Salt can be separated from water. Saltwater is a solution where salt molecules are fully dissolved and dispersed throughout the water. This dissolving process occurs because polar water molecules surround and pull apart the ionic bonds of the salt, forming a stable, homogenous liquid. Because the salt is dissolved at a molecular level, separation requires methods that break these molecular bonds rather than simple filtration.
The Physical Principles of Separation
Separating salt from water relies on the fundamental differences in their physical properties. Salt (sodium chloride) is an ionic compound, while water is a highly polar molecule. When salt dissolves, water molecules surround the positive sodium ions and negative chloride ions, pulling them out of the crystal structure and forming hydration shells.
Separation requires exploiting their differing behaviors during a phase change. Water boils at 100°C (212°F) at standard atmospheric pressure, but salt remains solid until it melts at about 801°C (1,474°F). This vast difference in boiling points forms the basis for thermal separation methods. Heating the solution causes the water to change phase into a vapor while the salt stays behind.
Dissolved salt slightly elevates the water’s boiling point, a phenomenon known as boiling point elevation. This occurs because the dissolved ions interfere with the water molecules’ ability to escape into the vapor phase. Despite this small increase, water still turns to steam far below the temperature required to vaporize salt, making separation by boiling effective. The resulting steam is pure water, leaving the salt as a solid residue.
Basic Methods for Home Separation
The boiling point difference is applied in distillation. Distillation involves boiling saltwater to create pure water vapor (steam), then collecting and cooling that steam to condense it back into liquid freshwater. A basic home setup uses a pot to boil the water and a slanted lid to direct the condensed steam into a separate container.
Distillation effectively separates the components but requires a constant input of heat energy. The resulting product is distilled water, leaving the salt as a residue. While effective for small quantities, the high energy demand makes distillation impractical for large-scale purification.
A less energy-intensive, slower technique is solar evaporation. This process mimics historical salt harvesting by leaving saltwater exposed to the sun in a shallow container. The sun’s energy causes the water to slowly evaporate into the atmosphere over time.
As the water turns to vapor, it leaves the dissolved salt behind, which crystallizes as the solution becomes saturated. This method is effective for recovering the salt itself but is not an efficient way to produce purified water for immediate consumption. Both distillation and evaporation rely on the phase change of water but differ in speed and energy source.
Industrial Desalination Processes
Industrial-scale separation, known as desalination, uses highly engineered processes to produce potable water daily. The most widely used modern method is Reverse Osmosis (RO), which uses pressure rather than heat. RO overcomes osmosis, where water naturally moves across a semipermeable membrane from low to high salt concentration.
In RO, powerful pumps apply pressure to the saltwater, forcing water molecules against the osmotic flow and through a specialized semipermeable membrane. The membrane pores are large enough for water molecules but small enough to block dissolved salt ions and other impurities. The required pressure must exceed the solution’s natural osmotic pressure, often requiring 50 to 60 bars for seawater.
The RO process yields two streams: highly purified water (with 95% to 99% of dissolved salts removed) and a concentrated brine stream, typically returned to the sea. RO plants are favored globally because they generally require less energy input than thermal methods, utilizing mechanical energy to force water through the membrane.
Thermal desalination methods are scaled-up, optimized versions of boiling. Processes like Multi-Stage Flash (MSF) and Multiple-Effect Distillation (MED) reduce boiling energy by operating under a vacuum. In MSF, preheated saltwater enters a series of chambers, or stages, each maintained at a progressively lower pressure.
The sudden pressure drop causes a portion of the water to instantly “flash” into steam without reaching the standard 100°C boiling point. The steam is condensed on heat exchange tubes to collect freshwater. The remaining hot brine passes into the next, lower-pressure stage to repeat the process. This multi-stage approach, often coupled with power plants to use waste heat, significantly improves efficiency by reusing heat energy.