Water generally becomes cleaner when it evaporates. This natural process forms the basis of many purification methods by separating water molecules from most impurities.
The Evaporation Process
Evaporation is a process where liquid water changes into a gaseous state, known as water vapor. This transformation happens when water molecules at the liquid’s surface gain sufficient energy, often from heat, to overcome the attractive forces holding them together. These energized molecules then break free and move into the air as individual gas molecules.
During this phase change, only water molecules typically acquire enough energy to transition into vapor. Other substances present in the water, such as dissolved solids or suspended particles, usually do not have the same properties or sufficient energy to evaporate. This difference in behavior is fundamental to the purification effect of evaporation.
What Happens to Contaminants
When water evaporates, various impurities are left behind in the original liquid. Dissolved solids, like salts and minerals, do not vaporize with the water. As water molecules escape, these dissolved substances become more concentrated in the remaining liquid and eventually precipitate out, forming solid residues. This is evident in salt production, where seawater is evaporated to harvest salt crystals.
Suspended particles, such as dirt, sediment, and microorganisms, are also too large and heavy to evaporate with water. They remain in the original container, unable to transition into the gaseous phase. Most non-volatile organic compounds, which have higher boiling points than water, are similarly left behind as the water turns into vapor.
Distillation: Harnessing Evaporation for Purity
The principle of evaporation for purification is intentionally applied in a process called distillation. Distillation involves heating water to produce vapor, then cooling this vapor to condense it back into liquid form. This collected liquid, known as distillate, is significantly purer than the original water because non-volatile impurities are left behind during evaporation.
Controlled distillation provides a highly effective method for water purification. This process separates water from most contaminants by leveraging the different vaporization points of water and impurities.
What Evaporation Doesn’t Remove
While evaporation is effective at removing many impurities, it does not result in perfectly pure water in all cases. Certain volatile organic compounds (VOCs) and dissolved gases can evaporate along with water. If these substances have boiling points similar to or lower than water, they may co-evaporate and thus be present in the collected vapor.
For instance, some chemicals and pesticides might evaporate with water, meaning the resulting vapor would not be entirely free of them. Therefore, while evaporated water is considerably cleaner than its source, it may still contain traces of these volatile contaminants. The degree of purity depends on the specific impurities present in the original water and the method of evaporation.