You can boil salt out of water, but only if the resulting steam is captured and condensed back into a liquid. Merely boiling salt water in an open pot will not purify it; this process only converts the liquid water into a gas, leaving all the dissolved salt behind. The separation of water from salt requires distillation, a controlled method of vaporization and condensation. This physical process uses the fundamental differences between the components in the solution to achieve separation.
The Science of Evaporation vs. Dissolved Solids
Water and common table salt (sodium chloride) possess vastly different physical properties, allowing boiling to separate them. Water is a volatile compound, meaning it easily changes from a liquid to a gas at a relatively low temperature, specifically 100°C (212°F) at standard atmospheric pressure. In contrast, sodium chloride is a non-volatile solid, remaining stable in its dissolved form at that temperature.
The boiling point of sodium chloride is extremely high, far beyond the temperature needed to boil water. When salt water is heated, only the water molecules gain enough kinetic energy to transition into steam. The salt ions are bound by strong ionic forces and are too heavy and stable to vaporize alongside the water.
If salt water is continuously boiled, the liquid water escapes as steam, but the salt remains in the pot. This steadily increases the concentration of salt in the remaining liquid, slightly raising the solution’s boiling point—a phenomenon called boiling point elevation. If all the water is boiled away, a solid crust of salt residue is left at the bottom of the container.
Distillation: The Method for Collecting Pure Water
The practical method for separating and collecting pure water from a salt solution is distillation, a three-part process. Distillation is highly effective because the difference between the boiling points of water and salt is so significant.
Vaporization
This step is achieved by heating the salt water until it reaches its boiling point and turns into steam. This steam consists almost entirely of pure water molecules, leaving non-volatile impurities like salt behind in the boiling chamber.
Condensation
This involves moving the hot steam away from the boiling water and cooling it rapidly. The steam is directed to a cooled surface, such as a metal plate or a glass coil. As the steam touches the cooler surface, it loses thermal energy and reverts from a gas back into a liquid state.
Collection
The newly condensed liquid water, known as the distillate, is gathered in a separate container. This collected water is now free from the dissolved salts and minerals that were left behind in the boiling vessel.
Practical Considerations and Water Purity
While simple distillation is excellent at removing salts and other dissolved solids, it has limitations for complete water purification. The process successfully removes heavy metals, inorganic compounds, and non-volatile impurities because their boiling points are much higher than water. The boiling action also effectively kills biological contaminants like bacteria and viruses, which are left behind with the salt residue.
However, some contaminants are volatile organic compounds (VOCs), which have boiling points similar to or lower than water. These substances may vaporize along with the water, traveling with the steam and re-contaminating the collected distillate. Simple distillation setups should incorporate a vent to release these lower-boiling gases or use an activated carbon filter to adsorb them from the collected water.
The final product of distillation is essentially pure water, often referred to as demineralized water. This water lacks naturally occurring minerals, such as calcium and magnesium, that give drinking water its familiar taste. Consequently, distilled water has a noticeably “flat” or bland taste compared to tap water or spring water.