it depends on whether there are clouds, on the angle of observation, and on the continuous oscillation of the surface. However, it is estimated that for a calm sea under a blue sky, the contribution of the sky's blue colour does not exceed 2% of the water's colour.
But we don't always see water blue. There are other factors that affect our perception of the water's colour. For example, many rivers and estuaries have murky waters, predominantly brown in colour. That indicates that they have a large amount of suspended sediment, typically silt and clay. A high concentration of minerals or heavy metals can also colour the water, such as it happens in Río Tinto, a river in the province of Huelva, Spain.
Water colours caused by the growth of certain microorganisms are even more striking. Lakes, ponds and oceans are full of microorganisms, algae and bacteria, although they are not visible to the naked eye, since they range in size from a micron (one thousandth of a millimetre) to a few tens of microns. But there are times when microorganisms proliferate and reach huge concentrations (up to 10 million individuals per millilitre of water) and can stain water white, green or red. So, can we say that the colour we see in the water is caused by its contents?
Water absorbs mostly the lower frequencies, associated to red and orange, whereas blue and green are completely transmitted
Up to this point, we have talked about water in a liquid state, but water is not always found like that in nature. There is also snow, frozen water. So, after looking into why water is blue, we ask ourselves, why is snow white?
If we could look at snow through a magnifying glass, we would see it is formed by flakes, water crystals frozen around a dust particle. They have a star-like shape with six arms. As the flakes get together, air is trapped, and it is that air that gives snow its white colour. Air causes the scattering of light, that it, it absorbs light and emits it in all directions, since white light is just a collection of all the wavelengths in visible light.
But frozen water is not just white, there are also blue glaciers. Ice in glaciers has been under huge pressure for millions of years; it has become compacted and is free of air bubbles, so it absorbs better the range of the electromagnetic spectrum comprising red and yellow hues, thus enhancing the intense blue colour we sometimes see in the ice.
As we have been able to see, the colour palette of water is quite diverse, based on the laws of physics, but also on the purity and constituents of water.