The process of evaporation, which creates water vapor or steam, is a remarkably effective method for purifying water. This simple phase change is the foundation of both the Earth’s massive natural water cycle and sophisticated engineered distillation systems designed to produce ultra-pure water. The resulting “evaporated water,” once condensed, is clean because the physical mechanism of vaporization acts as a separation barrier at the molecular level. This process allows thermal energy to transform contaminated liquid into highly purified vapor, explaining why most impurities are physically unable to make the journey from liquid to gas.
The Physics of Water Vaporization
The cleanliness of evaporated water begins with the molecular structure of the water molecule itself. Liquid water molecules (H₂O) are held together by strong intermolecular forces known as hydrogen bonds. These bonds require a substantial input of thermal energy for water to change its state from liquid to gas.
When water is heated, this thermal energy increases the kinetic energy of the individual water molecules. To transition into a gas, these molecules must gain enough energy to completely break free from the network of hydrogen bonds holding them in the bulk liquid. Only the H₂O molecules that reach this energy threshold, known as the latent heat of vaporization, escape the liquid surface and become individual gas molecules, or vapor.
Impurity Exclusion: Non-Volatile Contaminants
The primary reason evaporated water is so pure is that most common contaminants are categorized as non-volatile. This means they have a boiling point significantly higher than water’s 100°C (212°F) at standard pressure. Substances like salts, heavy metals (such as lead or mercury), and dissolved minerals simply do not possess the necessary volatility to escape with the steam. For instance, sodium chloride does not vaporize until it reaches a temperature exceeding 1,400°C.
This vast difference in boiling points forces a physical separation, leaving the non-volatile compounds behind in the original boiling vessel. The presence of these dissolved solids can even raise the boiling point of the bulk liquid, an effect known as boiling point elevation.
Biological contaminants, including bacteria, viruses, and protozoa, are also completely excluded during this process. These microorganisms are either destroyed by the prolonged heat exposure or are simply too large and heavy to be carried aloft by the escaping water vapor. The evaporation process acts as a size and volatility filter, physically barring the vast majority of inorganic and biological impurities from transitioning into the gaseous state.
The Role of Volatility in Contamination
While evaporation is highly effective against non-volatile substances, it is not a perfect purification method and has a significant limitation related to volatility. This caveat involves contaminants known as Volatile Organic Compounds (VOCs) and certain dissolved gases, which have boiling points near or even below that of water. When water is heated, these compounds can easily transition into the gaseous phase alongside the water vapor.
Examples of problematic VOCs include industrial solvents, some pesticides, and trihalomethanes, a common byproduct of chlorine disinfection in municipal water. If the boiling process is not carefully managed, these highly volatile substances will co-evaporate, travel with the steam, and re-contaminate the resulting purified water upon condensation. Professional distillation systems often incorporate a venting mechanism and a post-purification activated carbon filter to capture any remaining VOCs that make it through.
Natural and Engineered Distillation
The principle of purification through evaporation is constantly at work in the natural world, forming the basis of the water cycle. Solar energy drives the evaporation of surface water, leaving behind salts and other impurities in oceans and lakes. This pure water vapor then condenses into clouds, eventually falling back to Earth as rain, which is naturally distilled water.
Engineered distillation mirrors this natural process in a controlled environment. It typically involves three stages: boiling, condensation, and collection. In a commercial or home distillation unit, the contaminated water is boiled, and the resulting steam is guided into a separate, cooled chamber called a condenser. This cooling causes the pure water vapor to revert to liquid form, which is then collected.