Definition - What does Hard Water mean?
Hard water is water that has high mineral content (as opposed to "soft water"). Hard water is used to describe any water that contains mineral ions with a charge of +2. It is formed when water percolates through deposits of calcium and magnesium-containing minerals such as limestone, chalk and dolomite.
Hard water can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling towers and other equipment that handles water. Wherever water hardness is a concern, water softening is commonly used to reduce hard water's adverse effects.
Corrosionpedia explains Hard Water
Water hardness is caused by natural minerals that are dissolved in water. Calcium and magnesium are the most common minerals that make water hard. When heated, these minerals precipitate out of water and encrust themselves onto items as scale or mineral deposits, affecting the performance of equipment. It also clogs pipes, which reduces water flow, and it can cause accumulation of film and scale on bath and kitchen fixtures, reducing the life of clothing.
Scaling tends to be the result of water with a high hardness. Hard water typically contains calcium compounds which can precipitate out as calcium carbonate. However, if the hardness in the water is primarily noncarbonate, the chlorate and sulfate ions tend to keep the calcium in solution and prevent scale formation.
Pipes can become clogged with scale that reduces water flow and ultimately requires pipe replacement. Limescale has been known to increase energy bills by up to 25%.
In pipes, pitting is associated with hard or moderately hard water, with a pH between 7 and 7.8, and it is most likely to occur in cold water. The pitting is deep and narrow, and results in pipe failure. Though hard water creates scaling problems, it is less corrosive than softer water. The scale-forming properties of hard water tend to form a protective film on the surface of metals, providing corrosion protection.
Hard water can be softened (have its minerals removed) by treating it with lime or by passing it over an ion exchange resin. Water can be deionized by using a resin that replaces cations with hydrogen and anions with hydroxide.