Water quality is often described as “hard” or “soft,” a classification based on its mineral makeup. When water undergoes purification, its characteristics change dramatically, leading to confusion about its final classification. Understanding whether purified water is hard or soft requires examining the chemical properties that define these terms and how purification alters the water’s composition.
What Defines Hard and Soft Water?
Water hardness is a measure based on the concentration of specific dissolved minerals, primarily divalent metallic cations like calcium (Ca2+) and magnesium (Mg2+). Water acquires these minerals as it travels through geological formations, such as limestone and gypsum. The amount of these minerals dictates whether the water is classified as hard or soft.
Hardness is commonly measured in milligrams per liter (mg/L) of calcium carbonate equivalent. Water with a concentration below 60 mg/L is considered soft. Conversely, water is classified as moderately hard starting at 61 mg/L and very hard above 180 mg/L.
How Purification Affects Mineral Content
Purified water is chemically defined as soft because the processes used to create it are designed to strip away virtually all dissolved solids. Methods like Reverse Osmosis (RO) and distillation effectively remove the calcium and magnesium ions that cause hardness. This removal ensures the resulting water fits the “soft” classification, often with mineral content far below the 60 mg/L threshold.
Reverse Osmosis
Reverse Osmosis forces water through a semipermeable membrane under pressure. The membrane’s small pores physically block larger ions, such as calcium and magnesium, while allowing water molecules to pass. The rejected mineral ions are flushed away in a concentrated waste stream, leaving water with significantly reduced mineral content.
Distillation
Distillation achieves the same result using a phase change. Water is boiled, turning it into steam, which is then cooled and condensed back into liquid form. Since hardness-causing minerals are non-volatile, they do not evaporate with the water and are left behind in the boiling chamber.
The Practical Difference of Using Soft Water
The low mineral content of purified, soft water leads to several practical differences compared to hard water. Soft water allows soap to lather more effectively during cleaning. Hard water minerals react with soap to form soap scum, requiring more detergent for a good clean. Soft water minimizes this reaction, resulting in richer suds and cleaner rinsing.
Another important difference is the absence of scale buildup in appliances and plumbing. When hard water is heated, dissolved calcium carbonate precipitates out, forming limescale. Purified water lacks these minerals, preventing this damaging scale from forming in kettles, coffee makers, and water heaters. This prolongs their lifespan and maintains energy efficiency.
Highly purified water is often described as having a “flat” or “dull” taste because the dissolved minerals that contribute to flavor have been removed. However, this extreme purity can introduce a drawback regarding corrosivity. Water with very low total dissolved solids has little buffering capacity, meaning it can be slightly aggressive toward metals. This potentially leaches trace amounts of copper or lead from some pipes and fixtures over time.