Distilled water is a highly purified form of water that has undergone a specific thermal process to remove contaminants, making it fundamentally different from tap or spring water. In scientific contexts, this purification method produces a liquid that is nearly 100% water molecules (\(\text{H}_2\text{O}\)). It is valued in chemistry because its lack of dissolved substances prevents interference in sensitive reactions and analytical measurements. This level of purity establishes it as a standard solvent for preparing chemical reagents and solutions.
Defining Chemical Purity
The defining characteristic of distilled water is the absence of dissolved solids, minerals, and ions that are common in natural water sources. These impurities typically include salts like sodium chloride, metal ions such as calcium and magnesium, and various inorganic compounds. The resulting chemical state is measured by its electrical conductivity, a metric that reflects the concentration of charged particles. Highly purified water is a poor conductor of electricity because it lacks these mobile ions.
For chemists, the purity of distilled water is verified by its extremely low conductivity, typically ranging from 0.5 to 3 microsiemens per centimeter (\(\mu \text{S}/\text{cm}\)). Conversely, tap water usually exhibits conductivity values between 50 and 800 \(\mu \text{S}/\text{cm}\) due to its high dissolved solid content. The removal of dissolved atmospheric gases and mineral ions also results in a neutral state, meaning the water possesses a \(\text{pH}\) very close to 7.0.
The Science of Distillation
The process of distillation relies on the physical principle of phase change to separate water from non-volatile impurities. It is a multi-step technique designed to exploit the significant difference between the boiling point of water and the contaminants. The first step involves heating the source water to its boiling point, 100°C (212°F) at standard atmospheric pressure. At this temperature, the water changes state into steam, leaving behind substances that have higher boiling points, such as salts, heavy metals, and non-volatile organic compounds.
The second stage involves guiding the pure water vapor through a cooling apparatus, known as a condenser. This reduction in temperature causes the steam to lose energy and revert back into its liquid state, a process called condensation. The final step is the collection of this condensed liquid into a separate, clean container. This physical separation technique is highly effective because the vast majority of minerals, bacteria, and non-volatile materials cannot vaporize with the water.
Essential Laboratory Applications
The high level of purity achieved through distillation makes the resulting water indispensable for a variety of laboratory applications. Distilled water is routinely used to prepare standard solutions, where the exact concentration of a solute must be known and verified. Using contaminated water in this process would introduce unknown variables, skewing the final concentration and invalidating the experiment. It is also employed as a blank or control sample in analytical instrumentation, establishing a baseline measurement free from background interference.
Furthermore, the water’s purity is necessary for cleaning sensitive, high-precision glassware used in quantitative analysis. Even trace amounts of residual ions or organic residue from tap water could interfere with subsequent experiments. While deionized (DI) water is effective at removing charged ions, distilled water is often preferred for sensitive chemical work because the boiling process also eliminates non-volatile organic contaminants and biological substances.