Water quality is assessed using two fundamental metrics: mineral content (hardness) and a measure of acidity or alkalinity (pH). Understanding the relationship between these two characteristics is important for maintaining household plumbing integrity and the overall quality of water used daily. The interaction between low mineral content and pH determines how water behaves within a distribution system and affects the consumer experience. The condition of the water supply is a major factor in the longevity of infrastructure and the safety of the water itself.
Defining Water Hardness and Softness
Water hardness measures the concentration of dissolved divalent cations, primarily calcium and magnesium. These minerals enter the water supply as it flows through rocks and soil, such as limestone or gypsum. Water is classified as soft when the concentration of these minerals is very low.
Classification uses two standard units: parts per million (ppm) or grains per gallon (gpg). Water is considered soft if it contains less than 1.0 gpg (equivalent to less than 17.1 ppm of calcium carbonate). Conversely, hard water is categorized by higher mineral content, often exceeding 7.0 gpg. This distinction is based solely on mineral content, not acidity or alkalinity.
Understanding the pH Range of Soft Water
The pH scale determines how acidic or alkaline water is, ranging from 0 (highly acidic) to 14 (highly alkaline), with 7 being neutral. While pure water has a neutral pH of 7, natural water supplies typically fall between 6.5 and 8.5. Naturally soft water often leans slightly toward the acidic side, sometimes ranging from 6.0 to 7.0.
Soft water tends toward a lower pH because of its low mineral content. Hard water contains dissolved minerals like calcium and magnesium bicarbonates that act as buffers, resisting changes in pH. Soft water lacks these buffers, meaning that even small amounts of naturally occurring acids, such as dissolved carbon dioxide, can cause the pH to drop below 7.0 more easily. This low-alkalinity condition makes the water chemically less stable.
Corrosivity Why pH is Critical in Soft Water Systems
The most significant consequence of soft water with a low pH is its corrosivity, the tendency to dissolve metal. Soft water that is slightly acidic, particularly with a pH below 7.0, becomes aggressive toward metallic plumbing materials. This occurs because the water lacks the mineral content necessary to form a protective layer, known as a passivation film, on the interior surface of pipes.
Without this film, low-pH water accelerates the degradation of pipes made from copper, brass, and galvanized steel. This chemical reaction, known as leaching, causes pipe materials to dissolve into the water supply. A major concern involves the leaching of heavy metals, such as copper and lead, into the drinking water.
The presence of lead, found in solder joints or older service lines, poses a significant health risk. Corrosive water increases the concentration of these heavy metals, which can be high when water has been sitting stagnant in the pipes. Addressing the pH of soft water is an important public health measure to protect both plumbing infrastructure and human safety.
Daily Life and Consumer Experience
For the consumer, soft water presents several noticeable differences in daily use, largely due to the absence of mineral ions. One immediate benefit is enhanced soap lathering, resulting in greater cleaning efficiency and less detergent usage. Since the water does not contain scale-forming minerals, it also prevents the build-up of soap scum on fixtures and appliances.
The sensation of soft water on the skin is often described as slick or slippery after showering. This feeling is the smooth texture of soap rinsing more completely and the skin’s natural oils being left undisturbed, not a residue. Soft water can also affect the sensory experience of drinking water.
Some people find that soft water has a “flat” or “thin” taste compared to the mineral-rich profile of hard water. This perceived difference is related to the lack of dissolved minerals that contribute to the water’s overall flavor.