Evaporation is the process where liquid water transitions into a gaseous state, known as water vapor. While it is a highly effective natural purification method, it is not flawless. Purification relies entirely on the vast difference in physical properties, such as boiling points and molecular attraction, between water and the substances dissolved within it. This difference determines which contaminants are left behind and which travel with the vapor.
The Physics of Phase Change
Evaporation occurs when individual water molecules absorb sufficient thermal energy to overcome the forces holding them in the liquid mass. This energy input breaks the hydrogen bonds linking neighboring water molecules. Once a molecule possesses enough kinetic energy, it escapes the liquid surface and enters the atmosphere as an invisible gas.
This mechanism is the basis of purification because it is a highly selective process. Only the water molecules transition into the gaseous phase, making evaporation a phase separation technique. Any larger substances or molecules with significantly different physical properties remain in the liquid state because they cannot gain the energy required to vaporize at the same temperature as the water.
Impurities Excluded During Evaporation
Evaporation is highly effective at removing non-volatile contaminants, which are substances that do not easily turn into a gas. These impurities have boiling points far higher than water’s boiling point of 100°C (212°F) or exist as solid particles. When water turns to vapor, these substances are physically excluded and left behind as a concentrated residue.
A primary example is dissolved mineral salts, such as sodium chloride, which have strong ionic bonds requiring extremely high temperatures to break. Heavy metals, including lead and mercury, are also effectively removed because of their negligible vapor pressure at water’s boiling point. Furthermore, large particulates, sediments, bacteria, and viruses are physically incapable of entering the vapor phase. The resulting water vapor is inherently free of these high-boiling-point and solid impurities.
Contaminants That Carry Over with Vapor
The purification power of evaporation is limited by volatile contaminants, which are substances that vaporize at or below the boiling point of water. These compounds have weak molecular attractions, allowing them to escape the liquid alongside the water molecules. When water turns to steam, these volatile impurities travel with the vapor and re-contaminate the water upon condensation.
A major category of these carry-over contaminants is Volatile Organic Compounds (VOCs), which include common industrial solvents, gasoline components, and certain cleaning agents. Many VOCs have boiling points lower than water, meaning they vaporize even more readily than water itself. Similarly, certain pesticides and herbicides can be volatile enough to co-distill with the water.
Dissolved gases also present a challenge to purification, as they may remain dissolved in the water vapor or condense back into the purified water. Gases like ammonia, which has a very low boiling point, will vaporize easily and recombine with the condensed water. Even carbon dioxide and sulfur dioxide can dissolve into the purified vapor, slightly acidifying the resulting liquid.
Distillation and the Natural Water Cycle
The principle of evaporation followed by condensation is the basis for both controlled distillation and the natural water cycle. In industrial or laboratory distillation, water is intentionally boiled and the resulting steam is captured in a closed system and cooled back into pure liquid water. This controlled environment ensures that the clean vapor is immediately condensed, preventing external contamination.
The natural water cycle operates on the same principle, as the sun’s heat causes water to evaporate from oceans and lakes, leaving the salt and heavy minerals behind. The resulting clouds contain highly purified water vapor. However, the efficiency of this natural process is compromised by the atmosphere through which the vapor must travel. As the water vapor condenses into rain, it can absorb atmospheric pollutants, such as dust, soot, and chemical compounds that contribute to acid rain. The initial act of evaporation purifies the water, but the subsequent journey through a polluted atmosphere can reintroduce contaminants before the water falls back to Earth.