The simple act of boiling water does not, by itself, result in distilled water. While both processes involve heat, distillation is a two-step purification method that requires the collection of condensed steam to separate water molecules from contaminants. Merely boiling water only addresses biological threats, leaving behind dissolved solids. For water to be considered distilled, it must undergo vaporization and subsequent collection of the purified liquid, a process that yields water with up to 99.5 percent of impurities removed. Distillation is often used for applications requiring extremely clean water, such as in laboratories or medical settings.
Boiling is Only the First Step
Boiling water serves the primary function of inactivating biological contaminants, such as bacteria, viruses, and cysts. This heat effectively sterilizes the water, making it microbiologically safe to drink. However, boiling leaves behind all dissolved solids and non-volatile impurities. The concentration of these substances actually increases as the volume of water decreases through steam loss.
Distillation relies on the principle that water has a much lower boiling point than most dissolved substances. Once the water turns into steam, it is pure water vapor, leaving behind heavier compounds like salts and minerals in the boiling vessel. Purification occurs when this steam is captured and converted back into a liquid state through cooling, physically separating the pure water molecules from contaminants.
Setting Up a Home Distillation System
A basic home distillation system, often called a pot still, can be constructed using common kitchen items. Equipment includes a large pot for boiling the source water, an inverted lid to create a collection surface, and a separate vessel to collect the final product. The pot is filled partway with source water, and the collection vessel is placed inside, supported above the boiling water line.
Heat is applied, bringing the water to a gentle boil, which creates steam that rises beneath the inverted lid. The lid acts as a condenser; placing ice or a cold compress on its exterior speeds up the cooling process. As the hot steam contacts the cold underside, it changes phase back into liquid water, called the distillate.
The concave shape of the lid causes the water droplets to flow toward the center and drip directly into the collection vessel below. Maintaining a steady, gentle boil is important to prevent the original, contaminated water from splashing up and contaminating the purified condensate. This setup effectively separates the pure water that vaporized from the remaining concentrated impurities.
Impurities Removed During Distillation
Distillation is highly effective at removing contaminants that do not vaporize easily because they have boiling points significantly higher than water. These non-volatile compounds are left behind as residue in the boiling chamber. A wide array of inorganic minerals, salts, and heavy metals are successfully separated using this method.
Examples of removed contaminants include:
- Sodium, calcium, and magnesium, which cause scaling in appliances.
- Heavy metals, such as lead, arsenic, and mercury.
- Nitrates and fluoride.
- Microorganisms, including bacteria, viruses, and protozoan cysts.
The removal rate for these non-volatile impurities is high, often eliminating up to 99.5 percent of dissolved solids from the source water. The remaining concentrated impurities must be regularly cleaned from the boiling vessel to maintain system efficiency. This method of separation results in one of the purest forms of water available.
Volatile Compounds That Remain
A limitation of simple distillation is its inability to effectively remove certain volatile organic compounds (VOCs) and chemicals. These compounds have boiling points similar to or lower than water, meaning they vaporize along with the water molecules. When the steam is condensed and collected, these volatile substances are carried over into the final distilled product, reintroducing contamination.
Examples of these problematic compounds include certain pesticides, volatile solvents, and chlorine treatment byproducts like trihalomethanes. Since these substances vaporize at or near the temperature of boiling water, they can compromise the purity of the collected water. Their presence is often the reason distilled water may still contain trace amounts of the original impurities.
To address this issue, commercial distillers often incorporate methods to mitigate VOC carryover. These methods include using gas vents to allow lighter compounds to escape before condensation. Post-distillation filtration using activated carbon is also a common and effective way to adsorb any remaining VOCs from the condensed water, ensuring the final product reaches the highest possible level of purity.