When water evaporates, individual water molecules escape into the air while nearly everything else stays behind. Salts, minerals, bacteria, heavy metals, and other contaminants are too heavy or too strongly bonded to make that jump. A properly run distillation system, which mimics natural evaporation, removes up to 99.5% of impurities from water. The reason comes down to a simple physical reality: water molecules are small and light enough to become gas at 100°C, while almost everything dissolved or suspended in water is not.
How Water Molecules Leave Contaminants Behind
Water evaporates when individual H₂O molecules gain enough energy to break free from the liquid surface and enter the air as vapor. Each molecule that escapes is just water, nothing more. The dissolved minerals, salts, metals, and organic matter in the original water don’t have enough energy to make the same transition. They remain in the liquid or, once all the water is gone, sit as a dry residue.
Think of it like boiling a pot of saltwater on your stove. The steam rising off the pot is pure water vapor. The salt stays in the pot. If you kept boiling until the pot was dry, you’d find a crust of salt at the bottom. That separation happens because sodium chloride doesn’t boil until around 1,413°C, over fourteen times hotter than water’s boiling point. The same principle applies to virtually every mineral and heavy metal found in drinking water. Lead, arsenic, mercury, calcium, and magnesium all have boiling points hundreds or thousands of degrees above water’s. They simply cannot become gas under normal conditions.
This boiling point gap is the core reason evaporated water is so clean. It’s not a filter catching particles. It’s a phase change that only water molecules can complete at the relevant temperature.
What Gets Removed
The list of contaminants that evaporation leaves behind is extensive. Heavy metals like lead, arsenic, and mercury stay in the liquid. Dissolved minerals (calcium, magnesium, iron) that make water “hard” don’t evaporate. Chlorine, chloramines, and radioactive particles are also left behind. Even bacteria and viruses, which are enormous compared to a water molecule, cannot travel into the vapor phase during normal evaporation.
You can measure this difference with total dissolved solids (TDS), a standard water quality metric. Typical drinking water contains 25 to 500 milligrams per liter of dissolved solids. Freshly distilled water contains just 0.5 to 1.5 milligrams per liter. That’s a reduction of roughly 97% to 99.9%, depending on the starting water quality.
Bacteria face a double problem during evaporation. They’re far too large to enter the vapor phase, and the drying process itself kills most of them. Research on bacterial survival in evaporating droplets shows that most microorganisms die when a droplet fully dries out. Some hardy species can survive as dried residue, but they remain stuck in that residue rather than traveling with the water vapor.
Nature’s Version: The Water Cycle
This same purification happens on a planetary scale every day. The sun heats ocean water, lakes, and rivers. Water molecules evaporate, rise into the atmosphere, and eventually condense into clouds. When that water falls as rain, it’s essentially distilled. The ocean’s salt, the lake’s sediment, and the river’s bacteria all stayed behind.
Rainwater isn’t perfectly pure by the time it reaches the ground, though. It picks up gases and particles as it falls through the atmosphere, including dust, pollen, and dissolved carbon dioxide. But the evaporation step itself produced clean water. Any contamination happened after the phase change, not during it.
What Evaporation Does Not Remove
Here’s the important caveat: not every contaminant stays behind. Volatile organic compounds (VOCs) are chemicals with boiling points low enough that they evaporate alongside water or even more easily. These include solvents like benzene and toluene, certain pesticides, and industrial chemicals. The EPA classifies compounds with boiling points below about 245°C as volatile, meaning many of them will travel right along with water vapor during distillation.
This is why distillation alone isn’t always enough to produce perfectly pure water. Industrial distillation systems often pair the evaporation step with activated carbon filters that catch VOCs the vapor carried over. If your water source is contaminated with gasoline, paint thinner, or certain industrial pollutants, simple evaporation won’t fully separate them.
Another quirk: freshly distilled water exposed to air becomes slightly acidic. Pure water readily absorbs carbon dioxide from the atmosphere, forming a weak carbonic acid solution. At room temperature in normal air, distilled water settles to a pH of about 5.7, mildly acidic compared to the neutral 7.0 you might expect. This isn’t a health concern, but it’s why distilled water can taste flat or slightly sharp compared to mineral-rich tap water.
Why It Tastes Different
The same process that makes evaporated water clean also strips out the minerals that give water its familiar taste. Calcium and magnesium, the minerals responsible for “hard” water, contribute a subtle flavor most people are accustomed to. Distilled water lacks these entirely, which is why many people describe it as tasting bland or oddly empty. Some bottled water companies add minerals back after purification specifically to improve the flavor.
This mineral-free quality makes distilled water ideal for situations where purity matters more than taste: filling steam irons, topping off car batteries, running laboratory equipment, and use in medical devices like CPAP machines. In all these cases, the absence of dissolved minerals prevents the buildup and scaling that tap water would cause.