Hard water
Hard water has high mineral content, affecting soap and plumbing.
Hard water contains a high concentration of dissolved minerals, setting it apart from soft water. It develops when rainwater moves through underground deposits like limestone, chalk, or gypsum, picking up calcium and magnesium carbonates, bicarbonates, and sulfates along the way.
Drinking hard water may offer modest health advantages because of these minerals. However, it creates serious issues in industrial settings, where facilities monitor hardness levels to prevent damaging limescale buildup in boilers, cooling towers, and other water-handling equipment. At home, hard water reveals itself through poor soap lather and the white scale that forms inside kettles and water heaters. To counteract these problems, water softening is often employed.
**Origins**
Rain, snow, and other precipitation naturally contain very few divalent cations like calcium and magnesium, though they may carry small amounts of sodium, chloride, or sulfate from sea spray. When this precipitation falls in drainage basins made of hard, impervious, calcium-poor rocks, the water remains low in divalent cations and is classified as soft. Examples include Snowdonia in Wales and the Western Highlands of Scotland. In regions with complex geology, water hardness can vary significantly over short distances.
**Types**
**Permanent Hardness**
Permanent hardness comes from the concentration of cations with a charge of 2+ or higher—most often Ca²⁺ and Mg²⁺. These ions typically enter water by leaching from minerals in aquifers, such as calcite and gypsum (calcium sources) or dolomite (a magnesium and calcium mineral). Rainwater and distilled water are soft because they lack these ions.
A reversible reaction governs the dissolving and re-formation of calcium carbonate and calcium bicarbonate. Rain containing dissolved carbon dioxide can react with calcium carbonate, carrying calcium ions away. As carbon dioxide escapes into the atmosphere, calcium carbonate may redeposit as calcite, sometimes forming stalactites and stalagmites. Water softeners can remove calcium and magnesium ions, but boiling does not easily eliminate permanent hardness. This type is usually caused by calcium sulfate, calcium chloride, magnesium sulfate, or magnesium chloride, which do not precipitate when heated. Instead, ion-exchange columns or water softeners are needed.
- Common cations
- Ca2+ and Mg2+
- Common minerals
- calcite, gypsum, dolomite
- Temporary hardness cause
- dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate)
- Permanent hardness cause
- calcium sulfate/calcium chloride and/or magnesium sulfate/magnesium chloride
- Soap scum component
- calcium stearate
- Scale components
- calcium carbonate (CaCO3), magnesium hydroxide (Mg(OH)2), calcium sulfate (CaSO4)
- Recommended calcium level
- 40–80 ppm
- Recommended magnesium level
- 20–30 ppm
Lore & Background
Hard water originates when precipitation falls on drainage basins formed of hard, impervious and calcium-poor rocks, producing soft water, or when water percolates through limestone, chalk or gypsum deposits, picking up calcium and magnesium ions. Areas with complex geology can produce varying degrees of hardness over short distances. Permanent hardness is caused by divalent cations such as Ca2+ and Mg2+ from minerals like calcite and gypsum, and is generally difficult to remove by boiling. Temporary hardness is caused by dissolved bicarbonate minerals and can be reduced by boiling or lime softening.
Reader's Guide
Hard water is significant in both domestic and industrial settings. In domestic settings, it is indicated by a lack of foam formation when soap is agitated in water and by limescale in kettles and water heaters. In industrial settings, water hardness is monitored to avoid problematic limescaling in boilers, cooling towers, and other equipment. Hard water forms deposits called scale, composed mainly of calcium carbonate, magnesium hydroxide, and calcium sulfate, which clog plumbing and reduce heating efficiency. The World Health Organization states there is no convincing evidence that water hardness causes adverse health effects, and the United States National Research Council found it serves as a dietary supplement for calcium and magnesium. Water softening is commonly used to reduce hard water's adverse effects, often through ion-exchange resins or washing soda.
Did You Know?
- Hard water is formed when water percolates through deposits of limestone, chalk or gypsum.
- Temporary hardness can be reduced by boiling the water or by adding lime (calcium hydroxide).
- Soft water produces no calcium deposits in water heating systems.
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