The question of what chemical turns water blue is complex because the most common answer involves the water molecule itself rather than an added compound. The vibrant blue color seen in oceans and deep lakes results from the inherent properties of the H2O molecule interacting with light. Specific chemical additives are also commonly used to achieve a striking blue hue in engineered environments like swimming pools. Understanding water coloration requires exploring both the physical mechanisms of the pure substance and the effects of dissolved chemical compounds.
The Physics Behind Water’s Natural Blue Color
The natural blue tint of large bodies of water is an intrinsic property of the water molecule, not a reflection of the sky or an impurity. Sunlight, which appears white, is composed of a spectrum of colors, each corresponding to a different wavelength. When light penetrates water, the H2O molecules selectively absorb the longer wavelengths of the visible spectrum.
The colors at the red end of the spectrum—red, orange, and yellow—are absorbed more readily than the shorter, blue wavelengths. This absorption is due to the energy of the photons promoting transitions to high overtone and combination states of the nuclear motions of the water molecule. This process involves highly excited vibrations within the molecular structure.
After the red light is absorbed, the remaining light contains a greater proportion of blue. This blue light then scatters in various directions, allowing it to return to the observer’s eye from within the water column. The blue color is the result of the water acting like a filter, absorbing the complementary color (red) and scattering the unabsorbed color (blue).
Chemical Compounds That Cause Blue Coloration
While pure water has a physical mechanism for appearing blue, many human-made systems, like pools or decorative fountains, owe their color to specific chemical additives. The most common chemical agent used to create a vibrant blue color is a copper-based compound, particularly hydrated copper sulfate. Hydrated copper sulfate (CuSO4 · 5H2O), often known commercially as bluestone, is a crystalline solid that produces a vivid blue color when dissolved in water.
The coloration arises from the interaction between the copper(II) cation (Cu2+) and the surrounding water molecules. In solution, the copper ion forms a coordination complex where water molecules act as ligands surrounding the central Cu2+ ion. This arrangement splits the energy levels of the copper ion’s d-orbitals.
The energy difference between these split orbitals corresponds to the energy of photons in the red-orange region of the visible light spectrum. When white light passes through the solution, the complex absorbs this red-orange light, causing an electronic transition (a d-d transition) within the copper ion. The remaining light that is transmitted or reflected is the complementary color, which is blue.
In practical applications, copper sulfate is frequently added to swimming pools and reservoirs primarily to control algae growth. The blue coloration is a secondary effect of its use as an algaecide. Other trace minerals, such as specific iron compounds or industrial dyes, may also contribute to blue coloration, but copper compounds are the most recognized chemical additive for creating this hue.
The Role of Depth and Volume
The visibility of water’s intrinsic blue color is directly related to the volume and depth of the water being observed. A small glass of pure water appears colorless because there are not enough H2O molecules within the light path to absorb a detectable amount of red light. The inherent filtering effect of the water is too weak to be noticed over a short distance.
For the blue color to become apparent, the light must travel through an extended column of water, typically several meters deep. This extended path length ensures that the cumulative effect of selective absorption by countless H2O molecules is sufficient to remove the red wavelengths from the light spectrum. The deeper the water, the more pronounced the blue color appears as the cumulative absorption increases.
This concept explains why a swimming pool with white plaster walls appears blue, while a sample taken from that pool in a bucket appears clear. The blue color arises from the light that penetrates the depth of the pool, is filtered by the water, and is then scattered back to the surface. The volume of water is the factor that allows the subtle molecular interaction to manifest as a visible color.