Separating salt (sodium chloride) from water is a widely recognized challenge, especially when purifying seawater. When salt dissolves, it forms a homogeneous mixture known as a true solution, meaning the components are uniformly distributed at a molecular level. The separation of the water and salt molecules is accomplished through physical means, as the dissolving process itself does not involve a permanent chemical change. Various techniques are employed to achieve this separation, each exploiting a fundamental difference in the physical properties of the two substances.
Understanding the Salt-Water Solution
The ability of water to dissolve salt is rooted in the molecular structure of both compounds. A water molecule is considered polar because the oxygen atom attracts electrons more strongly than the two hydrogen atoms, creating a slight negative charge near the oxygen and a slight positive charge near the hydrogens. This uneven distribution of charge makes water an excellent solvent for many substances, including salt.
Sodium chloride is an ionic compound, consisting of positively charged sodium ions and negatively charged chloride ions held in a crystal lattice. When salt is introduced to water, the polar water molecules surround the salt crystal. The negative ends of the water molecules attract the positive sodium ions, while the positive ends attract the negative chloride ions, pulling them out of the crystal structure.
This process, called solvation, effectively breaks the ionic bonds of the salt. The individual salt ions become uniformly dispersed and surrounded by a shell of water molecules, which is why the solution appears clear. Since the original chemical identities of water and salt remain intact, the components maintain their unique properties, allowing for their eventual physical separation.
Separation Using Heat: Distillation and Evaporation
Thermal methods for separating salt and water rely on the vast difference in the boiling points of the two substances. Water boils at \(100^{\circ}\text{C}\) (\(212^{\circ}\text{F}\)), while the boiling point of sodium chloride is much higher, around \(1,413^{\circ}\text{C}\) (\(2,575^{\circ}\text{F}\)). This difference allows the water to be converted into vapor while the salt remains in its original liquid or solid state.
The simplest thermal method is evaporation, which can happen naturally over time or be accelerated by heating the solution. As the water absorbs heat, its molecules gain kinetic energy and escape the liquid surface as water vapor. The salt, being non-volatile, cannot vaporize and is left behind as a solid residue once all the water has departed. Evaporation is a straightforward method for recovering the salt, but it does not allow for the collection of the purified water.
Distillation is a more controlled process that recovers both the pure water and the salt. The salt solution is heated to boil the water, converting it into steam, which is then directed away from the remaining salt. This steam is channeled into a separate apparatus, known as a condenser, where it is cooled. The cooling causes the water vapor to change state back into pure liquid water, a process called condensation.
This collected liquid, known as distillate, is essentially free of the dissolved salt and other non-volatile impurities. Distillation is widely used in laboratories and for creating highly purified water, but it requires significant energy input to maintain the necessary boiling temperature.
Separation Using Pressure: Reverse Osmosis
Reverse osmosis (RO) is an advanced, non-thermal separation technique commonly employed in large-scale industrial desalination. To understand RO, one must first consider the natural process of osmosis, where water molecules naturally move across a semi-permeable membrane from an area of low salt concentration to an area of high salt concentration. This movement occurs to equalize the concentration on both sides and generates an internal pressure called osmotic pressure.
Reverse osmosis works by applying external pressure to the salt-rich side of the membrane, overcoming the natural osmotic pressure. This forced pressure pushes the water molecules in the opposite direction of their natural flow, through the semi-permeable membrane and toward the low-salt side. This actively separates the solvent (water) from the solute (salt).
The semi-permeable membrane is the central component of the RO system, acting as a highly selective filter. The membrane’s structure allows the relatively small water molecules to pass through its microscopic pores. The membrane is designed to block the much larger hydrated salt ions, which are too big to fit through, effectively rejecting 95% to 99% of the dissolved salts.
The result is a continuous flow of purified water on one side and a concentrated brine solution, containing the rejected salt, on the other. Because RO does not require heating the entire volume of water to its boiling point, it is often a more energy-efficient alternative to distillation for large-scale water purification, particularly in regions converting seawater to potable drinking water.