Hard water is water that contains large amounts of calcium (Ca), magnesium (Mg), or iron (Fe) ions; as opposed to soft water that does not contain large amounts of such minerals. Hard water is undesirable since it often has an unpleasant taste, interferes with the ability of soaps to dissolve (although some synthetic detergents dissolve well in hard water), and can cause scaling (the building up of insoluble precipitates) in pipes and hot water systems. On the other hand, hard water is generally not dangerous to human safety or health.
Water hardness is most commonly the result of dissolved calcium or magnesium ions, often caused by limestone or dolomite dissolving slightly when acidic water containing carbon dioxide runs through these minerals. These dissolved minerals lead to an increase in the amounts of calcium and magnesium ions. Water hardness where the negative ion (anion) is bicarbonate (as in the cases above) is sometimes called temporary hardness, since the unwanted ions can be reduced by boiling the water. If the anion is not bicarbonate, but is instead sulfate or chloride, then permanent hardness is said to result, and this condition cannot be remedied by merely boiling the water.
In either case, the calcium and carbonate ions (or calcium and sulfate ions) may deposit along the inside of pipes and water heating systems, leading to boiler scale. This scaling can significantly reduce the efficiency of a heating system and can build up to such an extent that the entire pipe is plugged, often leading to overheating of the boiler.
Hard water can be treated either by boiling the water (a method effective only for small quantities) or by precipitating the calcium or magnesium ions from the water (this method is also not practical for large quantities of water). A more efficient method is to use ion exchangers, in which the unwanted calcium and magnesium ions are exchanged or traded for sodium ions that do not form any insoluble precipitates and, thus, do not cause scaling. Most water softeners work by the ion exchange method. The soft water produced is not free of ions, only of undesirable ions.
Other methods are available for removing ions, including reverse osmosis and magnetic water conditioning. Reverse osmosis removes almost 100% of undesirable materials from water, including the hard water ions. This method uses pressure to force water to flow from a solution of concentrated minerals to one of dilute mineral content, the reverse direction of natural osmosis. The water flows through a semi-permeable membrane, which allows the water molecules to pass while filtering out unwanted molecules. This procedure requires several steps and is not as common in the home as ion exchange. Magnetic water conditioning occurs when electromagnets are attached to water pipes. These electromagnets create a strong magnetic field within the pipe, which keeps the hard water minerals from precipitating into the plumbing. This method has not been scientifically proven, and it is unknown if and how it actually works. Electromagnets have been installed in thousands of homes in the United States, but are not as common or reliable as ion exchange water softeners. One drawback to the ion exchange method is that the water produced is slightly acidic and contains a large amount of sodium ions. The acidic water can damage metal pipes over time, and there is an established link between sodium consumption and heart disease.
See also Ion and Ionization.