Color of water

Lore & Background

The intrinsic color of liquid water can be demonstrated by looking at a white light source through a long pipe filled with purified water and closed with transparent windows. The light sky blue color is caused by weak absorption in the red part of the visible spectrum. Water's electronic absorptions occur only in the ultraviolet, so visible color arises from vibrational overtones: the harmonic v1 + 3v3 at 14,318 cm⁻¹ (698 nm) and, in liquid at 20 °C, red-shifted absorptions at 740 nm and 660 nm due to hydrogen bonding. Heavy water (D2O) lacks this red absorption because its vibrational transitions have lower energy, so large bodies of D2O would not appear sky blue.

Lakes and oceans appear cyan for multiple reasons. The surface reflects the sky, but most light penetrates and interacts with water molecules, which absorb red, orange, and yellow wavelengths, leaving green, cyan, and blue. Scattering from suspended particles also plays an important role; without scattering, all bodies of water would appear black. In extremely pure mountain lakes, scattering from water molecules themselves contributes a cyan color. Glaciers appear deep blue up close because long path lengths of internal reflected light occur after air bubbles are squeezed out by pressure, increasing ice density.

Dissolved and particulate material can cause water to appear green, tan, brown, or red. Tannins produce dark brown colors; algae impart green. Standard color scales include the Forel-Ule scale and the Platinum-Cobalt scale (Hazen units). Apparent color includes reflected surface color and dissolved/suspended components, while true color is measured after purification. Color testing can reflect organic material or inorganic components like iron or manganese. In drinking water, green may indicate copper or algae, blue may indicate copper or backflowing cleaners, reds may indicate rust or airborne bacteria, and black water may indicate sulfur-reducing bacteria in hot water tanks.

Reader's Guide

The article establishes that water's intrinsic blue color is a fundamental physical property arising from weak absorption of red light by vibrational overtones of the water molecule, not from electronic transitions. This understanding is significant because it explains why large bodies of pure water appear cyan or blue, and why heavy water would lack this color. The article clarifies that the common belief that water simply reflects the sky is only a partial explanation; the primary cause is absorption of red wavelengths within the water itself. This knowledge has practical implications for water quality assessment: color can reveal physical, chemical, and bacteriological conditions, such as copper leaching, algae growth, rust, or sulfur-reducing bacteria. The distinction between apparent and true color provides a standardized method for measuring water purity. The article also notes that color does not necessarily indicate water is undrinkable; particulate color can be filtered, while dissolved substances like tannins are only toxic in large concentrations. The legacy of this understanding is its use in environmental monitoring, drinking water treatment, and the interpretation of natural water body colors, from mountain lakes to glaciers.

Did You Know?

• Heavy water (D2O) does not absorb red light, so large bodies of it would lack the characteristic sky blue of light water.
• Without scattering from suspended particles, all bodies of water would appear black because a few tens of meters of water absorb all light.
• The harmonic v1 + 3v3 at 14,318 cm⁻¹ (698 nm) is the main absorption responsible for water's visible color.