Seawater is a complex mixture, primarily composed of water molecules and various dissolved substances, most notably salts. Ocean water contains about 3.5% dissolved material by mass, with the majority being ionic compounds like sodium chloride. Boiling initiates a phase change, converting the liquid water into steam. This process separates the components based on how they react to heat energy.
The Immediate Result: Separating Water from Salt
Applying heat to seawater causes water molecules to gain enough energy to break their intermolecular bonds and escape as vapor. Evaporation occurs rapidly once the water reaches its boiling point of 100°C (212°F) at sea level. The resulting steam is composed almost entirely of pure \(\text{H}_2\text{O}\) molecules. Dissolved salts, such as sodium and chloride ions, are non-volatile and possess extremely high boiling points, far exceeding that of water. This differential in volatility allows boiling to separate the water from its dissolved salts.
What Remains: Concentrated Brine and Mineral Residue
As the water continues to boil and turn into steam, the volume of the liquid steadily decreases. The non-volatile salts and minerals become increasingly concentrated in the remaining liquid, forming a highly saline, super-saturated solution known as brine. If boiling continues until all the water has evaporated, a solid, crusty residue will be left. This residue is a thick layer of crystallized minerals, primarily sodium chloride, magnesium, and calcium salts. This mineral deposit, sometimes referred to as scaling, can be corrosive and difficult to remove.
The Fate of Non-Salt Contaminants
Boiling is highly effective at killing biological contaminants like bacteria, viruses, and protozoa, making water biologically safe. However, the process does not remove chemical contaminants, which behave differently based on their properties. Non-volatile inorganic substances, such as heavy metals like lead, mercury, and arsenic, cannot evaporate and remain in the boiling liquid. As the water volume reduces, the concentration of these contaminants increases dramatically, making the remaining liquid significantly more hazardous. Conversely, volatile organic contaminants (VOCs), including certain pesticides or industrial solvents, have low boiling points and may vaporize along with the steam, potentially contaminating the air or the collected water vapor.
Basic Principles of Solar and Stove-Top Distillation
Purification of seawater for drinking requires distillation, which is the complete process of capturing and condensing the pure steam. Distillation mimics the Earth’s natural water cycle of evaporation and condensation, ensuring the purified vapor is collected before mixing with concentrated contaminants. A simple stove-top still involves boiling the seawater and directing the resulting steam to a cooler surface, where it condenses back into liquid freshwater droplets for collection. Solar stills operate on the same principle but use the sun’s energy instead of an external heat source. A solar still consists of a sealed box with a transparent, angled cover where pure water vapor condenses and trickles down for collection.