While both types of water undergo specific treatment processes designed to remove certain substances, they achieve fundamentally different chemical states and levels of purity. Understanding the distinct processes used to create soft water and distilled water clarifies their unique compositions and appropriate applications.
Defining Soft Water and the Softening Process
Soft water is defined primarily by its low concentration of dissolved divalent metal ions, most notably calcium (Ca\(^{2+}\)) and magnesium (Mg\(^{2+}\)). Water containing high levels of these minerals is known as hard water, which is recognized for its tendency to form scale deposits and inhibit the lathering of soap. The process used to create soft water is known as ion exchange, which is typically carried out using a water softener unit.
During ion exchange, the hard mineral ions are chemically traded for monovalent ions, usually sodium (Na\(^{+}\)) or sometimes potassium (K\(^{+}\)) ions. The water passes through a bed of resin beads that are charged with these sodium ions. As the water flows over the resin, the calcium and magnesium ions detach from the water molecules and bind to the resin, simultaneously releasing sodium ions into the water. This substitution effectively reduces the water’s hardness, but it does not remove all other dissolved solids or contaminants.
Defining Distilled Water and the Distillation Process
Distilled water is created through distillation, one of the oldest methods of water treatment, which achieves a much higher level of purification. This process involves heating the water until it vaporizes into steam, leaving behind nearly all non-volatile impurities in the boiling chamber. These discarded substances include minerals, heavy metals, inorganic compounds, and most large organic molecules.
The purified steam is then collected and cooled in a separate condenser unit, where it converts back into liquid water. Because the minerals and dissolved solids do not transition into the steam phase, the resulting distilled water is characterized by an extremely low concentration of Total Dissolved Solids (TDS). This rigorous purification method also effectively kills microorganisms such as bacteria and viruses due to the boiling temperatures involved.
The Crucial Difference in Chemical Composition
Soft water is treated selectively to remove only the hardness-causing minerals, meaning it still contains other dissolved solids and electrolytes. Specifically, soft water contains an elevated amount of sodium or potassium salts, which are the byproduct of the ion exchange process. Distilled water, conversely, is virtually pure H\(_{2}\)O, with almost all dissolved substances, including the sodium salts found in soft water, removed.
This comprehensive removal of ions results in water with near-zero electrical conductivity, a key indicator of its purity. The goal of softening is to manage scale and improve soap performance, while the goal of distillation is to achieve maximum purity by removing nearly all contaminants for specialized applications.
When to Use Each Type of Water
The distinct compositions of each water type dictate their appropriate uses, linking the chemical outcome to the practical application. Soft water is the preferred choice for general household purposes, where the reduction of calcium and magnesium prevents scale buildup in plumbing and appliances like washing machines and dishwashers. Its ability to enhance the effectiveness of soaps and detergents makes it ideal for laundry, bathing, and cleaning, resulting in less soap scum and softer fabrics.
Distilled water, with its exceptional purity, is reserved for applications where even trace minerals could interfere or cause damage. This includes use in scientific laboratories for preparing reagents and solutions, as well as in medical devices like Continuous Positive Airway Pressure (CPAP) machines and humidifiers to prevent mineral inhalation. Furthermore, it is recommended for use in car batteries, steam irons, and engine cooling systems, where the absence of dissolved solids prevents corrosive mineral deposits from forming.