When water evaporates from the ocean, the resulting water vapor is essentially fresh, leaving the dissolved solids behind. The ocean is a complex solution, with sodium chloride—common table salt—being the most abundant dissolved mineral compound. Evaporation is a physical process that acts as nature’s large-scale purification system, separating water molecules from the non-volatile components of the seawater. This separation is fundamental to the hydrological cycle that continuously supplies freshwater across the globe.
The Science of Separation
Evaporation is the phase transition where liquid water turns into water vapor below the boiling point. This process is driven by solar energy, which gives water molecules enough kinetic energy to break free from the liquid surface and escape into the atmosphere. The difference in volatility between water and salt enables this separation. Water has a relatively low boiling point, meaning it readily changes into a gas at ambient temperatures near the ocean surface.
Dissolved salts, however, are crystalline solids with extremely high boiling and melting points. For example, sodium chloride requires approximately 1,413 degrees Celsius (2,575 degrees Fahrenheit) to vaporize. Since ocean surface temperatures are nowhere near this requirement, the salt compounds remain in the liquid phase when water molecules transition to a gas, increasing the salinity of the remaining surface water.
Molecular Explanation for Salt Exclusion
The mechanism behind salt exclusion is rooted in the strong electrostatic forces holding the salt ions in the solution. In seawater, salt exists as individual, electrically charged ions, primarily positive sodium ions (\(\text{Na}^+\)) and negative chloride ions (\(\text{Cl}^-\)). Water molecules are polar, meaning they form strong attraction shells, known as hydration shells, around each charged ion.
To evaporate, an ion would need to break free from the liquid water and drag this entire cluster of strongly attached water molecules with it. The energy required to break these electrostatic bonds and propel such a large, heavy cluster into the atmosphere is vastly greater than the energy needed for a single water molecule to break its weaker hydrogen bonds.
Therefore, the thermal energy supplied by the sun is sufficient only to break the relatively weak intermolecular forces between water molecules. This allows the small, light, and neutral \(\text{H}_2\text{O}\) molecules to escape as vapor. The much heavier, charged ions are effectively trapped by the surrounding water, ensuring the evaporated vapor is highly purified water.
How Salt Actually Enters the Atmosphere
While evaporation produces pure water vapor, salt enters the atmosphere through a distinct physical process called sea spray aerosolization. When wind blows over the ocean, especially in rougher conditions, it creates whitecaps and causes air bubbles to form and burst at the surface.
When these bubbles rupture, they physically eject tiny droplets of liquid seawater into the air. These small droplets are carried aloft by the wind and contain the same concentration of dissolved salt as the ocean surface. As the liquid water in these airborne droplets quickly evaporates, the dissolved salt is left behind as a minute, solid salt crystal particle suspended in the atmosphere.
These tiny salt particles, or sea salt aerosols, are the source of the salty taste sometimes detected near the coast and can be transported thousands of miles inland.
Implications for the Global Water Cycle
The purification of water during ocean evaporation is a fundamental driver of the planet’s hydrological cycle. The ocean is the source of an estimated 86% of global evaporation, continuously feeding freshwater vapor into the atmosphere. This vapor eventually condenses to form clouds and falls back to Earth as freshwater precipitation, such as rain and snow, sustaining terrestrial ecosystems and human populations.
The sea salt aerosols created by sea spray play an indirect but important part in this cycle. These airborne salt crystals act as cloud condensation nuclei (CCN), which are necessary for water vapor to condense and form cloud droplets. Without these particles, clouds would struggle to form.