Distilled water is categorized as extremely soft water. This classification is a direct result of the purification process that removes the dissolved minerals responsible for water hardness. Water that has been processed to remove these components naturally falls to the lowest point on the hardness scale.
Defining Water Hardness
Water hardness is defined by the concentration of specific dissolved multivalent metallic cations, primarily calcium (\(text{Ca}^{2+}\)) and magnesium (\(text{Mg}^{2+}\)) ions, acquired as water passes through geological deposits like limestone and gypsum. The concentration of these ions is typically measured and reported in terms of milligrams per liter (\(text{mg/L}\)) of equivalent calcium carbonate (\(text{CaCO}_3\)). Water is generally classified as “soft” if it contains less than \(60 text{ mg/L}\) of \(text{CaCO}_3\), while concentrations exceeding \(180 text{ mg/L}\) are considered “very hard.”
The presence of these mineral ions affects how water interacts with other substances, most notably soap and heat. Hard water reacts with soap to form an insoluble precipitate, often referred to as soap scum, which reduces the soap’s ability to lather. This reaction necessitates the use of more soap or detergent to achieve effective cleaning. Furthermore, heating hard water causes the dissolved minerals to precipitate out of the solution, forming a solid deposit known as scale inside pipes and appliances like kettles and water heaters, which can reduce their efficiency over time.
The Distillation Process and Mineral Removal
The purification method of distillation is a phase-change process that effectively separates water molecules from virtually all dissolved contaminants, including the hardness-causing minerals. This process is carried out in three distinct stages: vaporization, separation, and condensation. The initial step involves heating the source water in a boiling chamber until it reaches its boiling point of \(100^{circ} text{C}\).
As the water turns into steam, it leaves behind any substance with a significantly higher boiling point, including inorganic minerals and salts like calcium and magnesium. These compounds remain as residue in the boiling chamber because their boiling points are much higher than that of water. The steam, which is now pure water vapor, rises and is physically separated from the contaminated liquid remaining below.
In the final stage, the pure steam is directed into a condenser, where it is cooled. This temperature drop causes the vapor to revert to its liquid state, yielding the purified product. Since the condensed liquid water did not carry the dissolved minerals through the process, the resulting distilled water has an extremely low concentration of total dissolved solids, often removing more than \(99.9\) percent of the original material.
Distilled Water Versus Other Purified Types
Distilled water is often confused with other types of purified water, such as those treated by Reverse Osmosis (RO) or Deionization (DI), but the level of purity achieved by distillation is distinct. Reverse Osmosis systems purify water by forcing it under pressure through a semipermeable membrane that acts as a physical filter. This process is highly effective, removing up to \(99\) percent of total dissolved solids, but it is less comprehensive than distillation and may still leave trace amounts of certain smaller ions or volatile organic compounds.
Deionized water, conversely, is created using ion exchange resins that selectively remove charged mineral ions, replacing them with hydrogen and hydroxyl ions. While this method is excellent at removing salts and minerals, it is specifically designed to target only charged particles. Therefore, \(text{DI}\) water may not effectively remove uncharged particles, bacteria, or organic compounds unless the water is pre-treated.
Distillation physically changes the water’s state, leaving virtually everything non-water behind, which makes it a comprehensive purification method. The resulting product is characterized by a measurement of total dissolved solids that is nearly zero. This confirms its status as the softest form of water commercially available.