Distilled water is water that has been purified by turning it into steam by boiling and then condensing it back into a liquid. This simple but effective process removes almost all dissolved minerals, salts, metals and many other impurities. The result is an ultra pure, consistent form of water that behaves in a predictable way in many applications.

Although distilled water looks the same as tap water in a glass, it is very different on a microscopic level. Ordinary tap water contains dissolved minerals such as calcium and magnesium, as well as traces of metals, chlorine, organic matter and other substances from pipes and treatment systems. Distilled water has had almost all of these removed.

How Distilled Water Is Made

The process used to produce distilled water is called distillation. It works by using the natural water cycle as a model: evaporation, condensation and collection.

Water is first heated in a stainless steel vessel until it begins boils. As it boils, water molecules leave the surface of the liquid as steam. Most dissolved solids and many contaminants do not turn into vapour at this stage. They are left behind in the boiling chamber.

The steam is then directed into a separate, cooler chamber via a reflux condensing column. Here it passes through coils or a condenser where it loses heat. As it cools, the steam turns back into liquid water. This liquid is now highly purified because the majority of non-volatile impurities were left behind.

The condensed water is collected in a clean container as distilled water. In industrial and laboratory settings, the equipment is designed to ensure that the steam does not pick up contaminants from the air or from the surface of the condenser. In many systems, the water may be distilled more than once to achieve even higher levels of purity.

Distilled Water and Other Types of Water

It is helpful to understand how distilled water compares with other common types of water.

Tap water is treated to be safe to drink. It usually contains chlorine or chloramine to control bacteria, as well as dissolved minerals picked up from reservoirs and pipes. It is suitable for general household use, but its composition varies from place to place.

Spring water and mineral water are taken from natural sources. They contain high levels of minerals such as calcium, magnesium and bicarbonates. These minerals can give water a pleasant taste but also leave scale in kettles and appliances.

Filtered water has passed through a basic filter to remove certain contaminants. Depending on the filter, this may reduce chlorine, some metals, or particles. However, most dissolved solids and microbes remain.

Deionised water is treated using ion-exchange resins to remove charged particles, such as mineral ions. It can be very pure, but will still contain organic compounds, bacteria, or other non-ionic contaminants.

Distilled water has been purified by phase change, from liquid to vapour and back again. This allows it to remove both dissolved solids and many other impurities. It is widely used where predictable, low-mineral water is essential.

Why Purity Matters

The very low mineral content of distilled water makes it useful in settings where ordinary water could cause problems.

Mineral deposits are a common issue with tap water. When water containing calcium and magnesium is heated, it leaves limescale behind. Over time this can coat heating elements, pipes and jets. It reduces efficiency and can cause equipment to fail. Distilled water greatly reduces this build-up.

Electrical conductivity is another important factor. Dissolved salts allow water to conduct electricity. In sensitive electronic equipment, even a small amount of conductivity can be problematic. Because it has very low levels of dissolved ions, distilled water is much less conductive than tap water.

Consistency is also valuable. The composition of tap water changes with the source, the season and the treatment process. Distilled water is produced in a controlled way, so each batch should meet the same standard. This is important for laboratories, manufacturing, medical devices and many technical processes.

Common Uses of Distilled Water

Distilled water is used in many settings where purity and consistency are important.

In laboratories, it is used to prepare solutions, clean equipment and feed instruments. Impurities in water can interfere with chemical reactions, affect measurements and damage delicate components.

In medical and dental environments, distilled water is used in autoclaves, sterilisers and some patient equipment. This helps protect instruments from scale and supports reliable sterilisation cycles.

In industry, distilled water can be used in cooling systems, battery top-ups, steam irons, humidifiers and cleaning processes. Using distilled water reduces scale and residue, which can improve performance and extend the life of equipment.

In the home, many people use distilled water in steam irons, garment steamers, CPAP machines, facial steamers, aquariums (when mixed and balanced correctly) and for cleaning items such as camera sensors or glass surfaces where streaks and spots are not acceptable.

How Quality Is Measured

The quality of distilled water is often assessed using measures such as conductivity and total dissolved solids (TDS). These give an indication of how many ions or particles remain in the water after distillation.

Low conductivity and low TDS values suggest that very few dissolved substances are present. In many technical applications, specifications will state maximum acceptable levels for these measures. For example, laboratory-grade distilled water is held to stricter limits than water used in general industrial settings.

Microbiological quality can also be important. Although the distillation process reduces many microorganisms, storage conditions matter. Containers and handling equipment should be clean and suitable for contact with pure water. For this reason, distilled water for critical applications is produced, stored and transported under controlled conditions.

Storing and Handling Distilled Water

Distilled water is more 'active' in some ways than ordinary water because it has so few dissolved substances. Over time, it can slowly dissolve small amounts of materials from the surfaces it touches. This is one reason it is usually stored in suitable plastic or glass containers that are designed for pure water.

Containers should be kept sealed when not in use. This helps prevent dust, airborne particles and gases from dissolving back into the water. For example, carbon dioxide from the air can gradually increase the conductivity of distilled water.

It is good practice to store distilled water away from direct sunlight and sources of heat. This helps maintain its quality and reduces the risk of microbial growth in long-term storage. For sensitive applications, many users prefer to open smaller containers as needed, rather than a single large container that is repeatedly opened and closed.

By understanding what distilled water is, how it is produced and where its purity matters most, it becomes clear why it is chosen for so many technical, medical, industrial and household tasks where ordinary tap water is not ideal.