When saltwater is heated, the salt remains behind because the boiling process is a mechanism of separation. Saltwater is a solution where water acts as the solvent and the salt, typically sodium chloride, is the dissolved solute. Heating this mixture isolates the two components by exploiting the vast difference in their physical properties, a principle used in large-scale purification processes.
The Salt Residue
As the saltwater solution is heated, water molecules gain enough energy to transition from a liquid to a gas (vaporization). This change leaves the non-evaporating salt particles behind. As the volume of water decreases, the remaining liquid becomes increasingly concentrated, often referred to as brine. The temperature of the boiling solution will also slowly increase because the growing salt concentration elevates the boiling point of the remaining water. Eventually, if the heat source is maintained long enough, all of the water will escape as vapor. Once the water has boiled away, the salt particles will coalesce, leaving a solid, white, crusty residue of crystallized salt coating the container.
The Science Behind Separation
The separation of water and salt occurs due to a fundamental difference in their chemical structures and resulting volatility. Water molecules are held together by covalent bonds, but the intermolecular forces between them are relatively weak. When enough energy is supplied to reach the boiling point (approximately 100°C or 212°F), the water molecules overcome these forces and escape as a gas. In contrast, table salt (sodium chloride or NaCl) is an ionic compound held together by powerful electrostatic forces. These ionic bonds are significantly stronger than the forces holding water molecules together. The boiling point of pure water is about 100°C, but the temperature required to vaporize sodium chloride is drastically higher, melting around 801°C and boiling at approximately 1413°C (2575°F). This immense difference in boiling points means that at the temperature where water turns to steam, the salt remains stable in its condensed state. The salt is classified as a non-volatile solute, meaning it does not easily enter the gaseous phase under normal boiling conditions.
Is the Steam Salty?
The resulting steam is essentially pure water vapor because the salt ions do not evaporate along with the water molecules. When this water vapor cools and condenses back into a liquid, the resulting product is distilled water, which is free of the dissolved salts and minerals. This separation principle is used in thermal desalination, a process that creates potable water from seawater. Large-scale plants heat saltwater to create steam, leaving concentrated brine behind, and then collect the fresh condensate. A minor caveat is that extremely vigorous boiling can carry small droplets of liquid brine (aerosols) upward into the steam. These are physically entrained liquid particles, not vaporized salt, but for typical applications, the steam is considered pure water vapor.