What Ions Make Water Hard?
The phenomenon of hard water, although not dangerous to human health, causes several nuisances. The minerals responsible for hard water tend to form insoluble salts, also known as lime scale, in water pipes. They also form insoluble compounds in the presence of surfactants, such as those found in laundry detergents and bath soap, that result in soap scum. The severity of water hardness varies with geographic region. Modern water-softening systems are highly effective at reducing or eliminating hard-water ions.
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Calcium and Magnesium
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Limestone consists primarily of calcium carbonate, or CaCO3. Calcium carbonate does not exhibit particularly high solubility in water, but rainwater exhibits slightly acidic properties and reacts with the calcium carbonate to produce calcium hydrogen carbonate. Calcium hydrogen carbonate exhibits much better solubility than calcium carbonate, and as it dissolves, it undergoes a process called dissociation in which the ions separate into calcium ions, Ca(2+) and bicarbonate ions, HCO3(-). As such, regions with numerous or large limestone deposits will tend to exhibit harder water than regions with less limestone. Many rocks and minerals that contain calcium carbonate also contain somewhat smaller quantities of magnesium carbonate, which is prone to the same reactions as calcium.
Other Metals
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Chemists have identified several other "trace" metals that contribute to hard water, primarily iron and manganese. Iron tends to form reddish-brown deposits, whereas manganese deposits exhibit a brownish-black color. Like magnesium and calcium, their presence does not pose human health concerns, but they may impart a perceptible metallic taste to drinking water.
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Quantifying Hard Water
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The simplest tests to determine the extent of water hardness, sometimes called "alkalinity" do not discriminate between the various metals present in the water. Chemists instead determine the "equivalents" of calcium carbonate in units of milligrams of calcium carbonate per liter or water, or mg CaCO3/L. This method essentially assumes that the only hard-water species present is calcium carbonate. Or, more specifically, a solution that contained the specified number of grams of calcium carbonate per liter would be equivalent in hardness to the analyzed sample. A hardness in excess of 100 mg CaCO3/L generally constitutes hard water.
Water Softening
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Water softening systems use a method called "ion exchange" to remove hard-water ions. The systems work by passing the hard water through a resin impregnated with either sodium ions, Na+, or potassium ions, K+. The hard-water ions exhibit a stronger affinity for binding to the resin than do the sodium and potassium ions. Hence, the system "exchanges" hard-water ions for sodium or potassium ions. The sodium and potassium ions do not cause hard water. A sodium exchange system, however, inevitably increases the sodium ion content of a household's drinking water, which could be problematic for anyone on a low-sodium diet.
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