Boiling saltwater to create a safe source of drinking water relies on the physical properties of the substances involved. This method can be a lifesaver in emergency or resource-limited settings where freshwater is unavailable. The goal is to separate water molecules from dissolved salt, turning undrinkable seawater or brackish water into clean, potable liquid through the application of heat and careful collection of the resulting vapor.
The Principle of Thermal Separation
Separating water from salt using heat works because of the vast difference in their boiling temperatures. Water boils at 100°C (212°F) at sea level, turning into steam. Sodium chloride, or common table salt, is a non-volatile solid with a boiling point of 1413°C (2575°F), meaning it does not evaporate with the water.
When saltwater is heated, only the water changes phase from liquid to gas, leaving the salt and other non-volatile minerals behind in the boiling vessel. The resulting steam is pure water molecules (H₂O) free of dissolved solids. This process of boiling the liquid and collecting the condensed vapor is known as distillation.
The steam must be cooled to turn it back into liquid water, which is collected in a separate container. This phase change is highly effective at removing all non-volatile impurities, including salts, heavy metals, and most microorganisms. The concentrated salt solution remaining in the original pot is discarded once the process is complete.
Constructing a Simple Distillation Setup
Creating a functional still requires a few common items: a large pot for boiling, a heat source, a lid or dome to capture the steam, and a clean receptacle for the purified water. The boiling vessel should be clean and made of a material like stainless steel or glass that can withstand high heat.
Fill the pot approximately halfway with saltwater, leaving enough space so the boiling water does not splash into the collection vessel. Place a clean, heat-safe collection container, such as a glass bowl or cup, inside the large pot. Ensure the container’s rim is above the level of the saltwater, and the collection vessel must not touch the bottom of the pot.
Cover the pot with a domed or convex lid, placing it upside-down. The center of the lid should be slightly lower than the edges and positioned directly over the collection bowl. To increase efficiency, place a small weight, such as a clean rock or marble, on the center of the inverted lid. This weight helps direct the condensed water droplets toward the center, causing them to drip into the collection vessel.
Apply a steady, low-to-medium heat to the pot, bringing the saltwater to a gentle simmer rather than a rolling boil. Gentle heat prevents the raw saltwater from splashing up and contaminating the collection bowl. The steam rises, hits the cooler surface of the lid, and begins to condense back into liquid water. For faster condensation, place a cold, wet cloth or ice on the exterior surface of the lid, which creates a larger temperature differential. The condensed freshwater runs down the inverted lid and drips into the clean bowl below.
Assessing Water Purity and Safety
Water produced by distillation is highly pure, removing nearly 99.5 percent of dissolved solids and contaminants. While distillation effectively removes salt, minerals, and microorganisms, a potential concern is the presence of volatile organic compounds (VOCs). VOCs are chemicals, such as certain solvents or pesticides, that have boiling points near or below that of water.
These volatile substances can vaporize along with the steam and pass into the collection container. To minimize this risk, some recommend discarding the first small amount of water collected, as it may contain the highest concentration of these compounds. If VOCs are suspected, distillation alone may not be sufficient for complete purification.
The resulting distilled water is nearly mineral-free, giving it a somewhat bland taste. Long-term, exclusive consumption of mineral-free water is not recommended, as the body needs the trace minerals found in natural water sources. For short-term survival or emergency use, however, the purity benefits of removing salt and other harmful solids far outweigh the lack of minerals.