Evaporation, the process where liquid water turns into a gas, is the foundation of the Earth’s natural water cycle. This phase change is an effective purification mechanism, which is why rain and snow are naturally free of most contaminants. Evaporation is a powerful method for separating pure water from most dissolved and suspended impurities. However, the resulting water vapor is not perfectly pure because certain contaminants can also vaporize and travel with the water.
The Physics of Phase Change
The purification power of evaporation lies in the fundamental difference between the physical properties of water molecules and most contaminants. Evaporation occurs when individual water molecules gain enough kinetic energy to break their hydrogen bonds and escape as water vapor. This process happens at any temperature, using energy from the surroundings.
When water transitions into a gaseous state, only the relatively small H₂O molecules easily escape. Most dissolved substances, known as solutes, have different vapor pressures and molecular weights. The energy required for these impurities to transition into a gas is far higher than that required for water. This difference in volatility separates the pure water vapor from the bulk of the impurities left behind in the residual liquid.
Contaminants Left Behind
Evaporation is highly effective at removing non-volatile substances, which do not easily turn into a gas at water’s temperature. These substances remain concentrated in the liquid phase as the pure water escapes. The non-volatile category includes all inorganic salts, such as sodium chloride, calcium, and magnesium, which cause water hardness and salinity.
Heavy metals, including lead, mercury, and arsenic, are also left behind because they are too dense and non-volatile to travel with the water vapor. This physical exclusion is also effective against virtually all biological contaminants. Bacteria, viruses, protozoa, and cysts are physically much larger than water molecules and cannot become airborne in the water vapor phase.
Volatile Compounds That Remain
The limitation of evaporation comes from volatile organic compounds (VOCs). VOCs are carbon-containing chemicals with high vapor pressure and low boiling points, meaning they vaporize easily near water’s temperature. If water containing VOCs is evaporated, these compounds co-evaporate alongside the water molecules. When the resulting water vapor is collected and condensed, the VOCs travel with it, re-contaminating the purified water.
Examples of these problematic compounds include:
- Benzene and toluene, which are components of gasoline.
- Industrial solvents like trichloroethylene and perchloroethylene, used in dry cleaning.
- Certain pesticides and herbicides.
- Disinfection byproducts, like trihalomethanes, formed when chlorine reacts with organic matter.
Specialized methods are often needed to eliminate these co-evaporating volatile compounds, even though evaporation removes virtually all dissolved solids and biological threats.