Water described as “hard” is high in dissolved minerals, specifically calcium and magnesium. Hard water is not a health risk, but a nuisance because of mineral build up on fixtures and poor soap and/or detergent performance.
Water is a good solvent and picks up impurities easily. Pure water — tasteless, colorless, and odorless — is often called the universal solvent. As water moves through soil and rock, it dissolves very small amounts of minerals and holds them in solution. Calcium and magnesium dissolved in water are the two most common minerals that make water “hard.” The degree of hardness becomes greater as the calcium and magnesium content increases.
If soft water tastes bad, then why might you use a water softener? The answer is that extremely hard water may shorten the life of plumbing and lessen the effectiveness of certain cleaning agents. When hard water is heated, the carbonates precipitate out of solution, forming scale in pipes and tea kettles. In addition to narrowing and potentially clogging the pipes, scale prevents efficient heat transfer, so a water heater with scale will have to use a lot of energy to give you hot water. Soap is less effective in hard water because its reacts to form the calcium or magnesium salt of the organic acid of the soap. These salts are insoluble and form grayish soap scum, but no cleansing lather. Detergents, on the other hand, lather in both hard and soft water. Calcium and magnesium salts of the detergent’s organic acids form, but these salts are soluble in water.
Hard water can be softened (have its minerals removed) by treating it with lime or by passing it over an ion exchange resin. The ion exchange resins are complex sodium salts. Water flows over the resin surface, dissolving the sodium. The calcium, magnesium, and other cations precipitate onto the resin surface. Sodium goes into the water, but the other cations stay with the resin. Very hard water will end up tasting saltier than water that had fewer dissolved minerals.
Most of the ions have been removed in soft water, but sodium and various anions (negatively charged ions) still remain. Water can be deionized by using a resin that replaces cations with hydrogen and anions with hydroxide. With this type of resin, the cations stick to the resin and the hydrogen and hydroxide that are released combine to form pure water.
Is hard water harmful to health? No, not even very hard water. In fact there is evidence that calcium and other minerals in hard waters may be generally beneficial to health. Some studies have shown that hard water may contribute to a worsening of existing eczema in some individuals. The reason for this is uncertain and the evidence is very limited, if a water softener is installed for this reason, it is recommend that a drinking water tap that is not softened is retained.
What are the effects of hard water? Hard water can cause scale to form in kettles, steam irons and around taps and shower heads. Also, using hard water for washing can require slightly more soap or washing powder and can leave a scum around basins and baths. Washing powders and detergents are designed to wash perfectly well with hard water and most automatic dishwashers have built-in water softeners, to avoid “spotting” of crockery and glassware.