Distilled water and deionized water are distinct forms of purified water created through fundamentally different processes. While both aim to remove impurities, they target different types of contaminants, resulting in unique chemical profiles. Understanding these differences is necessary for selecting the correct water type for specific applications, from laboratory science to household use.
Understanding Distilled Water
Distilled water is produced by a process that mimics the natural water cycle: evaporation and condensation. Source water is heated to its boiling point, turning it into steam, which physically separates the water molecules from non-volatile impurities.
The water vapor leaves behind dissolved solids such as minerals, salts, and heavy metals in the boiling chamber. The steam is then cooled in a condenser unit, converting it back into purified liquid water. This method is highly effective at removing inorganic contaminants and kills most microorganisms due to the sustained high temperature.
While distilled water is exceptionally pure of non-volatile substances, it may still contain trace amounts of volatile organic compounds (VOCs). Contaminants with boiling points close to water may vaporize and re-contaminate the condensed product.
Understanding Deionized Water
Deionized (DI) water is created using a chemical process called ion exchange. This method specifically targets and removes charged particles, known as ions, from the water by passing it through columns containing specialized resin beads.
Cation resins attract positively charged ions (e.g., calcium and sodium), replacing them with hydrogen ions (\(\text{H}^+\)). Anion resins remove negatively charged ions (e.g., chloride and sulfate), substituting them with hydroxyl ions (\(\text{OH}^-\)). The exchanged hydrogen and hydroxyl ions then combine to form a pure water molecule (\(\text{H}_2\text{O}\)).
This process is remarkably efficient at removing dissolved mineral salts and metals. However, deionization is largely ineffective against uncharged substances, such as neutral organic molecules, bacteria, or viruses. DI water may still contain these non-ionic impurities unless an additional filtration step is used.
The Core Difference in Impurity Removal
The fundamental distinction lies in the type of contaminant each method eliminates. Distillation is a physical separation technique effective against non-volatile solids and microorganisms, while deionization is a chemical exchange process that removes substances based on their electrical charge.
The resulting DI water exhibits extremely low electrical conductivity because ions are the particles responsible for conducting electricity. High-purity DI water can reach a resistivity of 18.2 megaohm-centimeters, the theoretical maximum for water’s ionic purity.
Conversely, distilled water, while pure, may contain traces of dissolved atmospheric gases like carbon dioxide, which can ionize slightly and lead to measurable conductivity. Therefore, DI water is technically purer in terms of ionic content, but distilled water offers a broader reduction of both ionic and non-ionic contaminants, including biological agents.
Specific Uses for Each Water Type
The specific purity profile of each water type dictates its practical application. Deionized water is the preferred choice in industrial and laboratory settings where the absence of mineral ions is paramount.
Its low conductivity makes DI water ideal for electronics manufacturing, electroplating, and use in automotive cooling systems and lead-acid batteries, where mineral build-up could cause scaling or corrosion.
Distilled water is commonly used for purposes requiring a high degree of general purity, especially the removal of biological contaminants. It is often recommended for medical devices such as CPAP machines and humidifiers to prevent mineral scale deposition. Distilled water is also used in medical tool sterilization and in steam irons.