In chemistry, distilled water is water that has been purified through distillation, a process that uses evaporation and condensation to separate water molecules from dissolved minerals, salts, and other impurities. It serves as a near-pure form of H₂O, making it a standard baseline in labs where even trace contaminants could throw off results.
How Distillation Works
The process is straightforward and relies on a basic physical principle: water boils at 100°C (212°F), but most dissolved minerals and salts do not. When contaminated or impure water is heated to boiling, the water molecules escape as steam while inorganic compounds and large organic molecules stay behind in the original container. The steam is then directed into a cooling chamber, where it condenses back into liquid water, now separated from the impurities that couldn’t make the trip.
This is why distillation is sometimes called a phase-change purification method. The water changes from liquid to gas and back to liquid, and each phase change acts as a filter. Most dissolved solids simply can’t evaporate at water’s boiling point, so they get left behind.
What Distillation Removes (and What It Doesn’t)
Distillation is effective at removing dissolved minerals, salts, heavy metals, and particulates. It also eliminates bacteria and viruses, since these are far too large to travel with steam. For most practical purposes, the result is water with very few dissolved solids.
The notable exception is volatile organic compounds, or VOCs. These are chemicals with boiling points close to water’s, which means they can vaporize right alongside the steam and end up in the “purified” product. Certain pesticides, solvents, and some forms of mercury fall into this category. The purity of the source water matters for this reason. Starting with heavily contaminated water can mean some of those contaminants carry over, even after distillation.
Key Chemical Properties
Freshly distilled water has almost no dissolved ions, which gives it extremely low electrical conductivity, typically well below 100 microsiemens per centimeter. For comparison, regular tap water often ranges from 200 to 800 microsiemens per centimeter. This near-zero conductivity is one of the simplest ways to confirm water purity in a lab setting, since electrical current needs dissolved ions to travel through water.
The pH of distilled water is a point of common confusion. In theory, perfectly pure water should have a neutral pH of 7.0. In practice, distilled water typically measures between 5.5 and 7.5. The reason is carbon dioxide. As soon as distilled water is exposed to air, it absorbs CO₂ from the atmosphere. That CO₂ reacts with the water to form a weak acid (carbonic acid), which nudges the pH slightly below 7. This doesn’t mean distilled water is dangerously acidic. It’s a minor shift, but it’s worth knowing about for experiments where pH precision matters.
Distilled Water vs. Deionized Water
These two are often confused because both produce very pure water, but they work through completely different mechanisms and the results aren’t interchangeable.
- Distilled water uses heat to evaporate and recondense water. It removes ionic impurities, particulates, bacteria, and most dissolved solids. It struggles with volatile organics.
- Deionized water passes water through electrically charged resin beads that swap out dissolved ions for hydrogen and hydroxide ions, which combine to form pure H₂O. It’s excellent at stripping out charged particles but does nothing to remove uncharged molecules like sugars, bacteria, or viruses.
In short, distillation is a broader purification method. Deionization is more targeted, removing only charged species. Some labs use both in sequence: distillation first to remove the widest range of contaminants, then deionization to strip out any remaining ions.
Why Labs Use Distilled Water
The core reason is control. In any chemistry experiment, you need to know that the results you’re seeing come from the chemicals you added, not from mystery minerals hiding in your water. Tap water contains dissolved calcium, magnesium, chloride, fluoride, and dozens of other substances that vary by location and even by season. Any of those could react with your reagents, interfere with a titration, or skew a measurement.
Distilled water eliminates those unknowns. When you dissolve a compound in distilled water, you know the only solute present is the one you put there. When you rinse glassware with it, you know you’re not depositing mineral residue. When you use it as a blank or control in an analysis, you know it won’t produce a false signal. This consistency is what makes it the default solvent and rinsing agent in chemistry labs worldwide.
Common specific uses include preparing standard solutions for titrations, diluting samples for spectroscopic analysis, calibrating instruments, and acting as a solvent for reactions where ionic interference would compromise results. In analytical chemistry especially, where measurements depend on detecting tiny concentrations of a substance, even small amounts of contamination from water can make the difference between a reliable result and a meaningless one.
Practical Grades of Distilled Water
Not all distilled water is equally pure. Single-distilled water is adequate for most general chemistry work, but for highly sensitive analyses, labs may use double-distilled or even triple-distilled water, where the process is repeated to further reduce trace contaminants. Some standards organizations define specific purity grades based on conductivity and total dissolved solids, with the highest grades reserved for work like trace metal analysis or molecular biology, where contamination at parts-per-billion levels can be problematic.
For routine chemistry coursework or standard lab procedures, single-distilled water is almost always sufficient. The key distinction from tap water is the removal of ionic impurities, and even a single distillation accomplishes that effectively.