Soft water and distilled water are often mistakenly believed to be interchangeable because both are forms of treated water. However, these two types of water are fundamentally different based on their creation processes and resulting chemical makeup. Understanding these distinctions is important for consumers, as the appropriate use for each type of water is governed by its final composition.
Understanding Soft Water
Soft water is defined by its low concentration of dissolved minerals, specifically calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)) ions. These two cations are responsible for water hardness, which causes limescale buildup in pipes and appliances. The process of creating soft water, known as water softening, targets the removal of these hardness-causing ions.
The most common method is ion exchange, typically using a water softening system. Hard water flows over resin beads charged with sodium (\(\text{Na}^{+}\)) or potassium (\(\text{K}^{+}\)) ions. The resin captures the calcium and magnesium ions, releasing sodium or potassium ions into the water in their place.
This process results in low levels of calcium and magnesium, often less than 17.1 parts per million (ppm). However, the treated water is not free of dissolved solids. It contains a higher concentration of the replacement ions, usually sodium, meaning soft water remains a solution containing salts and other trace elements.
Understanding Distilled Water
Distilled water represents a highly aggressive form of purification, designed to remove virtually all impurities, minerals, and contaminants. It is produced through distillation, which mimics the natural water cycle of evaporation and condensation. Raw water is heated to its boiling point, converting it into steam or vapor.
Since most dissolved solids, including minerals, salts, and heavy metals, have a higher boiling point than water, they are left behind in the boiling vessel. The pure water vapor is collected in a separate, cooled chamber, where it condenses back into a liquid state. This collected liquid is the distilled water, which is essentially pure \(\text{H}_2\text{O}\).
The result is water with an extremely low level of total dissolved solids, often less than 1 ppm, making it de-mineralized and de-ionized. This method effectively removes hardness ions, trace elements, organic compounds, and many other inorganic compounds. Distilled water is considered the highest standard of purity outside of specialized laboratory grades.
Key Differences and Appropriate Uses
The fundamental difference lies in their composition and purification method. Soft water is chemically treated water where specific divalent ions (\(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\)) are exchanged for monovalent ions (\(\text{Na}^{+}\) or \(\text{K}^{+}\)). Distilled water is physically purified water where almost all dissolved matter is separated from the \(\text{H}_2\text{O}\) molecule through phase change.
Soft water’s primary purpose is for general household use in areas with hard water. It improves laundry efficiency and appliance lifespan by preventing the formation of limescale inside water heaters, dishwashers, and plumbing systems. It is generally acceptable for drinking, though the increased sodium content can be a concern for individuals on restricted diets.
Distilled water is reserved for specialized applications where the presence of any dissolved solid, even sodium, could cause interference or damage. Its near-zero mineral content makes it the required choice for medical equipment, such as Continuous Positive Airway Pressure (CPAP) machines, to prevent mineral buildup in the humidifier. It is also used in laboratory testing, automotive cooling systems, and steam irons, as the absence of minerals ensures no residue or corrosion will occur.